Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
IV. Regulations and
Maintenance
25. Nutrition, Metabolism,
and Temperature
Regulation
© The McGraw−Hill
Companies, 2004
We are usuallymore concerned with
the taste of food than with itsnutri-
tional value when choosing from a
menu or when selecting food to pre-
pare. Knowing about nutrition is impor-
tant, however, because we literally are
what we eat. Food providesus with energy
and the building blocksnecessary to synthesize
new molecules. What happens if we don’t obtain
enough vitamins, or ifwe eattoo much sugar and fats? Health claims about foods
and food supplements bombard us every day. Which onesare ridiculous, and
which oneshave merit? A basic understanding of nutrition can help usto answer
these and other questionsso that we can develop a healthy diet.
Thischapter explains nutrition (912), metabolism(920), carbohydrate me-
tabolism (922), lipid metabolism (929), protein metabolism (930), interconver-
sion of nutrient molecules (931), metabolic states (932), metabolic rate (934),
andbody temperature regulation (935).
Colorized scanning electron micrograph (SEM) of
a mitochondrion in the cytoplasm of an intestinal
epithelial cell. The mitochondrion hasan outer
and innermembrane. The inner membrane has
numerousfolds that project into the interior of the
mitochondrion. Enzymes, necessaryfor producing
ATP, are located in these folds.
CHAPTER
25
Nutrition,
Metabolism,
and
Temperature
Regulation
Part 4 Regulationsand Maintenance
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
IV. Regulations and
Maintenance
25. Nutrition, Metabolism,
and Temperature
Regulation
© The McGraw−Hill
Companies, 2004
Part4 Regulationsand Maintenance912
Benefitsof a Healthy Diet
Two studiescompleted in 2000 compared the eating habits of67,272
women and 51,529 men to the government’sHealthy Eating Index, a
measure ofhow well a diet conforms to dietary guidelines and the food
pyramid. Those who ate best, according to the index, were compared to
those who ate the worst. Men who ate besthad a 28% reduction in heart
disease and a 11% decrease in chronicdiseasescompared to men who
ate worst. Women who ate besthad a 14% reduction in heart disease
butno significant decrease in chronic diseasescompared to women who
ate worst. There wasno significant difference in cancer between the men
and women who ate bestcompared to those who ate worst.
1. Define the terms nutrient and essential nutrient, and list the
sixmajor classes of nutrients.
2. List the six food groups shown in a food guide pyramid.
Whatfood groups are at the bottom and top of the pyramid?
Kilocalories
The energy stored within the chemical bonds ofcertain nutrients is
used by the body.A calorie (kalo¯-re¯;cal)is the amount of energy
(heat) necessary to raise the temperature of 1 g of water 1°C. A
kilocalorie(kilo¯-kal-o¯-re¯;kcal) is 1000 calories and is used to ex-
press the larger amounts ofenerg y supplied by foods and released
through metabolism.
WhatIs a Calorie?
A kilocalorie isoften called a Calorie (with a capitalC ). Unfortunately,
thisusage has resulted in confusion between the terms calorie(with a
lowercase c) and Calorie(with a capital C). It’s common practice on food
labelsand in nutrition books to use calorie when Calorie (kilocalorie) is
the proper term.
Nutrition
Objectives
■ Define nutrients, and describe the food guide pyramid.
■ Describe for carbohydrates, lipids, and proteinstheir
dietarysources, their uses in the body, and the daily
recommended amountsof each in the diet.
■ List the vitamins and minerals, and indicate the function of
each. Define the termsDaily Values and % Daily Value.
Nutrition is the process by which certain components of
food are obtained and used by the body.The process includes di-
gestion, absorption, transportation, and cell metabolism. Nutri-
tion is also defined as the evaluation of food and drink
requirements for normal body function.
Nutrients
Nutrients are the chemicals taken into the body that are used to
produce energy,provide building blocks for new molecules, or
function in other chemical reactions.Some important substances
in food,such as nondigestible plant fibers, are not nutrients. Nutri-
ents are divided into six major classes: carbohydrates, proteins,
lipids,vitamins, minerals, and water. Carbohydrates,proteins, and
lipids are the major organic nutrients and are broken down by en-
zymes into their individual components during digestion.Many of
these subunits are broken down further to supply energy,whereas
others are used as building blocks for other macromolecules.Car-
bohydrates,proteins, lipids, and water are required in fairly sub-
stantial quantities,whereas vitamins and minerals are required in
only small amounts.Vitamins, minerals, and water are taken into
the body without being digested.
Essential nutrientsare nutrients that must be ingested be-
cause the body cannot manufacture them or is unable to manu-
facture adequate amounts of them. The essential nutrients
include certain amino acids, certain fatty acids, most vitamins,
minerals, water,and a minimum amount of carbohydrates. The
term essential doesn’t mean, however,that only the essential nu-
trients are required by the body.Other nutrients are necessary,
but,if they are not part of the diet,they can be synthesized from
other ingested nutrients.Most of this synthesis takes place in the
liver,which has a remarkable ability to transform and manufac-
ture molecules.
The U.S.Depart ment of Agriculture provides recommen-
dations for obtaining the proper amounts of carbohydrates,
lipids, proteins, vitamins, minerals,and fiber in the form of a
“food guide pyramid”(figure 25.1). The six food groups shown in
the pyramid are (1) bread,cereal, rice, and pasta; (2) vegetables;
(3) fruits;(4) milk, yogurt, and cheese; (5) meat, poultry,fish, dry
beans,eggs, and nuts; and (6) fats, oils, and sweets. The shape of
the pyramid suggests that grains,vegetables, and fruits should be
the main part of the diet. Fats, oils, and sweets can be used in
moderation to improve the flavor of foods.A balanced diet in-
cludes a variety of foods from each of the major food groups.Va-
riety is necessary because no one food contains all the nutrients
necessary for good health.
Fats, oils, and sweets
(use sparingly)
Meat, poultry, fish,
dry beans, eggs,
and nut group
(2–3 servings)
Fruit group
(2–4 servings)
Milk, yogurt, and
cheese group
(2–3 servings)
Vegetable group
(3–5 servings)
Bread, cereal, rice, and pasta group
(6–11 servings)
MILK
MILK
MILK
Figure 25.1
Food Guide Pyramid
The pyramid suggeststhree approaches to a healthy diet: eat different
amountsof foods from each basic food group, use fatsand sugars sparingly,
and choose varietyby eating the indicated number of servings per day ofthe
differentfoods from each major food group.
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
IV. Regulations and
Maintenance
25. Nutrition, Metabolism,
and Temperature
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© The McGraw−Hill
Companies, 2004
Chapter 25 Nutrition, Metabolism, and Temperature Regulation 913
Almost all of the kilocalories supplied by food come from
carbohydrates,proteins, or fats. For each gram of carbohydrate or
protein that the body metabolizes, about 4 kcal of energy is re-
leased.Fats contain more energy per unit of weight than carbohy-
drates and proteins and yield about 9 kcal/g. Table 25.1 lists the
kilocalories supplied by some typical foods. A typical diet in the
United States consists of50%–60% carbohydrates, 35%–45% fats,
and 10%–15% protein.Table 25.1 also lists the carbohydrate, fat,
and protein composition ofsome foods.
3. Define the term kilocalorie, and state the number of
kilocaloriessupplied by a gram of carbohydrate, lipid, and
protein.
Carbohydrates
Sourcesin the Diet
Carbohydrates include monosaccharides, disaccharides, and
polysaccharides (see chapter 2).Most of the carbohydrates humans
ingest come from plants. An exception is lactose (milk sugar),
which is found in animal and human milk.
The most common monosaccharides in the diet are glucose
and fructose.Plants capture the energy in sunlight and use the en-
ergy to produce glucose which can be found in vegetables.Fructose
(fruit sugar) and galactose are isomers ofg lucose (see figure 2.14).
Glucose is found in vegetables and fructose is found in fruits,
berries, honey, and high-fructose corn syrup, which is used to
sweeten soft drinks and desserts.Galactose is usually found in milk.
The disaccharide sucrose (table sugar) is what most people
think of when they use the term sugar.Sucrose is a glucose and a
fructose molecule joined together,and its pr incipal sources are
sugarcane, sugar beets, maple sugar,and honey. Maltose (malt
sugar),derived from germinating cereals, is a combination of two
glucose molecules,and lactose (in milk) consists of a glucose and a
galactose molecule (see figure 2.14).
Thecomplex carbohydrates are the polysaccharides: starch,
glycogen,and cellulose. These polysaccharides consist of many glu-
cose molecules bound together to form long chains.Starch is an
energy storage molecule in plants and is found primarily in vegeta-
bles,fruits, and grains. Glycogen is an energy storage molecule in
animals and is located in muscle and in the liver.By the time meats
Table 25.1 Food Consumption
Food
Energy Carbohydrate Fat Protein
Food Quantity (kcal) (g) (g) (g)
Dairy Products
Whole milk (3.3% fat) 1 cup 150 11 08 08
Low fat milk (2% fat) 1 cup 120 12 05 08
Butter 1 tablespoon 100 — 12 —
Grain
Bread, white enriched 1 slice 75 24 01 02
Bread, whole wheat 1 slice 65 14 01 03
Fruit
Apple 1 80 20 01—
Banana 1 100 26 — 01
Orange 1 65 16 — 01
Vegetables
Corn, canned 1 cup 140 33 01 04
Peas, canned 1 cup 150 29 01 08
Lettuce 1 cup 005 02——
Celery 1 cup 020 05—01
Potato, baked 1 large 145 33 — 04
Meat, Fish, and Poultry
Lean ground beef (10% fat) 3 ounces 185 — 10 23
Shrimp, french fried 3 ounces 190 09 0917
Tuna, canned 3 ounces 170 — 0724
Chicken breast, fried 3 ounces 160 01 0526
Bacon 2 slices 85 — 08 04
Hot dog 1 170 011507
continued
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
IV. Regulations and
Maintenance
25. Nutrition, Metabolism,
and Temperature
Regulation
© The McGraw−Hill
Companies, 2004
Part4 Regulationsand Maintenance914
needed.Because cells can store only a limited amount of glycogen,
any additional glucose is converted into fat,which is stored in adi-
pose tissue.
In addition to being used as a source ofenerg y,sugars have
other functions.They form part of deoxyribonucleic acid (DNA),
ribonucleic acid (RNA),and ATP molecules (see chapter 2); and
they combine with proteins to form glycoproteins,such as the gly-
coprotein receptor molecules on the outer surface of the plasma
membrane (see chapter 3).
Recommended Amounts
It’s recommended that 60% ofthe daily intake of kilocalories be
from carbohydrates.Although a minimum acceptable level of car-
bohydrate ingestion is unknown,consumption of too few carbohy-
drates per day results in overuse of proteins and fats for energy
sources.Because muscles are primarily protein, the use of proteins
for energy can result in the breakdown of muscle tissue, and the
use offats can result in acidosis (see chapter 27).
Complex carbohydrates are recommended because starchy
foods often contain other valuable nutrients like vitamins and
minerals.Although foods like soft drinks and candy are rich in car-
bohydrates,they are mostly sugar and they may have little other
nutritive value. For example, a typical soft drink contains 9
are processed and cooked,they contain little, if any,g lycogen.Cel-
lulose forms cell walls,which surround plant cells.
Usesin the Body
During digestion,polysaccharides and disaccharides are split into
monosaccharides,which are absorbed into the blood (see chapter
24).Humans can digest starch and glycogen because they can break
the bonds between the glucose molecules of starch and glycogen.
Humans are unable to digest cellulose because they can’t break the
bonds between its glucose molecules. Instead, cellulose provides
fiber,or “roughage,”thereby increasing the bulk of feces and pro-
moting defecation.
The liver converts fructose,galactose, and other monosac-
charides absorbed by the blood into glucose.Glucose, whether ab-
sorbed from the digestive tract or produced by the liver,is a
primary energy source for most cells, which use it to produce
adenosine triphosphate (ATP)molecules (see “Anaerobic Respi-
ration”on p.923 and “Aerobic Respiration”on p. 925). Because the
brain relies almost entirely on glucose for its energy,blood glucose
levels are carefully regulated (see chapter 18).
If excess amounts ofg lucose are present,the glucose is con-
verted into glycogen, which is stored in muscle and in the liver.
Glycogen can be rapidly converted back to glucose when energy is
Table 25.1 continued
Food
Energy Carbohydrate Fat Protein
Food Quantity (kcal) (g) (g) (g)
Fast Foods
McDonald’s Egg McMuffin 1 327 031 15 19
McDonald’s Big Mac 1 563 041 33 26
Taco Bell’s beef burrito 1 466 037 21 30
Arby’s roast beef 1 350 032 15 22
Pizza Hut Super Supreme 1 slice 260 023 13 15
Long John Silver’s fish 2 pieces 366 021 22 22
McDonald’s fish fillet 1 432 037 25 14
Dairy Queen malt, large 1 840 125 28 22
Desserts
Cupcake with icing 1 130 021 05 02
Chocolate chip cookie 4 200 029 09 02
Apple pie 1 piece 345 051 15 03
Dairy Queen cone, large 1 340 052 10 10
Beverage
Cola soft drink 12 ounces 145 037 — —
Beer 12 ounces 144 013 — 01
Wine 31⁄2 ounces 73 002——
Hard liquor (86 proof) 11⁄2 ounces 105 0———
Miscellaneous
Egg 1 80 001 06 06
Mayonnaise 1 tablespoon 100 0— 11 —
Sugar 1 tablespoon 45 012 — —
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
IV. Regulations and
Maintenance
25. Nutrition, Metabolism,
and Temperature
Regulation
© The McGraw−Hill
Companies, 2004
Chapter 25 Nutrition, Metabolism, and Temperature Regulation 915
teaspoons of sugar. In excess,the consumption of foods high in
sugar content can result in obesity and tooth decay.
4. What are the most common monosaccharides in the diet?
Whatare sucrose, maltose, and lactose?
5. Give three examples of complex carbohydrates. How are
theyused by the body?
6. How does the body use glucose and other monosaccharides?
7. What quantities of carbohydrate should be ingested daily?
Lipids
Sourcesin the Diet
About 95% of the lipids in the human diet are triglycerides(trı¯-
gliser-ı¯dz). Triglycerides, which are sometimes called triacylglyc-
erols (trı¯-asil-gliser-olz),consist of three fatty acids attached to a
glycerol molecule (see chapter 2).Triglycerides are often referred to
as fats,which are divided into saturated and unsaturated fats. Satu-
rated fatshave only single covalent bonds between the carbon atoms
of their fatty acids (see figure 2.17). They are found in the fats of
meats (e.g.,beef, pork), dairy products (e.g., whole milk,cheese, but-
ter), eggs, coconut oil,and palm oil. Unsaturated fats have one
(monounsaturated) or more (polyunsaturated) double covalent
bonds between the carbon atoms oftheir fatty acids (see figure 2.17).
Monounsaturated fats include olive and peanut oils;and polyunsat-
urated fats occur in fish,safflower, sunflower,and corn oil.
Saturating Fats
Solid fats, such asshortening and margarine, work better than liquid oils
do for preparing some foods, such aspastries. Polyunsaturated
vegetable oilscan be changed from a liquid to a solid by making them
more saturated, thatis, by decreasing the number ofdouble covalent
bondsin their polyunsaturated fatty acids. Hydrogen gasis bubbled
through the oil. Ashydrogen binds to the fatty acids, double covalent
bondsare converted to single covalent bonds to produce a change in
molecular shape thatsolidifies the oil. The more saturated the product,
the harder itbecomes at room temperature.
The remaining 5% of lipids include cholesterol and phos-
pholipids like lecithin (lesi-thin). Cholesterol is a steroid (see
chapter 2) found in high concentrations in liver and egg yolks,but
it’s also present in whole milk,cheese, butter, and meats. Choles-
terol is not found in plants.Phospholipids are major components
of plasma membranes,and they are found in a variet y of foods.A
good source oflecithin is egg yolks.
Usesin the Body
Triglycerides are important sources ofenergy that are used to pro-
duce ATP molecules.A gram of triglyceride delivers more than twice
as many kilocalories as a gram of carbohydrate.Some cells, such as
skeletal muscle cells,derive most of their energy from triglycerides.
After a meal, excess triglycerides that are not immediately
used are stored in adipose tissue or the liver.Later,when energ y is
required,the t riglycerides are broken down,and their fatty acids
are released into the blood,where they are taken up and used by
various tissues. In addition to storing energy,adipose tissue sur-
rounds and pads organs,and under the skin adipose tissue is an in-
sulator,which prevents heat loss.
Cholesterol is an important molecule with many functions
in the body.It can either be obtained in food or manufactured by
the liver and most other tissues.Cholesterol is a component of the
plasma membrane, and it can be modified to form other useful
molecules, such as bile salts and steroid hormones.Bile salts are
necessary for fat digestion and absorption. Steroid hormones in-
clude the sex hormones estrogen, progesterone,and testosterone,
which regulate the reproductive system.
The eicosanoids (ı¯ko¯-sa˘-noydz), which include prosta-
glandins and leukotrienes,are derived from fatty acids. The mole-
cules are involved in activities like inflammation,blood clotting,
tissue repair,and smooth muscle contraction. Phospholipids, such
as lecithin,are part of the plasma membrane and are used to con-
struct the myelin sheath around the axons ofnerve cells. Lecithin is
also found in bile and helps to emulsify fats.
Recommended Amounts
The American Heart Association recommends that fats account for
30% or less ofthe total daily kilocaloric intake, with 8%–10% com-
ing from saturated fats,up to 10% from polyunsaturated fats, and
up to 15% from monounsaturated fats.Furthermore, saturated fats
should contribute no more than 10% oftotal fat intake, and choles-
terol should be limited to 300 mg (the amount in an egg yolk) or less
per day.These guidelines reflect the belief that excess amounts of
fats,especially saturated fats and cholesterol, contribute to cardio-
vascular disease.Evidence also suggests that high-fat intake is asso-
ciated with colon cancer.The typical U.S. diet derives 35%–45% of
its kilocalories from fats, indicating that most Americans need to
reduce fat consumption.On the other hand, fat intake may account
for as little as 10% ofthe kilocalories in a healthy person’s diet.
Most ofthe lecithin consumed in the diet is broken down in
the digestive tract.The liver has the ability to manufacture all of the
lecithin necessary to meet the body’s needs,and it’s not necessary
to consume lecithin supplements.
Linoleic (lin-o¯-le¯ik)acid and -linolenic (lin-o¯-lenik)
acid are essential fatty acids because the body cannot synthesize
them and they must be ingested.They are found in plant oils, such
as canola or soybean oils.
FattyAcids and Blood Clotting
The essentialfatty acids are used to synthesize prostaglandinsthat affect
blood clotting. Linoleicacid can be converted to arachidonic (a˘-rak-i-
donik)acid, which is used to produce prostaglandinsthat increase blood
clotting. Alpha-linolenicacid can be converted to eicosapentaenoic
(ı¯ko¯-sa˘-pen-ta˘-no¯ik)acid (EPA), which is used to produce prostaglandins
thatdecrease blood clotting. Normally, mostprostaglandins are
synthesized from linoleicacid because it’s more plentiful in the body.
Individualswho consume foods rich in EPA, however, such asherring,
salmon, tuna, and sardines, increase the synthesisofprostaglandins from
EPA. Individualswho eat these fish twice or more timesper week have a
lower riskof heart attackthan those who don’t, possibly because of
reduced blood clotting. Although EPA can be obtained using fish oil
supplements, thisis not currently recommended because fish oil
supplementscontain high amounts of cholesterol, vitaminsA and D, and
uncommon fattyacids, all of which can cause health problemswhen taken
in large amounts.
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
IV. Regulations and
Maintenance
25. Nutrition, Metabolism,
and Temperature
Regulation
© The McGraw−Hill
Companies, 2004
8. What is the major source of lipids in the diet? What are
othersources?
9. Define saturated and unsaturated fats.
10. How are triglycerides, cholesterol, prostaglandins, and
lecithin used bythe body?
11. Describe the recommended dietary intake of lipids. List the
essential fattyacids.
Proteins
Sourcesin the Diet
Proteinsare chains of amino acids (see chapter 2). Proteins in the
human body are constructed of20 different kinds of amino acids,
which are divided into two groups: essential and nonessential
amino acids. The body cannot synthesize essential amino acids,
so they must be obtained in the diet.The nine essential amino acids
are histidine, isoleucine, leucine,lysine, methionine, phenylala-
nine,threonine, try ptophan,and valine. The body can synthesize
nonessential amino acids from other molecules. If adequate
amounts ofthe essential amino acids are ingested, they can be con-
verted to the nonessential amino acids. Like the essential amino
acids,the nonessential amino acids are necessary for good health.
A complete protein food contains adequate amounts of all
nine essential amino acids, whereas an incomplete proteinfood
does not. Examples of complete proteins are meat,fish, poultry,
milk, cheese, and eggs,and examples of incomplete proteins are
leafy green vegetables,grains, and legumes (peas and beans).
Usesin the Body
Essential and nonessential amino acids are used to synthesize pro-
teins.Proteins perform numerous functions in the human body as the
following examples illustrate.Collagen provides structural strength in
connective tissue as does keratin in the skin,and the combination of
actin and myosin makes muscle contraction possible.Enzymes are re-
sponsible for regulating the rate of chemical reactions, and protein
hormones regulate many physiologic processes (see chapter 18).Pro-
teins in the blood act as buffers to prevent changes in pH,and hemo-
globin transports oxygen and carbon dioxide in the blood.Proteins
also function as carrier molecules to move materials across plasma
membranes,and other proteins in the plasma membrane function as
receptor molecules and ion channels.Antibodies, lymphokines, and
complement are part of the immune system response that protects
against microorganisms and other foreign substances.
Proteins are also used as a source ofenergy,y ielding the same
amount ofenergy as car bohydrates.If excess proteins are ingested,
the energy in the proteins can be stored by converting their amino
acids into glycogen or fats.
Recommended Amounts
The recommended daily consumption of protein for a healthy
adult is 0.8 g/kg ofbody weight, or about 10% of total kilocalories
(55 g protein/day for a 2000 kcal/day intake).A cup of skim milk
contains 8 g protein,1 ounce of meat contains 7 g protein, and a
slice ofbread provides 2 g protein. If two incomplete proteins, such
as rice and beans are ingested,each can provide amino acids lack-
ing in the other.Thus, a correctly balanced vegetarian diet can pro-
vide all ofthe essential amino acids.
Part4 Regulationsand Maintenance916
When protein intake is adequate,the synthesis and break-
down of proteins in a healthy adult occurs at the same rate.The
amino acids ofproteins contain nitrogen; so saying that a person is
in nitrogen balance means that the nitrogen content of ingested
protein is equal to the nitrogen excreted in urine and feces.A starv-
ing person is in negative nitrogen balance because the nitrogen
gained in the diet is less than that lost by excretion.In other words,
when proteins are broken down for energy,more nitrogen is lost
than is replaced in the diet.A growing child or a healthy pregnant
woman,on the other hand, is in positive nitrogen balance because
more nitrogen is going into the body to produce new tissues than is
lost by excretion.
12. Distinguish between essential and nonessential amino
acids. Between complete and incomplete protein foods.
13. Describe some of the functions performed by proteins in the
body.
14. What is the recommended daily consumption of proteins?
Define the term nitrogen balance.
Vitamins
Vitamins (vı¯ta˘-minz; life-giving chemicals) are organic mole-
cules that exist in minute quantities in food and are essential to
normal metabolism (table 25.2). Essential vitamins cannot be
produced by the body and must be obtained through the diet.Be-
cause no single food item or nutrient class provides all the essential
vitamins,it’s necessary to maintain a balanced diet by eating a va-
riety of foods. The absence of an essential vitamin in the diet can
result in a specific deficiency disease.A few vitamins, such as vita-
min K,are produced by intestinal bacteria,and a few can be formed
by the body from substances called provitamins.A provitamin is a
part of a vitamin that can be assembled or modified by the body
into a functional vitamin.Beta carotene is an example of a provit-
amin that can be modified by the body to form vitamin A. The
other provitamins are 7-dehydrocholesterol (de¯-hı¯dro-ko¯-
lester-ol),which can be converted to vitamin D, and tryptophan
(tripto¯-fan), which can be converted to niacin.
Vitamins are not broken down by catabolism but are used by
the body in their original or slightly modified forms. After the
chemical structure ofa vitamin is destroyed, its function is usually
lost.The chemical structure of many vitamins is destroyed by heat,
such as when food is overcooked.
Many vitamins function as coenzymes,which combine with
enzymes to make the enzymes functional (see chapter 2).Without
coenzymes and their enzymes,many chemical reactions would oc-
cur too slowly to support good health and even life.For example,
vitamins B
2
and B
3
, biotin (bı¯o¯-tin),and pantothenic (pan-to¯-
thenik) acid are critical for the chemical reactions necessary to
produce energy.Folate (fo¯la¯t) and vitamin B
12
are involved in nu-
cleic acid synthesis.Vitamins A, B
1
,B
6
,B
12
,C, and D are necessary
for growth.Vitamin K is necessary for the synthesis of proteins in-
volved in blood clotting (see table 25.2).
Vitamins are either fat-soluble or water-soluble.Fat-soluble
vitamins, such as vitamins A, D,E, and K, dissolve in lipids. They
are absorbed from the intestine along with lipids. Some of them
can be stored in the body for a long time. Because they can be
stored,it’s possible to accumulate these vitamins in the body to the
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
IV. Regulations and
Maintenance
25. Nutrition, Metabolism,
and Temperature
Regulation
© The McGraw−Hill
Companies, 2004
Chapter 25 Nutrition, Metabolism, and Temperature Regulation 917
Table 25.2 The Principal Vitamins
Reference
Daily
Intake (RDI)s
*
1000 RE
†
1.5 mg
1.7 mg
20 mg
10 mg
0.3 mg
2.0 mg
0.4 mg
6µg
60 mg
400 IU
‡
30 IU
80µg
Fat (F)– or
Water (W)–
Soluble
F
W
W
W
W
W
W
W
W
W
F
F
F
Vitamin
A (retinol)
B
1
(thiamine)
B
2
(riboflavin)
B
3
(niacin)
Pantothenic acid
Biotin
B
6
(pyridoxine)
Folate
B
12
(cobalamins)
C (ascorbic acid)
D (cholecalciferol,
ergosterol)
E(tocopherol,
tocotrienols)
K(phylloquinone)
Source
From provitamin carotene
found in yellow and green
vegetables: preformed in
liver, egg yolk, butter, and
milk
Yeast, grains, and milk
Green vegetables, liver, wheat
germ, milk, and eggs
Fish, liver, red meat, yeast,
grains, peas, beans, and
nuts
Liver, yeast, green vegetables,
grains, and intestinal
bacteria
Liver, yeast, eggs, and
intestinal bacteria
Fish, liver, yeast, tomatoes,
and intestinal bacteria
Liver, green leafy vegetables,
and intestinal bacteria
Liver, red meat, milk, and eggs
Citrus fruit, tomatoes, and
green vegetables
Fish liver oil, enriched milk,
and eggs; provitamin D
converted by sunlight to
cholecalciferol in the skin
Wheat germ, cotton seed,
palm, and rice oils; grain,
liver, and lettuce
Alfalfa, liver, spinach,
vegetable oils, cabbage,
and intestinal bacteria
Function
Necessary for rhodopsin
synthesis, normal health
of epithelial cells, and
bone and tooth growth
Involved in carbohydrate and
amino acid metabolism,
necessary for growth
Component of flavin adenine
dinucleotide; involved in
citric acid cycle
Component of nicotinamide
adenine dinucleotide;
involved in glycolysisand
citric acid cycle
Constituent of coenzyme-A,
glucose production from
lipids and amino acids,
and steriod hormone
synthesis
Fatty acid and nucleic acid
synthesis; movement of
pyruvic acid into citric acid
cycle
Involved in amino acid
metabolism
Nucleic acid synthesis,
hematopoiesis; prevents
birth defects
Necessary for red blood cell
production, some nucleic
acid and amino acid
metabolism
Collagen synthesis; general
protein metabolism
Promotes calcium and
phosphorus use; normal
growth and bone and teeth
formation
Prevents the oxidation of
plasma membranes and
DNA
Required for synthesis of a
number of clotting factors
Symptoms of
Deficiency
Rhodopsin defiency, night
blindness, retarded
growth, skin disorders and
increase in infection risk
Beriberi—muscle weakness
(including cardiac muscle),
neuritis, and paralysis
Eye disorders and skin
cracking, especially at
corners of the mouth
Pellagra—diarrhea, dermatitis,
and nervous system
disorder
Neuromuscular dysfunction
and fatigue
Mental and muscle
dysfunction, fatigue, and
nausea
Dermatitis, retarded growth,
and nausea
Macrocytic anemia (enlarged
red blood cells) and spina
bifida
Pernicious anemia and
nervous system disorders
Scurvy—defective bone
formation and poor wound
healing
Rickets—poorly developed,
weak bones,
osteomalacia; bone
reabsorption
Hemolysis of red blood cells
Excessive bleeding due to
retarded blood clotting
* RDIs for people over 4 years of age; IU international units.
† Retinol equivalents (RE). 1 retinol equivalent 1 µg retinol or 6 µg beta carotene.
‡ As cholecalciferol. 1 µg cholecalciferol 40 IU (international units) vitamin D.
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point of toxicity.Water-soluble vitamins, such as the B vitamins
and vitamin C,dissolve in water. They are absorbed from the water
in the intestinal tract and remain in the body only a short time be-
fore being excreted.
Vitamins were discovered at the beginning ofthe twentieth
century.They were found to be associated with certain foods that
were known to protect people from diseases like rickets and
beriberi. In 1941, the first Food and Nutrition Board established
the Recommended Dietary Allowances (RDAs), which are the
nutrient intakes sufficient to meet the needs ofnearly all people in
certain age and gender groups. RDAs have been established for
different-aged males and females,starting with infants and contin-
uing on to adults. RDAs are also set for pregnant and lactating
women.The RDAs have been reevaluated every 4–5 years and up-
dated,when necessary, on the basis of new information.
The RDAs establish a minimum intake ofvitamins and min-
erals that should protect almost everyone (97%) in a given group
from diseases caused by vitamin or mineral deficiencies.Although
personal requirements can vary,the RDAs are a good benchmark.
The further dietary intake is below the RDAs,the more likely a nu-
tritional deficiency can occur.On the other hand, the consumption
of too large a quantity of some vitamins and minerals can have
harmful effects. For example, the long-term ingestion of 3–10
times the RDA for vitamin A can cause bone and muscle pain,skin
disorders, hair loss, and increased liver size.The long-term con-
sumption of 5–10 times the RDA of vitamin D can result in the
deposition ofcalcium in the kidneys, heart, and blood vessels, and
the regular consumption of more than 2 g of vitamin C daily can
cause stomach inflammation and diarrhea.
Free Radicalsand Antioxidants
Damage from free radicalsmay contribute to aging and certain diseases,
such asatherosclerosis and cancer. Free radicalsare molecules,
produced aspart of normal metabolism, that are missing an electron.
Free radicalscan replace the missing electron bytaking an electron from
cellmolecules, such as fats, proteins, or DNA, resulting in damage to the
cell. The lossof an electron from a molecule is called oxidation.
Antioxidantsare substances that prevent oxidation ofcell components
bydonating an electron to free radicals. Examples of antioxidants
include beta carotene (provitamin A), vitamin C, and vitamin E.
Manystudies have been done to determine whether or not taking
large dosesof antioxidants is beneficial. Although future research may
suggestotherwise, the consensus among scientists establishing the
RDAsis that the best evidence presently available doesn’tsupport the
claimsthat taking large doses of antioxidantsprevents chronic disease
or otherwise improveshealth. On the other hand, the amount of
antioxidantsnormally found in a balanced dietthat includes fruits and
vegetablesrich in antioxidants, combined with the complexmix of other
chemicalsfound in food, can be beneficial.
15. What are vitamins, essential vitamins, and provitamins?
Name the water-soluble vitaminsand the fat-soluble
vitamins.
16. List some of the functions of vitamins.
17. What are Recommended Dietary Allowances (RDAs)? Why
are theyuseful?
Part4 Regulationsand Maintenance918
PREDICT
Whatwould happen if vitamins were broken down during the process
ofdigestion rather than being absorbed intact into the circulation?
Minerals
Minerals(min er-a˘lz) are inorganic nutrients that are necessary
for normal metabolic functions. They constitute about 4%–5%
ofthe total body weight and are components of coenzymes, a few
vitamins,hemoglobin, and other organic molecules. Minerals are
involved in a number ofimportant functions, such as establish-
ing resting membrane potentials and generating action poten-
tials,adding mechanical strength to bones and teeth, combining
with organic molecules,or acting as coenzymes, buffers, or regu-
lators of osmotic pressure. Table 25.3 lists important minerals
and their functions.
Minerals are ingested by themselves or in combination with
organic molecules. Minerals are obtained from animal and plant
sources.Mineral absorption from plants, however,can be limited be-
cause the minerals tend to bind to plant fibers.Refined breads and
cereals have hardly any minerals or vitamins because they are lost in
the processing ofthe seeds used to make them. The seeds are crushed
and the outer parts ofthe seeds, which contains most of their miner-
als and vitamins,are removed. The inner part of the seeds, which has
few minerals and vitamins, is used to make the refined breads and
cereals.Minerals and vitamins are often added to refined breads and
cereals to compensate for their loss during the refinement process.A
balanced diet can provide all the necessary minerals,with a few pos-
sible exceptions. For example,women who suffer from excessive
menstrual bleeding may need an iron supplement.
18. What are minerals? List some of the important functions of
minerals.
CaloricIntake and Life Span
Studiesin mice, rats, and other animals indicate that life span can be
increased byapproximately one-third by decreasing normalcaloric
intake 30%-50%, provided the dietincludes enough protein, fat,
vitamins, and minerals. Whylife span increases is not understood, but
one proposed explanation for thisphenomenon is that decreased caloric
intake in some wayreduces free radicaldamage to mitochondria. It has
been suggested thathumans might derive a similar benefit by reducing
caloricinput, starting at age 20. Unlike laboratory animals, however,
humanswould have to voluntarily restrict their caloricintake by
30%–50%, which isan unlikely behavioralchange for most humans.
Much more needsto be learned before it will be known if the restriction
ofcaloric intake to increase longevity isbeneficial to humans.
DailyValues
Daily Valuesare dietary reference values now appearing on food
labels to help consumers plan a healthy diet.Daily Values are based
on two other sets of reference values:Reference Daily Intakes and
Daily Reference Values.The Reference Daily Intakes (RDIs) are
based on the 1968 RDAs for certain vitamins and minerals.RDIs
have been set for four categories ofpeople: infants, toddlers, people
over 4 years of age, and pregnant or lactating women.Generally,
the RDIs are set to the highest 1968 RDA value ofan age category.
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Chapter 25 Nutrition, Metabolism, and Temperature Regulation 919
For example,the highest RDA for iron in males over 4 years of age
is 10 mg/day and for females over 4 years ofage is 18 mg/day.Thus,
the RDI for iron is set at 18 mg/day.The Daily Reference Values
(DRVs)are set for total fat, saturated fat, cholesterol, total carbo-
hydrate,dietary fiber, sodium, potassium, and protein.
Having two standards on food labels,RDIs for vitamins and
minerals and DRVs for other nutrients, was thought to be more
confusing for consumers than having one standard.Therefore,the
RDIs and DRVs are combined to form the Daily Values.In addi-
tion,not all possible Daily Values are required to be listed.
The Daily Values appearing on food labels are based on a
2000 kcal reference diet,which approximates the weight mainte-
nance requirements ofpostmenopausal women, women who exer-
cise moderately,teenage girls, and sedentary men (figure 25.2). On
large food labels,additional information is listed based on a daily
intake of2500 kcal, which is adequate for young men.
The Daily Values for energy-producing nutrients are deter-
mined as a percentage ofdaily kilocaloric intake: 60% for carbohy-
drates, 30% for total fats, 10% for saturated fats,and 10% for
proteins.The Daily Value for fiber is 11.5 g for each 1000 kcal ofin-
take.The Daily Values for a nutrient in a 2000 kcal/day diet can be
calculated on the basis ofthe recommended daily percentage of the
nutrient and the kilocalories in a gram of the nutrient. For exam-
ple,carbohydrates should be 60% of a 2000 kcal/day diet, or 1200
kcal/day (0.60 2000). Since there are 4 kcal in a gram of carbo-
hydrate,the Daily Value for carbohydrate is 300 g/day (1200/4).
The Daily Value for some nutrients is the uppermost limit
considered desirable because of the link between these nutrients
and certain diseases.Thus, the Daily Values for total fats is less than
65 g,saturated fats is less than 20 g, and cholesterol is less than 300
mg because oftheir association with increased risk of hear t disease.
The Daily Value for sodium is less than 2400 mg because ofits as-
sociation with high blood pressure in some people.
For a particular food,the Daily Value is used to calculate the
Percent Daily Value (% Daily Value) for some of the nutrients in
one serving of the food (see figure 25.2). For example,if a serv ing
of food has 3 g of fat and the Daily Value for total fat is 65 g,then
the % Daily Value is 5% (3/65 0.05, or 5%).The Food and Drug
Administration (FDA) requires % Daily Values to be on food labels
so that the public has useful and accurate dietary information.
Table 25.3 Important Minerals
Reference Daily
Intake (RDIs)
*
1 g
3.4 g
120µg
Unknown
2.0 mg
2.5 mg
150µg
18 mg
400 mg
3.5 mg
75µg
1 g
2 g
55µg
500 mg
†
Unknown
15 mg
Mineral
Calcium
Chloride
Chromium
Cobalt
Copper
Fluorine
Iodine
Iron
Magnesium
Manganese
Molybdenum
Phosphorus
Potassium
Selenium
Sodium
Sulfur
Zinc
Function
Bone and teeth formation, blood clotting, muscle activity,
and nerve function
Blood acid–base balance; hydrochloric acid production
in stomach
Associated with enzymesin glucose metabolism
Component of vitamin B
12
; red blood cell production
Hemoglobin and melanin production, electron-transport
system
Provides extra strength in teeth; prevents dental caries
Thyroid hormone production, maintenance of normal
metabolic rate
Component of hemoglobin; ATP production in electron-
transport system
Coenzyme constituent; bone formation; muscle and
nerve function
Hemoglobin synthesis; growth; activation of several
enzymes
Enzyme component
Bone and teeth formation; important in energy transfer
(ATP); component of nucleic acids
Muscle and nerve function
Component of many enzymes
Osmotic pressure regulation; nerve and muscle function
Component of hormones; several vitamins, and proteins
Component of several enzymes; carbon dioxide transport
and metabolism; necessary for protein metabolism
Symptoms of Deficiency
Spontaneous action potential generation in
neurons and tetany
Acid–base imbalance
Unknown
Anemia
Anemia and loss of energy
No real pathology
Goiter and decrease in normal metabolism
Anemia, decreased oxygen transport, and
energy loss
Increased nervous system irritability,
vasodilation, and arrhythmias
Tremors and convulsions
Unknown
Loss of energy and cellular function
Muscle weakness, abnormal
electrocardiogram, and alkaline urine
Unknown
Nausea, vomiting, exhaustion, and dizziness
Unknown
Deficient carbon dioxide transport and
deficient protein metabolism
* RDIs for people over 4 years of age, except for sodium.
† The estimated minimum for people over 10 years of age. The maximum Daily Value for sodium is 2400 mg.
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Anatomy and Physiology,
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IV. Regulations and
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25. Nutrition, Metabolism,
and Temperature
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PREDICT
One serving ofa food has 30 g of carbohydrate. What % DailyValue for
carbohydrate ison the food label for this food?
The % Daily Values for nutrients related to energy consump-
tion are based on a 2000 kcal/day diet.For people who maintain their
weight on a 2000 kcal/day diet,the total of the % Daily Values for
each ofthese nutrients should add up to no more than 100%. For in-
dividuals consuming more or fewer kilocalories per day than 2000
kcal,however, the total of the % Daily Values can be more or fewer
than 100%.For example, for a person consuming 2200 kcal/day,the
total ofthe % Daily Values for each of these nutrients should add up
to no more than 110% because 2200/2000 1.10, or 110%.
Part4 Regulationsand Maintenance920
PREDICT
Suppose a person consumes1800 kcal/day. Whattotal % Daily Values
for energy-producing nutrientsis recommended?
When using the % Daily Values ofa food to determine how
the amounts ofcertain nutrients in the food fit into the overall diet,
the number ofser vings in a container or package needs to be con-
sidered. For example,suppose a small (2.25-ounce) bag of corn
chips has a % Daily Value of16% for total fat. One might suppose
that eating the bag ofchips accounts for 16% of total fat for the day.
The bag,however, contains 2.5 servings. Therefore, if all the chips
in the bag are consumed,they account for 40% (16% 2.5) of the
maximum recommended total fat.
19. What are the Reference Daily Intakes and the Daily
Reference Values? When combined, whatreference setof
valuesis established?
20. Define % Daily Values. The % Daily Values appearing on
food labelsare based on how many kilocalories per day?
Metabolism
Objective
■ Describe the energy changes that take place in metabolism.
Metabolism(me˘-tabo¯-lizm; change) is the total of all the
chemical changes that occur in the body. It consists of
anabolism (a˘-nabo¯-lizm), the energy-requiring process by
which small molecules are joined to form larger molecules,and
catabolism (ka˘-tabo¯-lizm), the energy-releasing process by
which large molecules are broken down into smaller molecules.
Anabolism occurs in all cells of the body as they divide to form
new cells, maintain their own intracellular structure, and pro-
duce molecules like hormones,neurotransmitters, or extracellu-
lar matrix molecules for export. Catabolism begins during the
process of digestion and is concluded within individual cells.
The energy derived from catabolism is used to drive anabolic re-
actions and processes such as active transport and muscle
contraction.
The cellular metabolic processes are often referred to as cel-
lular metabolism or cellular respiration.The digestive products of
carbohydrates, proteins, and lipids taken into body cells are
Figure 25.2
Food Label
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IV. Regulations and
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25. Nutrition, Metabolism,
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Chapter 25 Nutrition, Metabolism, and Temperature Regulation 921
catabolized,and the released energy is used to combine adenosine
diphosphate (ADP) and an inorganic phosphate group (P
i
) to
form ATP (figure 25.3).
ADP P
i
Energy → ATP
ATP is often called the energy currency ofthe cell because
when it is spent,or broken down to ADP,energy becomes available
for use by the cell.
The chemical reactions responsible for the transfer ofenergy
from the chemical bonds of nutrient molecules to ATP molecules
involve oxidation–reduction reactions (see chapter 2).A molecule
is reduced when it gains electrons and is oxidized when it loses
electrons. A nutrient molecule has many hydrogen atoms cova-
lently bonded to the carbon atoms that form the “backbone”of the
molecule.Because a hydrogen atom is a hydrogen ion (proton) and
an electron,the nutrient molecule has many electrons and is, there-
fore,highly reduced. When a hydrogen ion and an associated elec-
tron are lost from the nutrient molecule,the molecule loses energy
and becomes oxidized.The energy in the electron is used to syn-
thesize ATP.The major events of cellular metabolism are summa-
rized in figure 25.4.
21. Define metabolism, anabolism, and catabolism. How is the
energyderived from catabolism used to drive anabolic
reactions?
22. How does the removal of hydrogen atoms from nutrient
moleculesresult in a loss of energy from the nutrient
molecule?
Adenosine
Energy
ATP
ADP + P
i
Energy
Adenosine
The energy released during catabolism
can be used to synthesize ATP.
ATP Production
Catabolism is the energy-
releasing reactions resulting
from the breakdown of
larger molecules to smaller
ones. Ingested food
is the source of molecules
used in catabolic reactions.
Anabolism is the energy-
requiring reactions that join
smaller molecules to form
larger ones. Anabolic
reactions result in the
synthesis of the molecules
necessary for life.
Anabolism
The energy released from the
breakdown of ATP can be used during
anabolism to synthesize other
molecules and to provide energy for
cellular process such as active
transport and muscle contraction.
ATP Breakdown
Catabolism
PPP
PPP
1
2
Figure 25.3
ATP Coupling ofCatabolic and AnabolicReactions
Energyreleased by catabolic reactions is used to form ATP, which releasesthe energy for use in anabolicreactions.
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Carbohydrate Metabolism
Objectives
■ Describe the use of glucose to produce ATP withoutoxygen
and with oxygen.
■ Describe the chemical reactions occurring in glycolysis,
acetyl-CoA formation, and the citricacid cycle.
■ Describe the electron-transport chain and howATP is
produced in the process.
Part4 Regulationsand Maintenance922
Glycolysis
Carbohydrate metabolism begins with glycolysis (glı¯-koli-sis),
which is a series ofchemical reactions in the cytosol that results in
the breakdown ofglucose into two pyruv ic (pı¯-roovik)acidmol-
ecules (figure 25.5).
Glycolysis is divided into four phases.
1. Input ofATP.The first steps in glycolysis require the input of
energy in the form of two ATP molecules.A phosphate
2 lactic
acid
2 NADH
2 NADH
2 CO
2
4 CO
2
O
2
6 NADH
2 FADH
2
2 ATP
Glycolysis
Oxygen
absent
Oxygen
present
Citric
acid
cycle
2 ATP
34 ATP
H
2
O
Electron-transport system
Glucose
2 pyruvic
acid
(see figure 25.6)
(see figure 25.7)
(see figure 25.5)
(see figure 25.8)
Figure 25.4
Cellular Metabolism
Overview ofcellular metabolism, including glycolysis, citric acid cycle, and electron-transportsystem.
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Chapter 25 Nutrition, Metabolism, and Temperature Regulation 923
group is transferred from ATP to the glucose molecule,a
process called phosphorylation(fosfo¯r-i-la¯shu˘n), to form
glucose-6-phosphate.The glucose-6-phosphate atoms are
rearranged to form fructose-6-phosphate,which is then
converted to fructose-1,6-bisphosphate by the addition of
another phosphate group from another ATP.
2. Sugar cleavage.Fructose-1,6-bisphosphate is cleaved into two
three-carbon molecules,glyceraldehyde (glis-er-alde˘-hı¯d)-
3-phosphate and dihydroxyacetone (dı¯hı¯-drok-se¯-ase-to¯n)
phosphate.Dihydroxyactone phosphate is rearranged to
form glyceraldehyde-3-phosphate;consequently, two
molecules ofglyceraldehyde-3-phosphate result.
3. NADH production.Each glyceraldehyde-3-phosphate
molecule is oxidized (loses two electrons) to form 1,3-
bisphosphoglyceric (bizphos-fo-glise¯rik) acid, and
nicotinamide adenine (nik-o¯-tina˘-mı¯d ade˘-ne¯n)
dinucleotide (NAD
ⴙ
)is reduced (gains two electrons) to
NADH. Glyceraldehyde-3-phosphate also loses two
hydrogen ions,one of which binds to NAD
.
NAD
2 e
+ 2 H
→NADH H
NAD
is the oxidized form ofnicotinamide adenine
dinucleotide,and NADH is the reduced form. NADH is a
carrier molecule with two high-energy electrons (e
) that
can be used to produce ATP molecules through the electron-
transport chain (see “Electron-Transport Chain”on p.926).
4. ATP and pyruvic acid production.The last four steps of
glycolysis produce two ATP molecules and one pyruvic acid
molecule from each 1,3-bisphosphoglyceric acid molecule.
The events of glycolysis are summarized in table 25.4.Each
glucose molecule that enters glycolysis forms two glyceraldehyde-
3-phosphate molecules at the sugar cleavage phase. Each
glyceraldehyde-3-phosphate molecule produces two ATP mole-
cules,one NADH molecule, and one pyruvic acid molecule. Each
glucose molecule,therefore, forms four ATP,two NADH, and two
pyruvic acid molecules.Because the start of glycolysis requires the
input of two ATP molecules,however, the final yield of each glu-
cose molecule is two ATP,two NADH,and two pyruvic acid mole-
cules (see figure 25.4).
If the cell has adequate amounts of oxygen,the NADH and
pyruvic acid molecules are used in aerobic respiration to produce
ATP.In the absence of sufficient oxygen,they are used in anaero-
bic respiration.
23. Describe the four phases of glycolysis. What are the
productsof glycolysis?
24. What determines whether the pyruvic acid produced in
glycolysisis used in aerobic or anaerobic respiration?
AnaerobicRespiration
Anaerobic(an-a¯r-o¯bik)respiration is the breakdown of glucose
in the absence of oxygen to produce two molecules of lactic
(laktik) acid and two molecules of ATP (figure 25.6).The ATP
thus produced is a source of energy during activities such as in-
tense exercise, when insufficient oxygen is delivered to tissues.
Anaerobic respiration can be divided into two phases.
1. Glycolysis.The first phase of anaerobic respiration is
glycolysis,in which glucose undergoes several reactions to
produce two pyruvic acid molecules and two NADH.
There’s also a net gain oftwo ATP molecules.
2. Lactic acid formation.The second phase is the conversion of
pyruvic acid to lactic acid,a reaction that requires the input
ofenerg y from the NADH produced in phase 1 of
anaerobic respiration.
Lactic acid is released from the cells that produce it and is
transported by the blood to the liver.When oxygen becomes
available,the lactic acid in the liver can be converted through a
series ofchemical reactions into glucose. The glucose then can be
released from the liver and transported in the blood to cells that
use glucose as an energy source.This process of converting lactic
acid to glucose is called the Cori cycle.Some of the reactions in-
volved in converting lactic acid into glucose require the input of
ATP (energy) produced by aerobic respiration.The oxygen neces-
sary for the synthesis of the ATP is part ofthe oxygen debt (see
chapter 9).
25. Describe the two phases of anaerobic respiration. How
manyATP molecules are produced byanaerobic
respiration?
26. What happens to the lactic acid produced in anaerobic
respiration when oxygen becomesavailable?
Table 25.4 ATP Production from One
Glucose Molecule
Total ATP
Process Product Produced*
Glycolysis 4 ATP 02 ATP (4 ATP produced
minus 2 ATP
to start)
2 NADH 06 ATP (or 4 ATP;
see text)
Acetyl-CoA
production 2 NADH 06 ATP
Citric acid cycle 2 ATP 02 ATP
6 NADH 18 ATP
2 FADH
2
04 ATP
Total 38 ATP (or 36 ATP;
see text)
*NADH and FADH
2
are used in the production of ATP in the electron-transport chain.
Abbreviations: ATP adenosine triphosphate, NADH reduced nicotinamide adenine
dinucleotide, FADH
2
reduced flavin adenine diphosphate, acetyl-CoA acetyl
coenzyme A.
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Part4 Regulationsand Maintenance924
ADP
Glucose
1. Input of ATP.
Two ATP molecules are
required to start glycolysis,
and fructose-1,6-bisphosphate
is formed.
2. Sugar cleavage.
Fructose-1,6-bisphosphate
is split to form two three-carbon
glyceraldehyde-3-phosphate
molecules.
Glucose-6-phosphate
Fructose-6-phosphate
Fructose-1,6-bisphosphate
ATP
ADPATP
O
CH
2
CH
2
OH
H
2
C
H
H
OH
OH
HHO
O
P
P
O
H
2
C
H
H
OH
HO
OH
H
O
P
O
O
CH
2
P
O
CH
2
OH
P
C
CH
2
O
CH
2
P
CH OH
COH
To step 3
(top of next page)
Glyceraldehyde-3-phosphate
(2 molecules)
Dihydroxyacetone phosphate
H
H
HO
H
OH
OH
OH
H
H
CH
2
OH
O
H
H
HO
H
OH
OH
OH
H
H
O
Figure 25.5
Glycolysis
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Chapter 25 Nutrition, Metabolism, and Temperature Regulation 925
AerobicRespiration
Aerobic(a¯r-o¯bik)respiration is the breakdown of glucose in the
presence of oxygen to produce carbon dioxide,water, and 38 ATP
molecules.Most of the ATP molecules required to sustain life are
produced through aerobic respiration,which can be considered in
four phases:glycolysis, acetyl-CoA formation, the citric acid cycle,
and the electron-transport chain.The first phase of aerobic respi-
ration, as in anaerobic respiration, is glycolysis.The remaining
phases are acetyl-CoA formation, the citric acid cycle, and the
electron-transport chain.
Acetyl-CoA Formation
In the second phase of aerobic respiration, pyruvic acid moves
from the cytosol into a mitochondrion,which is separated into an
inner and outer compartment by the inner mitochondrial mem-
brane.Within the inner compartment, enzymes remove a carbon
and two oxygen atoms from the three-carbon pyruvic acid mole-
cule to form carbon dioxide and a two-carbon acetyl (ase-til)
group (figure 25.7).Energy is released in the reaction and is used to
reduce NAD
to NADH.The acetyl group combines with coen-
zyme A (CoA) to form acetyl-CoA. For each two pyruvic acid
O
COOH
C
CH
3
O
COOH
PC
CH
2
O
OH
COOH
PCH
CH
2
O
COOH
P
CH OH
CH
2
2ATP
4. ATP and pyruvic acid production.
Two ATP molecules and a pyruvic
acid molecule are produced for each
1,3-bisphosphoglyceric acid.
2ADP
2ATP2ADP
O
O
C
P
O
P
CH OH
CH
2
2 NAD
+
2 NADH
1,3-bisphosphoglyceric acid
(2 molecules)
3-phosphoglyceric acid
(2 molecules)
2-phosphoglyceric acid
(2 molecules)
Phosphoenolpyruvic acid
(2 molecules)
Pyruvic acid
(2 molecules)
3. NADH Production.
Glyceraldehyde-3-phosphate
is oxidized to
1,3-bisphosphoglyceric
acid, and NAD
+
is reduced
to NADH.
H
2
O
Figure 25.5
(continued)
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molecules from glycolysis,two acetyl-CoA molecules, two carbon
dioxide molecules,and two NADH are formed (see figure 25.4).
CitricAcid Cycle
The third phase of aerobic respiration is the citric acid cycle,
which is named after the six-carbon citric acid molecule formed
in the first step of the cycle (see figure 25.7).It is also called the
Krebs’cycle after its discoverer, the British biochemist Sir Hans
Krebs. The citric acid cycle begins with the production of citric
acid from the combination ofacetyl-CoA and a four-carbon mol-
ecule called oxaloacetic (oksa˘-lo¯-a˘-se¯tik) acid.A series of reac-
tions occurs, resulting in the formation of another oxaloacetic
acid, which can start the cycle again by combining with another
acetyl-CoA.During the reactions of the citric acid cycle, three im-
portant events occur.
1. ATP production.For each citric acid molecule, one ATP is
formed.
2. NADH and FADH
2
production.For each citric acid
molecule,three NAD
molecules are converted to NADH
molecules,and one flavin (f la¯vin) adenine dinucleotide
(FAD) molecule is converted to FADH
2
.The NADH and
FADH
2
molecules are electron carriers that enter the
electron-transport chain and are used to produce ATP.
Part4 Regulationsand Maintenance926
3. Carbon dioxide production.Each six-carbon citric acid
molecule at the start ofthe cycle becomes a four-carbon
oxaloacetic acid molecule at the end ofthe cycle. Two
carbon and four oxygen atoms from the citric acid molecule
are used to form two carbon dioxide molecules.Thus, some
ofthe carbon and oxygen atoms that make up food
molecules like glucose are eventually eliminated from the
body as carbon dioxide.We literally breathe out part of the
food we eat!
For each glucose molecule that begins aerobic respiration,
two pyruvic acid molecules are produced in glycolysis,and they are
converted into two acetyl-CoA molecules that enter the citric acid
cycle.To determine the number of molecules produced from glu-
cose by the citric acid cycle,two “turns” of the cycle must, there-
fore,be counted; the results are two ATP,six NADH, two FADH
2
,
and four carbon dioxide molecules (see figure 25.4).
Electron-TransportChain
The fourth phase of aerobic respiration involves the electron-
transport chain(figure 25.8), which is a series of electron carr iers
in the inner mitochondrial membrane. Electrons are transferred
from NADH and FADH
2
to the electron-transport carriers, and
hydrogen ions are released from NADH and FADH
2
.After the loss
of the electrons and the hydrogen ions, the oxidized NAD
and
FAD are reused to transport additional electrons from the citric
acid cycle to the electron-transport chain.
The electrons released from NADH and FADH
2
pass from
one electron carrier to the next through a series of oxidation–re-
duction reactions. Three of the electron carriers also function as
proton pumps that move the hydrogen ions from the inner mito-
chondrial compartment into the outer mitochondrial compart-
ment. Each proton pump accepts an electron,uses some of the
electron’s energy to export a hydrogen ion,and passes the electron
to the next electron carrier.The last electron carrier in the series
collects the electrons and combines them with oxygen and hydro-
gen ions to form water.
1/2 O
2
2 H
2 e
→H
2
O
Without oxygen to accept the electrons,the reactions of the
electron-transport chain cease, effectively stopping aerobic
respiration.
The hydrogen ions released from NADH and FADH
2
are
moved from the inner mitochondrial compartment to the outer
mitochondrial compartment by active transport. As a result,the
concentration ofhydrogen ions in the outer compartment exceeds
that of the inner compartment, and hydrogen ions diffuse back
into the inner compartment.The hydrogen ions pass through cer-
tain channels formed by an enzyme called ATP synthase.As the
hydrogen ions diffuse down their concentration gradient they lose
energy that is used to produce ATP.This process is called the
chemiosmotic (kem-e¯-os-motik)model because the chemical
formation ofATP is coupled to a diffusion force similar to osmosis.
2 lactic
acid
Oxygen
absent
4 ATP2 ATP
2 NAD
+
2 NAD
+
Glucose
2 pyruvic
acid
2 NADH
Figure 25.6
AnaerobicRespiration
In the absence ofoxygen, the pyruvic acid produced in glycolysisis converted
to lacticacid. The NADH produced in glycolysisis converted back to NAD
.
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Chapter 25 Nutrition, Metabolism, and Temperature Regulation 927
NADH
Acetyl group
Pyruvic acid
Acetyl-CoA
CoA – SH
NADH
NAD
+
CO
2
COOH
COOH
C
C
O
CH
2
COOH
CHO
COOH
COOH
H
OH
COOH
CH
2
CH
2
CH
2
C
H
COOH
COOH
COOH
CH
2
CH
2
C
H
COOH
HO
COOH
COOH
CH
2
SH
CoA
SH
CoA
C
COOH
O
COOH
CH
2
CH
2
C
H
COOH
O
S CoA
CH
2
CH
2
COOH
COOH
CH
2
CH
2
COOH
COOH
H
HOOC
H
C
C
C
NAD
+
NAD
+
NAD
+
FAD
ATP
ADP
FADH
2
NADH
NADH
Citric
acid
Isocitric
acid
␣-ketoglutaric
acid
Succinyl-CoA
Succinic
acid
Fumaric
acid
Malic
acid
Oxaloacetic
acid
1. Pyruvic acid is
produced in glycolysis.
2. Acetyl-CoA formation.
In the presence of oxygen,
pyruvic acid is converted to
acetyl-CoA, which enters the
citric acid cycle. CO
2
and
NADH are produced.
3. Citric acid cycle.
Citric acid is converted
through a series of reactions
to oxaloacetic acid, which can
combine with acetyl-CoA to
restart the cycle. In the
process, 1 ATP, 3 NADH, 1 FADH
2
,
and 2 CO
2
molecules are produced.
cis
-aconitic
acid
CO
2
CO
2
O
C
COOH
CH
3
O
C
H
CH
3
O
C
S CoA
CH
3
Coenzyme A
Figure 25.7
AerobicRespiration
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27. Define the term aerobic respiration, and list the products
produced byit. Describe the four phases of aerobic
respiration.
28. Why is the citric acid cycle a cycle? What molecules are
produced asa result of the citric acid cycle?
29. What is the function of the electron-transport chain?
Describe the chemiosmoticmodel of ATP production.
PREDICT
Manypoisons function by blocking certain stepsin the metabolic
pathways. For example, cyanide blocksthe laststep in the electron-
transportchain. Explain why this blockage would cause death.
Summaryof ATP Production
For each glucose molecule,aerobic respiration produces a net gain
of38 ATP molecules: 2 from glycolysis, 2 from the citric acid cycle,
and 34 from the NADH molecules and FADH
2
molecules that pass
through the electron-transport chain (see table 25.4). For each
NADH molecule formed,three ATP molecules are produced by the
electron-transport chain,and for each FADH
2
molecule,two ATP
molecules are produced.
Part4 Regulationsand Maintenance928
The number of ATP molecules produced is also reported
as 36 ATP molecules.The two NADH molecules produced by
glycolysis in the cytosol cannot cross the inner mitochondrial
membrane; thus,their elect rons are donated to a shuttle mole-
cule that carries the electrons to the electron-transport chain.
Depending on the shuttle molecule,each glycolytic NADH mol-
ecule can produce 2 or 3 ATP molecules.In skeletal muscle and
the brain,2 ATP molecules are produced for each NADH mole-
cule,resulting in a total number of 36 ATP molecules; but in the
liver,kidneys, and heart, 3 ATP molecules are produced for each
NADH molecule, and the total number of ATP molecules
formed is 38.
Six carbon dioxide molecules are produced in aerobic res-
piration.Water molecules are reactants in some of the chemical
reactions of aerobic respiration and products in others.Six wa-
ter molecules are used,but 12 are formed, for a net gain of 6 wa-
ter molecules. Thus, aerobic respiration can be summarized
asfollows:
C
6
H
12
O
6
6 O
2
6H
2
O 38 ADP 38 P
i
→
6 CO
2
12 H
2
O 38 ATP
2e
–
2e
–
2e
–
2e
–
2e
–
1. NADH or FADH
2
transfer their
electrons to the
electron-transport
chain.
2.As the electrons move through
the electron-transport chain,
some of their energy is used to
pump hydrogen ions into the
outer compartment, resulting in
a higher concentration of
hydrogen ions in the outer than
in the inner compartment.
3. The hydrogen ions diffuse back into
the inner compartment through special
channels (ATP synthase) that couple
the hydrogen ion movement with the
production of ATP. The electrons,
hydrogen ions, and oxygen combine to
form water.
4. ATP is transported out of the
inner compartment by a
carrier molecule that
exchanges ATP for ADP. A
different carrier molecule
moves phosphate into the
inner compartment.
NAD
+
NADH
2e
–
1
2
Inner
membrane
Outer
membrane
Outer
compartment
Inner
compartment
Cytosol
I II III IV
H
+
H
+
H
+
H
+
H
+
+ ADP
3
4
FADH
2
P
i
P
i
H
2
O
2H
+
ATP
synthase
Carrier
molecule
2
1
ADP
ATP
ATP
H
+
H
+
H
+
O
2
ProcessFigure 25.8
Electron-TransportChain
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Chapter 25 Nutrition, Metabolism, and Temperature Regulation 929
30. In aerobic respiration, how many ATP molecules are
produced from one molecule of glucose through glycolysis,
the citricacid cycle, and the electron-transport chain?
31. Why is the total number of ATP produced in aerobic
respiration listed as38 or 36?
The Quantityof ATP Produced from Glucose
The number ofATP molecules produced per glucose molecule isa
theoreticalnumber that assumes two hydrogen ions are necessaryfor
the formation ofeach ATP. Ifthe number required is more than two, the
efficiencyof aerobic respiration decreases. In addition, it’snow
understood thatit costs energy to get ADP and phosphates into the
mitochondria and to getATP out. Considering allthese factors, it’s
estimated thateach glucose molecule yields about25 ATP molecules
instead of38 ATP molecules.
Lipid Metabolism
Objective
■ Describe the basic steps involved in using lipids as an
energysource.
Lipids are the body’s main energy-storage molecules. In a
healthy person,lipids are responsible for about 99% of the body’s
energy storage, and glycogen accounts for about 1%.Althoug h
proteins are used as an energy source,they are not considered stor-
age molecules because the breakdown of proteins normally in-
volves the loss ofmolecules that perform other functions.
Lipids are stored primarily as triglycerides in adipose tissue.
There is constant synthesis and breakdown of triglycerides; thus,
the fat present in adipose tissue today isn’t the same fat that was
there a few weeks ago.Between meals when triglycerides are broken
down in adipose tissue, some of the fatty acids produced are re-
leased into the blood,where they are called free fatty acids. Other
tissues, especially skeletal muscle and the liver,use the free fatty
acids as a source ofenergy.
The metabolism of fatty acids occurs by beta-oxidation, a
series of reactions in which two carbon atoms at a time are re-
moved from the end ofa fatty acid chain to form acetyl-CoA. The
process of beta-oxidation continues to remove two carbon atoms
at a time until the entire fatty acid chain is converted into acetyl-
CoA molecules.Acetyl-CoA can enter the citric acid cycle and be
used to generate ATP (figure 25.9).
Acetyl-CoA is also used in ketogenesis(ke¯-to¯-jene˘-sis), the
formation of ketone bodies. In the liver when large amounts of
acetyl-CoA are produced,not all of the acetyl-CoA enters the cit-
ric acid cycle.Instead, two acetyl-CoA molecules combine to form
a molecule of acetoacetic (ase-to¯-a-se¯tik) acid,which is con-
verted mainly into -hydroxybutyric (hı¯-dro¯kse¯-bu¯-tirik) acid
and a smaller amount of acetone (ase-to¯n). Acetoacetic acid, -
hydroxybutyric acid,and acetone are called ketone (ke¯to¯n)bod-
ies and are released into the blood, where they travel to other
tissues,especially skeletal muscle. In these tissues, the ketone bod-
ies are converted back into acetyl-CoA that enters the citric acid
cycle to produce ATP.
32. Define the term beta-oxidation, and explain how it results in
ATP production.
33. What are ketone bodies, how are they produced, and for
whatare they used?
The Danger ofExcessive Amounts of Ketones
Normally, blood containsonlysmall amounts of ketone bodies. During
starvation (see “ClinicalFocus: Starvation” on p. 934), however, or in
patientswith diabetes mellitus, the rate of fat metabolism increases. As
a result, the quantityof ketone bodies increases to produce the
condition calledketosis. The increased number of ketone bodies can
exceed the capacityof the body’s buffering system, resulting in acidosis,
a decrease in blood pH (see chapter 27).
Triglycerides
Glyceraldehyde-
3-phosphate
Gluconeogenesis
Lipogenesis
Ketogenesis
Beta-oxidation
Lipogenesis
Free
fatty acids
Pyruvic
acid
Acetyl-CoA
Ketone
bodies
Some amino
acids
Citric
acid
cycle
Glucose
Glycerol
Figure 25.9
Lipid Metabolism
Triglyceride isbroken down into glycerol and fatty acids. Glycerolenters
glycolysisto produce ATP. The fattyacids are broken down by beta-
oxidation into acetyl-CoA, which entersthe citric acid cycle to produce ATP.
Acetyl-CoA can also be used to produce ketone bodies(ketogenesis).
Lipogenesisis the production of lipids. Glucose is converted to glycerol,
and amino acidsare converted to acetyl-CoA molecules. Acetyl-CoA
moleculescan combine to form fatty acids. Glycerol and fatty acids join to
form triglycerides.
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Protein Metabolism
Objective
■ Describe the metabolism of amino acids in the body.
Once absorbed into the body,amino acids are quickly taken
up by cells,especially in the liver. Amino acids are used to synthe-
size needed proteins (see chapter 3) or as a source ofenergy (figure
25.10). Unlike glycogen and triglycerides, amino acids are not
stored in the body.
The synthesis of nonessential amino acids usually begins
with keto acids (figure 25.11).A keto acid can be converted into an
amino acid by replacing its oxygen with an amine group.Usually
this conversion is accomplished by transferring an amine group
Part4 Regulationsand Maintenance930
from an amino acid to the keto acid,a reaction called transamina-
tion (trans-ami-na¯shu˘n). For example,-ketoglutaric acid (a
keto acid) reacts with an amino acid to form glutamic acid (an
amino acid; figure 25.12a). Most amino acids can undergo
transamination to produce glutamic acid. The glutamic acid is
used as a source of an amine group to construct most of the
nonessential amino acids. A few nonessential amino acids are
formed in other ways from the essential amino acids.
Amino acids can be used as a source ofenergy. In oxidative
deamination (de¯-am-i-na¯shu˘n; deaminization, de¯-ami-ni-
za¯shu˘n), an amine group is removed from an amino acid (usually
glutamic acid),leaving ammonia and a keto acid (figure 25.12b). In
the process, NAD
is reduced to NADH, which can enter the
Alanine, cysteine
glycine, serine,
threonine
Isoleucine,
leucine,
tryptophan
Phenylalanine,
tyrosine, leucine,
lysine, tryptophan
Arginine,
histidine, glutamine,
proline
Aspartate,
asparagine
Tyrosine,
phenylalanine
Isoleucine,
methionine,
valine
Acetoacetyl-
CoA
Oxaloacetic
acid
Citric
acid
Fumaric
acid
α-ketoglutaric
acid
Succinyl-
CoA
Pyruvic
acid
Glucose
Acetyl-
CoA
Figure 25.10
Amino Acid Metabolism
Variousentry points for amino acids into carbohydrate metabolism.
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electron-transport chain to produce ATP.Ammonia is toxic to cells.
An accumulation of ammonia to toxic levels is prevented because
the liver converts it into urea,which is carried by the blood to the
kidneys,where the urea is eliminated (figure 25.12c; see chapter 26).
Amino acids are also used as a source ofenerg y by convert-
ing them into the intermediate molecules ofcar bohydrate metab-
olism (see figure 25.10).These molecules are then metabolized to
yield ATP.The conversion of an amino acid often begins with a
transamination or oxidative deamination reaction, in which the
amino acid is converted into a keto acid (see figure 25.12).The keto
acid enters the citric acid cycle or is converted into pyruvic acid or
acetyl-CoA.
34. What is accomplished by transamination and oxidative
deamination?
35. How are proteins (amino acids) used to produce energy?
Interconversion ofNutrient
Molecules
Objective
■ Describe the processes by which nutrients are changed
from one type of nutrientinto another type.
Blood glucose enters most cells by facilitated diffusion and is
immediately converted to glucose-6-phosphate,which cannot re-
cross the plasma membrane (figure 25.13a).Glucose-6-phosphate
then continues through glycolysis to produce ATP. If, however, ex-
cess glucose is present (e.g.,after a meal), it’s used to form glycogen
through a process called glycogenesis (glı¯-ko¯-jene˘-sis). Most of
the body’s glycogen is contained in skeletal muscle and the liver.
Once glycogen stores, which are quite limited,are filled,
glucose and amino acids are used to synthesize lipids,a process
calledlipogenesis (lip-o¯-jene˘-sis;see figure 25.9).Glucose mol-
ecules can be used to form glyceraldehyde-3-phosphate and
acetyl-CoA. Amino acids can also be converted to acetyl-CoA.
Glyceraldehyde-3-phosphate is converted to glycerol,and the
two-carbon acetyl-CoA molecules are joined together to form
fatty acid chains.Glycerol and three fatty acids then combine to
form triglycerides.
C COOH
NH
2
R
H
C COOH
O
R
(a) Amino acid (b) Keto acid
Figure 25.11
GeneralFormulas of an Amino Acid and a
Keto Acid
(a) Amino acid with a carboxylgroup (—COOH), an amine group (NH
2
), a
hydrogen atom (H), and a group called “R” thatrepresents the rest of the
molecule. (b) Keto acid with a double-bonded oxygen replacing the amine
group and the hydrogen atom ofthe amino acid.
R
1
—CH— COOH + HOOC —CH
2
—CH
2
—C— COOH R
1
—C— COOH + HOOC —CH
2
—CH
2
—CH— COOH
NH
2
OO NH
2
—
—
—
—
—
—
Amino acid
α-ketoglutaric acid α-keto acid
Glutamic acid
HOOC—CH
2
—CH
2
—CH— COOH + H
2
O HOOC —CH
2
—CH
2
—C— COOH + NH
3
NH
2
—
OH
—
—
Glutamic acid NAD
+
α-ketoglutaric acid
Ammonia
2 NH
3
+ CO
2
C O + H
2
O
NH
2
NH
2
—
—
—
—
Ammonia Urea
NADH
Carbon dioxide Water
Enzymes
Enzymes
Enzymes
(a)Transamination
(b)Oxidative deamination
(c)Conversion of ammonia to urea
Figure 25.12
Amino Acid Reactions
(a) Transamination reaction in which an amine group istransferred from an amino acid to a keto acid to form a differentamino acid. (b) Oxidative deamination
reaction in which an amino acid losesan amine group to become a keto acid and to form ammonia. In the process, NADH, which can be used to generate ATP, is
formed. (c) Ammonia isconverted to urea in the liver. The actual conversion ofammonia to urea is more complex, involving a number of intermediate reactions that
constitute the urea cycle.
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Alcoholism and Cirrhosisof the Liver
Enzymesin the liver convert ethanol (beverage alcohol) into acetyl-CoA,
and in the processtwo NADH molecules are produced. The NADH
moleculesenter the electron-transport chain and are used to produce ATP
molecules. Each gram ofethanol provides 7 kcalof energy. Because of the
high levelof NADH in the cell that results from the metabolism ofethanol,
the production ofNADH by glycolysis and bythe citric acid cycle is
inhibited. Consequently, sugarsand amino acidsare not broken down but
are converted into fatsthat accumulate in the liver. Chronicalcohol abuse
can, therefore, resultin cirrhosis (sir-ro¯sis)ofthe liver, which involvesfat
deposition, celldeath, inflammation, and scar tissue formation. Death can
occur because the liver isunable to carryout its normal functions.
When glucose is needed,glycogen can be broken down into
glucose-6-phosphate through a set ofreactions called glycogenol-
ysis(glı¯ko¯-je˘-noli-sis;figure 25.13b).In skeletal muscle, glucose-
6-phosphate continues through glycolysis to produce ATP. The
liver can use glucose-6-phosphate for energy or can convert it to
glucose, which diffuses into the blood.The liver can release glu-
cose,but skeletal muscle cannot because it lacks the necessary en-
zymes to convert glucose-6-phosphate into glucose.
Release of glucose from the liver is necessary to maintain
blood glucose levels between meals.Maintaining these levels is es-
pecially important to the brain,which normally uses only g lucose
for an energy source and consumes about two-thirds of the total
glucose used each day.When liver glycogen levels are inadequate to
supply glucose, amino acids from proteins and glycerol from
triglycerides are used to produce glucose in a process called gluco-
neogenesis (glooko¯-ne¯-o¯-jene˘-sis).Most amino acids can be
Part4 Regulationsand Maintenance932
converted into citric acid cycle molecules,acetyl-CoA, or pyruvic
acid (see figure 25.10). Through a series of chemical reactions,
these molecules are converted into glucose.Glycerol enters glycol-
ysis by becoming glyceraldehyde-3-phosphate.
36. Define the terms glycogenesis, lipogenesis,
glycogenolysis, and gluconeogenesis.
Metabolic States
Objective
■ Differentiate between the absorptive and postabsorptive
metabolicstates.
Two major metabolic states have been described in the body.
The first is the absorptive state, the period immediately after a
meal when nutrients are being absorbed through the intestinal wall
into the circulatory and lymphatic systems (figure 25.14).The ab-
sorptive state usually lasts about 4 hours after each meal,and most
ofthe glucose that enters the circulation is used by cells to provide
the energy they require.The remainder of the glucose is converted
into glycogen or fats. Most of the absorbed fats are deposited in
adipose tissue.Many of the absorbed amino acids are used by cells
in protein synthesis,some are used for energy, and others enter the
liver and are converted into fats or carbohydrates.
The second state,the postabsorptive state, occurs late in the
morning,late in the afternoon, or during the night after each ab-
sorptive state is concluded (figure 25.15).Normal blood glucose
levels range between 70 and 110 mg/100 mL, and it’s vital to the
body’s homeostasis that this range be maintained, especially for
ATP
ADP
Gluconeogenesis
Glycog
enolys
is
Glycolysis
(Liver
only)
High blood glucose
Blood
glucose
Glucose
Glucose-6-phosphate
Energy
Glycogen
(energy storage)
Amino
acids,
glycerol
Energy
P
i
(a) When blood glucose levels are high, glucose enters
the cell and is phosphorylated to form glucose-6-phosphate,
which can enter glycolysis or glycogenesis.
Low blood glucose
Blood
glucose
Glucose-6-phosphate
Glycogen
(b) When blood glucose levels drop, glucose-6-phosphate can be
produced through glycogenolysis or gluconeogenesis.
Glucose-6-phosphate can enter glycolysis, or the phosphate group
can be removed in liver tissue and glucose released into the blood.
Glyco
lysis
Glycog
enesis
Cell Cell
Figure 25.13
Interconversion ofNutrient Molecules
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Chapter 25 Nutrition, Metabolism, and Temperature Regulation 933
Nutrients Absorbed
Nutrients are absorbed from the
digestive tract and carried by the blood
to the liver.
Nutrients Stored and Used
Nutrients are stored in adipose tissue
as triglycerides and in muscle as
glycogen. Nutrients also are a source
of energy for tissues. Amino acids
are used to synthesize proteins.
Most tissues
(including muscle and
adipose)
Nervous
tissue
Adipose
tissues
Muscle
Amino acids
Amino acids
Proteins
Glucose
Glycogen
Glucose Fatty acids Glucose Glucose
Energy
Energy
Fatty acids
Glycerol
Nutrients Processed
The liver converts nutrients into
energy-storage molecules, such as
glycogen, fatty acids, and
triglycerides. Amino acids are also
used to synthesize proteins, such as
plasma proteins.
Fatty acids and triglycerides
produced by the liver are released into
the blood. Nutrients not processed by
the liver are also carried by the blood
to tissues.
Proteins
α-keto acids Acetyl-CoA
Urea
Fatty
acids
Glycerol
Glycogen
Ammonia
Energy
Nonessential
amino acids
Triglycerides Glucose
Triglycerides
Figure 25.14
Eventsof the Absorptive State
Absorbed molecules, especiallyglucose, are used as sources of energy. Moleculesnot immediately needed for energy are stored: glucose is converted to glycogen
or triglycerides, triglyceridesare deposited in adipose tissue, and amino acids are converted to triglyceridesor carbohydrates.
Stored Nutrients Used
Stored energy molecules are used
as sources of energy: glycogen is
converted to glucose, and
triglycerides are converted to fatty
acids. Molecules released from
tissues are carried by the blood
to the liver.
Nutrients Processed
The liver processes molecules to
produce additional energy sources:
glycogen and amino acids are
converted to glucose and fatty
acids to ketones. Glucose and
ketones are released into the blood
and are transported to tissues.
Most tissues
(including muscle)
Nervous
tissue
Adipose
tissue
Muscle
Amino acids
Proteins
Glucose
Lactic acid
Glycogen
Glycerol
Glycerol
Fatty acids
Glycogen
Glucose
Ammonia UreaAmino acids
Energy
Energy
α-keto acid
Fatty acids
Triglycerides
Ketone bodies
Glucose
Energy
EnergyEnergy
Energy
Fatty acids
Acetyl-CoA Ketone bodies
Figure 25.15
Eventsof the Postabsorptive State
Stored energymolecules are used as sources of energy: glycogen is converted to glucose; triglyceridesare broken down to fatty acids, some of which are converted
to ketones; and proteinsare converted to glucose.
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Clinical Focus Starvation
Starvation results from the inadequate in-
take ofnutrients or the inability to metabo-
lize or absorb nutrients. It can have a
number of causes, such asprolonged fast-
ing, anorexia, deprivation, or disease. No
matter whatthe cause, starvation follows the
same course and consistsof three phases.
The eventsof the first two phases occur even
during relatively short periods of fasting or
dieting, but the third phase occursonly in
prolonged starvation and can end in death.
During the first phase of starvation,
blood glucose levels are maintained
through the production of glucose from
glycogen, proteins, and fats. Atfirst, glyco-
gen isbroken down into glucose; however,
onlyenough glycogen is stored in the liver
to last a few hours. Thereafter, blood glu-
cose levels are maintained by the break-
down of proteins and fats. Fats are
decomposed into fatty acids and glycerol.
Fattyacids can be used as a source of en-
ergy, especiallyby skeletal muscle, thusde-
creasing the use ofglucose by tissues other
than the brain. Glycerol can be used to
make a smallamount of glucose, but most
of the glucose is formed from the amino
acidsof proteins. In addition, some amino
acidscan be used directly for energy.
In the second stage, which can lastfor
several weeks, fatsare the primary energy
source. The liver metabolizes fatty acids
into ketone bodies that can be used asa
source ofenergy. After about a week of fast-
ing, the brain beginsto use ketone bodies,
as well as glucose, for energy. Thisusage
decreasesthe demand for glucose, and the
rate of protein breakdown diminishesbut
doesn’t stop. In addition, the proteins not
essentialfor survival are used first.
The third stage of starvation begins
when the fat reserves are depleted and
a switch to proteins as the major energy
source takes place. Muscles, the largest
source of protein in the body, are rapidly
depleted. Atthe end of this stage, proteins
essential for cellular functionsare broken
down, and cellfunction degenerates.
In addition to weight loss, symptoms
of starvation include apathy, listlessness,
withdrawal, and increased susceptibilityto
infectiousdisease. Few people die directly
from starvation because theyusually die of
some infectiousdisease first. Other signs of
starvation include changes in hair color,
flaky skin, and massive edema in the ab-
domen and lower limbs, causing the ab-
domen to appear bloated.
During the process of starvation, the
abilityof the body to consume normal vol-
umesof food also decreases. Foods high in
bulkbut low in protein content often cannot
reverse the processof starvation. Interven-
tion involves feeding the starving person
low-bulkfood that provides ample proteins
and kilocalories and is fortified with vita-
mins and minerals. The processof starva-
tion also results in dehydration, and
rehydration isan important part of interven-
tion. Even with intervention, a victim may
be so affected bydisease or weakness that
he or she cannotrecover.
normal functioning of the brain. During the postabsorptive state,
blood glucose levels are maintained by the conversion of other
molecules to glucose.The first source of blood glucose during the
postabsorptive state is the glycogen stored in the liver.This glyco-
gen supply,however, can provide glucose for only about 4 hours.
The glycogen stored in skeletal muscles can also be used during
times ofv igorous exercise.As glycogen stores are depleted, fats are
used as an energy source. The glycerol from triglycerides can be
converted to glucose.The fatty acids from fat can be converted to
acetyl-CoA,moved into the citric acid cycle, and used as a source of
energy to produce ATP.In the liver,acetyl-CoA is used to produce
ketone bodies that other tissues use for energy.The use of fatty
acids as an energy source partly eliminates the need to use glucose
for energy,resulting in reduced glucose removal from the blood
and maintenance ofblood glucose levels at homeostatic levels. Pro-
teins can also be used as a source ofglucose or can be used for en-
ergy production,again sparing the use of blood glucose.
37. What happens to glucose, fats, and amino acids during the
absorptive state?
38. Why is it important to maintain blood glucose levels during
the postabsorptive state? Name three sourcesfor this
glucose.
Part4 Regulationsand Maintenance934
Metabolic Rate
Objective
■ Define the term metabolic rate, and explain the three ways
metabolicenergy is used.
Metabolic rate is the total amount of energy produced and
used by the body per unit oftime. A molecule of ATP exists for less
than 1 minute before it’s degraded back to ADP and inorganic phos-
phate.For this reason,ATP is produced in cells at about the same rate
as it’s used.Thus, in examining metabolic rate,ATP production and
use can be roughly equated.Metabolic rate is usually estimated by
measuring the amount ofoxygen used per minute because most ATP
production involves the use of oxygen.One liter of oxygen con-
sumed by the body is assumed to produce 4.825 kcal ofenergy.
The daily input ofenergy should equal the metabolic expendi-
ture ofenergy; otherwise, a person will gain or lose weight. For a typ-
ical 23-year-old,70 kg (154-pound) male to maintain his weight, the
daily input should be 2700 kcal/day;for a typical 58 kg (128-pound)
female ofthe same age 2000 kcal/day is necessary. A pound of body
fat provides about 3500 kcal. Reducing kilocaloric intake by 500
kcal/day can result in the loss of1 pound of fat per week. Clearly, ad-
justing kilocaloric input is an important way to control body weight.
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Chapter 25 Nutrition, Metabolism, and Temperature Regulation 935
The Proportion ofFat in the Diet and Body Weight
Notonly the number of kilocalories ingested but also the proportion of
fatin the diet has an effect on body weight. To convertdietary fat into
bodyfat, 3% of the energy in the dietary fat is used, leaving 97% for
storage asfat deposits. On the other hand, the conversion of dietary
carbohydrate to fatrequires 23% of the energy in the carbohydrate,
leaving just77% as body fat. If two people have the same kilocaloric
intake, the one with the higher proportion offat in his or her diet is more
likelyto gain weight because fewer kilocalories are used to convert the
dietaryfat into body fat.
Metabolic energy is used in three ways:for basal metabolism,
for the thermic effect offood, and for muscular activity.
Basal Metabolic Rate
The basal metabolic rate (BMR) is the metabolic rate calculated
in expended kilocalories per square meter ofbody surface area per
hour.It’s determined by measuring the oxygen consumption of a
person who is awake but restful and has not eaten for 12 hours.The
liters of oxygen consumed are then multiplied by 4.825 because
each liter ofoxygen used results in the production of 4.825 kcal of
energy. A typical BMR for a 70 kg (154-pound) male is 38
kcal/m
2
/h.
BMR is the energy needed to keep the resting body func-
tional. In the average person, basal metabolism accounts for
about 60% of energy expenditure. Basal metabolism supports
active-transport mechanisms,muscle tone, maintenance of body
temperature,beating of the heart, and other activities. A number
of factors can affect the BMR. Muscle tissue is metabolically
more active than adipose tissue, even at rest.Younger people
have a higher BMR than older people because of increased cell
activity,especially dur ing growth.Fever can increase BMR 7%
for each degree Fahrenheit increase in body temperature.During
dieting or fasting, greatly reduced kilocaloric input can depress
BMR, which apparently is a protective mechanism to prevent
weight loss. Thyroid hormones can increase BMR on a long-
term basis,and epinephrine can increase BMR on a shor t-term
basis (see chapter 18). Males have a greater BMR than females
because men have proportionately more muscle tissue and less
adipose tissue than women do. During pregnancy,a woman’s
BMR can increase 20% because of the metabolic activity of
thefetus.
Thermic Effectof Food
The second component ofmetabolic energy concerns the assimila-
tion offood. When food is ingested, the accessory digestive organs
and the intestinal lining produce secretions,the motility of the di-
gestive tract increases,active transport increases, and the liver is in-
volved in the synthesis ofnew molecules. The energy cost of these
events is called the thermic effect offoo d and accounts for about
10% ofthe body’s energy expenditure.
Muscular Activity
Muscular activity consumes about 30% ofthe body’s energy. Phys-
ical activity resulting from skeletal muscle movement requires the
expenditure ofenergy. In addition, energy must be provided for in-
creased contraction ofthe hear t and of the muscles of respiration.
The number ofkilocalories used in an activit y depends almost en-
tirely on the amount ofmuscular work performed and on the du-
ration of the activity. Despite the fact that studying can make a
person feel tired, intense mental concentration produces little
change in the BMR.
Energy loss through muscular activity is the only component
of energy expenditure that a person can reasonably control. A
comparison ofthe number of kilocalor ies gained from food versus
the number of kilocalories lost in exercise reveals why losing
weight can be difficult.For example, walking (3 mph) for 20 min-
utes burns the kilocalories supplied by one slice of bread,whereas
jogging (5 mph) for the same time eliminates the kilocalories ob-
tained from a soft drink or a beer (see table 25.1). Nonetheless,
weight loss through exercise and dieting is possible.
39. Define the term metabolic rate.
40. What is BMR? What factors can alter BMR?
41. What is the thermic effect of food?
42. BMR, thermic effect of food, and muscular activity each
accountfor what percent of total energy expenditure?
43. How are kilocalorie input and output adjusted to maintain
bodyweight?
Body Temperature Regulation
Objective
■ Describe heat production and regulation in the body.
Humans are homeotherms(ho¯me¯-o¯-thermz;uniform warm-
ing),or warm-blooded animals, and can regulate body temperature
rather than have it adjusted by the external environment.Mainte-
nance of a constant body temperature is very important to homeo-
stasis.Most enzymes are very temperature sensitive and function only
in narrow temperature ranges.Environmental temperatures are too
low for normal enzyme function,and the heat produced by metabo-
lism helps maintain the body temperature at a steady,elevated level
that is high enough for normal enzyme function.
Free energy is the total amount of energy liberated by the
complete catabolism offood. It’s usually expressed in terms ofkilo-
calories (kcal) per mole offood consumed. For example, the com-
plete catabolism of1 mole of glucose (168 g; see chapter 2) releases
686 kcal of free energy.About 43% of the total energy released by
catabolism is used to produce ATP and to accomplish biologic
work,such as anabolism, muscular contraction, and other cellular
activities.The remaining energy is lost as heat.
PREDICT
Explain whywe become warm during exercise and why we shiver
when it’scold.
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Obesityis the presence of excess fat, and it
resultsfrom the ingestion of more food than
is necessary for the body’senerg yneeds.
Obesitycan be defined on the basis of body
weight, body massindex, or percent body
fat. “Desirable bodyweight” is listed in the
Metropolitan Life Insurance Table and indi-
cates, for any height, the weight that is
associated with a maximum life span. Over-
weight is defined as weighing 10% more
than the “desirable weight,” and obesityis
weighing 20% more than the “desirable
weight.” Bodymass index (BMI) can be cal-
culated by dividing a person’sweight (Wt)
in kilograms by the square of his or her
height(Ht) in meters: BMI Wt/Ht
2
. A BMI
greater than 25–27 is overweight, and a
value greater than 30 isdefined as obese.
About 10% of people in the United States
have a BMI of30 or greater. In terms of the
percentof the total body weight contributed
byfat, 15% body fat or lessin men and 25%
bodyfat or less in women is associated with
reduced health risks. Obesityis defined to
be more than 25% body fat in men and
30%–35% in women.
Obesity is classified according to the
number and size offat cells. The greater the
amount of lipids stored in the fat cells,
the larger their size. In hyperplastic obe-
sity, a greater-than-normal number of fat
cellsoccur that are also larger than normal.
Thistype of obesity is associated with mas-
sive obesityand begins at an early age. In
nonobese children, the number offat cells
triples or quadruples between birth and 2
yearsof age and then remains relativelysta-
ble untilpuberty, when a further increase in
the number occurs. In obese children, how-
ever, between 2 yearsof age and puberty,
an increase also occursin the number of fat
cells. Hypertrophic obesity results from a
normal number of fat cells that have in-
creased in size. Thistype of obesity is more
common, isassociated with moderate obe-
sityor being “overweight,” and typically de-
velopsin adults. People who were thin or of
average weightand quite active when they
were young become lessactive as they be-
come older. Theybegin to gain weight be-
tween age 20 and 40, and, although they
no longer use as many kilocalories, they
still take in the same amount of food as
when they were younger. The unused kilo-
caloriesare turned into fat, causing fat cells
to increase in size. Atone time, it was be-
lieved thatthe number of fat cellsdid not in-
crease after adulthood. It’snow known that
the number of fat cells can increase in
adults. Apparently, if all the existing fat
cellsare filled to capacity with lipids, new
fat cells are formed to store the excess
lipids. Once fatcells are formed, however,
dieting and weightloss don’t result in a de-
crease in the number offat cells—instead,
they become smaller in size astheir lipid
contentdecreases.
The distribution offat in obese individ-
ualsvaries. Fat can be found mainly in the
upper body, such asin the abdominal re-
gion, or it can be associated with the hips
and buttocks. These distribution differ-
encesare clinically significant because up-
per body obesity is associated with an
increased likelihood of diabetes mellitus,
cardiovascular disease, stroke, and death.
In some cases, a specificcause of obe-
sitycan be identified. For example, a tumor
in the hypothalamuscan stimulate overeat-
ing. In most cases, however, no specific
cause isapparent. In fact, obesity occurs for
manyreasons, and obesity in an individual
can have more than one cause. A genetic
componentto obesity seems to exist, and,
ifone or both parents are obese, their chil-
dren are more likelyto also be obese. Envi-
ronmental factors, such aseating habits,
however, can also play an important role.
For example, adopted children can exhibit
similaritiesin obesity to their adoptive par-
ents. In addition, psychologicfactors, such
as overeating asa means for dealing with
stress, can contribute to obesity.
Regulation ofbody weight is actually a
matter ofregulating body fat because most
changesin body weight reflect changes in
the amountof fat in the body. According to
the “setpoint” theory of weight control, the
body maintains a certain amountof body
fat. If the amount decreases below or in-
creases above this level, mechanismsare
activated to return the amountof body fatto
itsnormal value.
The two factorsthat affect the amount
ofadipose tissue in the body are energy in-
take and energy expenditure. The regula-
tion ofenerg yintake is poorly understood.
Apparently, neuronsoriginating in or pass-
ing through the hypothalamus continually
and spontaneously stimulate appetite and
food-seeking behaviors. After food is con-
sumed, several mechanismsare responsi-
ble for decreasing further food intake.
Neural mechanisms, such asdistension of
the stomach, are known to inhibitfeeding,
and a number of hormones released from
the gastrointestinal tract or pancreasalso
Clinical Focus Obesity
Part4 Regulationsand Maintenance936
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decrease appetite. For example, somato-
statin, cholecystokinin, glucagon, insulin,
and other hormones have been shown to
reduce food intake. The levelof fatty acids,
glucose, or amino acids in the blood also
providesthe brain with information neces-
sary to adjust appetite. Low levels of fatty
acids, glucose, and amino acidsstimulate
appetite, whereashigh levels of these sub-
stancesinhibit appetite.
Some scientistsbelieve that the num-
ber offat cells in the body also affects ap-
petite. According to thisline of reasoning,
fatcells maintain their size, and, once a “fat
plateau” isattained, the body stays at that
plateau. Fatcells accomplish this by effec-
tivelytaking up triglycerides and converting
them to fat. Consequently, lessenergy is
available for muscle and bodyorgans, and,
to compensate, appetite increases to pro-
vide needed energy. In support of thishy-
pothesis, it’sknown that obese individuals
have an increased amount of the enzyme
lipoprotein lipase, which isresponsible for
the uptake and storage oftriglyceridesin fat
cells. Furthermore, in obese individuals
who have lostweight, the levels of lipopro-
tein lipase increase even more.
It’s a common belief that the main
cause ofobesity is overeating. Certainly for
obesity to occur, atsome time, energ yin-
take must have exceeded energyexpendi-
ture. A comparison ofthe kilocaloric intake
ofobese and lean individuals at their usual
weights, however, revealsthat on a per kilo-
gram basis, obese people consume fewer
kilocaloriesthan lean people.
When people lose a large amount of
weight, their feeding behavior changes.
They become hyperresponsive to external
food cues, thinkof food often, and cannot
get enough to eat without gaining weight.
It’s now understood that this behavior is
typicalof both lean and obese individuals
who are below their relative set point for
weight. Other changes, such asa decrease
in basalmetabolic rate, take place in a per-
son who haslost a large amount of weight.
Most of thisdecrease in BMR probably re-
sultsfrom a decrease in muscle mass asso-
ciated with weight loss. In addition, some
evidence existsthat energy lost through ex-
ercise and the thermic effect of food are
also reduced.
Thus, a person who has lost a large
amount of weight is a person with an in-
creased appetite and a decreased abilityto
expend energy. It’sno surprise that only a
smallpercentage of obese people maintain
weightloss over the long term. Instead, the
typicalpattern is one of repeated cycles of
weightloss followed by a rapid regain of the
lostweight.
Current research isattempting to find
ways to help manage obesity. Unfortu-
nately, most appetite suppressants can
only be used for a short time. Dexfenflu-
ramine (deks-fen-flu¯ra˘-me¯n), which had
been approved by the FDA for long-term
use, was recalled because ofharmful side
effects.
Humanshave an obese (ob) gene that
codes for a protein called leptin(leptin),
which ismainly produced by adipose cells.
Leptin is released into the blood and af-
fectsappetite and body weight regulation.
When energy stores in adipose tissue de-
crease, leptin levelsdecrease, resulting in
an increased appetite and a decreased
metabolism. Decreased leptin levels may
be a signalthat helps the body to adjust to
fasting or starvation byincreasing food in-
take and reducing energyexpenditure. Two
populations of obese individuals provide
supporting evidence for thishypothesis. In
some obese people, leptin is inappropri-
ately low as a function of the amount of
bodyfat present. Thus, there is a mismatch
between the leptin signaland the amount
of energy stored in adipose tissue. Most
obesity, however, occursin the presence
of elevated levels of leptin. Thisobserva-
tion seems contradictory, because high
leptin levels should cause decreased ap-
petite, increased metabolism, and weight
loss. It turns out, however, that these
obese individuals are leptin-resistant.
Theymay have defective receptors for lep-
tin or in some other waydon’t respond ap-
propriately to leptin. Thisis analogous to
people with noninsulin-dependent dia-
betesmellitus (see chapter 18), who have
increased levels of insulin but don’t re-
spond to it. Future research maydetermine
the mechanism of leptin resistance and
the role ofleptin in obesity.
The message emerging from currentre-
search is that body weight results from
many complicated genetic and metabolic
factorsthat go awry in many different ways.
Obesityis being regarded as a chronic con-
dition that maysomeday respond to med-
ication in much the same waythat diabetes
does. Nonetheless, medication willonly be
partof the story. Drugs can help, but eating
lessand exercising more will still be neces-
saryfor optimal health.
Chapter 25 Nutrition, Metabolism, and Temperature Regulation 937
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Part4 Regulationsand Maintenance938
contractions ofshivering, whereas heat loss occurs through evapo-
ration. Heat gain or loss can occur by radiation,conduction, or
convection depending on the skin temperature and the environ-
mental temperature.If the skin temperature is lower than the envi-
ronmental temperature,heat is gained, but if the skin temperature
is higher than the environmental temperature,heat is lost.
The difference in temperature between the body and the en-
vironment determines the amount ofheat exchanged between the
environment and the body.The greater the temperature difference,
the greater the rate of heat exchange.Control of the temperature
difference is used to regulate body temperature.For example,if en-
vironmental temperature is very cold,like on a cold winter day, a
large temperature difference exists between the body and the envi-
ronment,and a large loss of heat occurs. The loss of heat can be de-
creased by behaviorally selecting a warmer environment, for
example,by going inside a heated house. Heat loss can also be de-
creased by insulating the exchange surface,such as by putting on
extra clothes. Physiologically,temperature difference can be con-
trolled through dilation and constriction of blood vessels in the
skin. When these blood vessels dilate,they bring warm blood to
thesurface of the body, raising skin temperature; conversely,vaso-
constriction decreases blood flow and lowers skin temperature.
PREDICT
Explain whyvasoconstriction of the skin’s blood vesselson a cool day
isbeneficial.
When environmental temperature is greater than body tem-
perature, vasodilation brings warm blood to the skin,causing an
increase in skin temperature that decreases heat gain from the en-
vironment.At the same time, evaporation carries away excess heat
to prevent heat gain and overheating.
Body temperature regulation is an example of a negative-
feedback system that is controlled by a “set point.”A small area in
the anterior part of the hypothalamus detects slight increases in
body temperature through changes in blood temperature (figure
25.17).As a result, mechanisms are activated that cause heat loss,
such as vasodilation and sweating, and body temperature de-
creases. A small area in the posterior hypothalamus can detect
slight decreases in body temperature and can initiate heat gain by
increasing muscular activity (shivering) and vasoconstriction.
Under some conditions,the set point of the hypothalamus is
actually changed. For example, during a fever,the set point is
raised,heat-conserving and heat-producing mechanisms are stim-
ulated,and body temperature increases. In recovery from a fever,
the set point is lowered to normal,heat loss mechanisms are initi-
ated,and body temperature decreases.
44. Define the terms homeotherm and free energy. How much
of the free energyis lost as heat from the body?
45. What are four ways that heat is exchanged between the
bodyand the environment?
46. How is body temperature behaviorally and physiologically
maintained in a cold and in a hotenvironment?
47. How does the hypothalamus regulate body temperature?
Radiation from
sun and water
Convection
from
cool breeze
Evaporation
Conduction from
hot sand
Radiation
from sand
Figure 25.16
HeatExchange
Heatexchange between a person and the environment occursby convection,
radiation, evaporation, and conduction. Arrowsshow the direction of net heat
gain or lossin this environment.
The average normal body temperature usually is considered
37°C (98.6°F) when measured orally and 37.6°C (99.7°F) when
measured rectally.Rectal temperature comes closer to the true core
body temperature,but an oral temperature is more easily obtained
in older children and adults and,therefore,is the preferred measure.
Heat can be exchanged with the environment in a number of
ways (figure 25.16).Radiation is the loss of heat as infrared radia-
tion,a type of electromagnetic radiation. For example, the coals in
a fire give offradiant heat that can be felt some distance away from
the fire.Conduction is the exchange of heat between objects in di-
rect contact with each other,such as the bottom of the feet and the
floor.Convection is a transfer of heat between the body and the
air.A cool breeze results in the movement of air over the body and
loss ofheat from the body. Evaporation is the conversion of water
from a liquid to a gaseous form,a process that requires heat. The
evaporation of 1 g of water from the body’s surface results in the
loss of580 cal of heat.
Body temperature is maintained by balancing heat gain with
heat loss. When heat gain equals heat loss,body temperature is
maintained. If heat gain exceeds heat loss,body temperature in-
creases, and if heat loss exceeds heat gain,body temperature de-
creases. Heat gain occurs through metabolism and the muscular
Seeley−Stephens−Tate:
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IV. Regulations and
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25. Nutrition, Metabolism,
and Temperature
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Chapter 25 Nutrition, Metabolism, and Temperature Regulation 939
Body temperature
(normal range)
Body temperature
decreases
Body temperature
increases
Body temperature
(normal range)
Body temperature
homeostasis
is maintained
• Constriction of skin blood vessels decreases
heat loss from the skin.
• Shivering increases heat production.
• Behavioral modifications, such as putting on
a jacket or seeking a warmer environment,
decrease heat loss.
A decrease in body temperature results
from increased heat loss.
• Increased sweating increases evaporative
heat loss.
• Dilation of skin blood vessels increases heat
loss from the skin.
• Behavioral modifications, such as taking off a
jacket or seeking a cooler environment,
increase heat loss.
A decrease in body temperature is detected
by receptors in the hypothalamus and skin.
The posterior hypothalamus responds to the
receptors and activates heat-conserving and
heat-generating mechanisms.
An increase in body temperature results
from decreased heat loss and increased
heat generation.
An increase in body temperature is detected by
receptors in the hypothalamus and skin.
The anterior hypothalamus responds to the
receptors and activates heat loss mechanisms.
HomeostasisFigure 25.17
Temperature Regulation
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Clinical Focus Hyperthermia and Hypothermia
Hyperthermia
Ifheat gain exceeds the ability of the body
to lose heat, body temperature increases
above normallevels, a condition called hy-
perthermia. Hyperthermia can result from
exercise, exposure to hot environments,
fever, and anesthesia.
Exercise increases body temperature
because of the heat produced as a by-
product of muscle activity(see chapter 9).
Normally, vasodilation and increased
sweating prevent body temperature in-
creases that are harmful. In a hot, humid
environment, the evaporation of sweat is
decreased, and exercise levelshave to be
reduced to preventoverheating.
Exposure to a hot environment nor-
mally results in the activation of heat
lossmechanisms, and body temperature is
maintained at normallevels. This is an ex-
cellent example of a negative-feedback
mechanism. Prolonged exposure to a hot
environment, however, can resultin heatex-
haustion. The normal negative-feedback
mechanisms for controlling bodytempera-
ture are operating, but they are unable to
prevent an increase in body temperature
above normallevels. Heavy sweating results
in dehydration, decreased blood volume,
decreased blood pressure, and increased
heart rate. Individuals suffering from heat
exhaustion have a wet, coolskin because of
the heavysweating. They usually feel weak,
dizzy, and nauseated. Treatmentincludes
reducing heatgain by moving to a cooler en-
vironment, ceasing activityto reduce heat
produced by muscle metabolism, and
restoring blood volume bydrinking fluids.
Heat stroke is more severe than heat
exhaustion because itresults from a break-
down in the normal negative-feedback
mechanisms oftemperature regulation. If
the temperature of the hypothalamus be-
comes too high, itno longer functions ap-
propriately. Sweating stops, and the skin
becomes dry and flushed. The person be-
comes confused, irritable, or even coma-
tose. In addition to the treatment for heat
exhaustion, heatloss from the skin should
be increased. Thiscan be accomplished by
increasing evaporation from the skin byap-
plying wet cloths or byincreasing conduc-
tive heatloss by immersing the person in a
coolbath.
Fever is the development of a higher-
than-normal body temperature following
the invasion of the body by microorgan-
ismsor foreign substances. Lymphocytes,
neutrophils, and macrophages release
chemicalscalled pyrogens(pı¯ro¯ -jenz) that
raise the temperature setpoint of the hy-
pothalamus. Consequently, bodytempera-
ture and metabolic rate increase. Fever is
believed to be beneficial because it
speeds up the chemical reactionsof the
immune system (see chapter 22) and in-
hibitsthe growth of some microorganisms.
Although beneficial, body temperatures
greater than 41°C(106°F) can be harmful.
Aspirin lowers body temperature by
inhibiting the synthesis of pyrogens
(prostaglandins).
Malignant hyperthermia is an inher-
ited muscle disorder. Certain drugsused to
induce generalanesthesia for surgery cause
sustained, uncoordinated muscle contrac-
tionsin individuals with this disorder. Con-
sequently, bodytemperature increases.
Therapeutic hyper thermia is an in-
duced local or general body increase in
temperature. It’s a treatment sometimes
used on tumorsand infections.
Hypothermia
Ifheat loss exceeds the ability of the body
to produce heat, body temperature de-
creasesbelow normal levels. Hypothermia
isa decrease in body temperature to 35°C
(95°F) or below. Hypothermia usually re-
sults from prolonged exposure to cold
environments. At first, normal negative-
feedbackmechanisms maintain body tem-
perature. Heat loss is decreased by
constricting blood vesselsin the skin, and
heatproduction is increased by shivering.
If body temperature decreases despite
these mechanisms, hypothermia devel-
ops. The individual’s thinking becomes
sluggish, and movements are uncoordi-
nated. Heart, respiratory, and metabolic
rates decline, and death results unless
body temperature is restored to normal.
Rewarming should occur ata rate of a few
degreesper hour.
Frostbite is damage to the skin and
deeper tissues resulting from prolonged
exposure to the cold. Damage resultsfrom
direct cold injury to cells, injury from ice
crystalformation, and reduced blood flow
to affected tissues. The fingers, toes, ears,
nose, and cheeksare most commonly af-
fected. Damage from frostbite can range
from rednessand discomfort to loss of the
affected part. The best treatment is im-
mersion in a warm water bath. Rubbing
the affected area and local, dry heat
should be avoided.
Therapeutic hypothermia is some-
times used to slow metabolic rate during
surgical procedureslike heart surgery. Be-
cause metabolicrate is decreased, tissues
don’t require as much oxygen as normal
and are lesslikely to be damaged.
Part4 Regulationsand Maintenance940
Nutrition
(p. 912)
Nutrition is the taking in and use offood.
Nutrients
1. Nutrients are the chemicals used by the body and consist of
carbohydrates,lipids, proteins, vitamins, minerals,and water.
2. Essential nutrients are nutrients that must be ingested because the
body cannot manufacture them or is unable to manufacture
adequate amounts ofthem.
Kilocalories
1. A calorie (cal) is the heat (energy) necessary to raise the temperature of
1 g ofwater 1°C. A kilocalorie (kcal) or Calorie (Cal) is 1000 calories.
SUMMARY
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
IV. Regulations and
Maintenance
25. Nutrition, Metabolism,
and Temperature
Regulation
© The McGraw−Hill
Companies, 2004
Chapter 25 Nutrition, Metabolism, and Temperature Regulation 941
2. A gram of carbohydrate or protein yields 4 kcal,and a g ram of fat
yields 9 kcal.
Carbohydrates
1. Carbohydrates are ingested as monosaccharides (glucose,fructose),
disaccharides (sucrose,maltose, lactose), and polysaccharides
(starch,glycogen, cellulose).
2. Polysaccharides and disaccharides are converted to glucose.Glucose
can be used for energy or stored as glycogen or fats.
3. About 125–175 g of carbohydrates should be ingested each day.
Lipids
1. Lipids are ingested as triglycerides (95%) or cholesterol and
phospholipids (5%).
2. Triglycerides are used for energy or are stored in adipose tissue.
Cholesterol forms other molecules,such as steroid hormones.
Cholesterol and phospholipids are part ofthe plasma membrane.
3. The recommended daily diet should derive no more than 30% ofits
kilocalories from lipids,and no more than 300 mg should be in the
form ofcholesterol.
Proteins
1. Proteins are ingested and broken down into amino acids.
2. Proteins perform many functions:protection (antibodies),
regulation (enzymes,hormones), structure (collagen), muscle
contraction (actin and myosin),and transportation (hemoglobin,
carrier molecules,ion channels).
3. An adult should consume 0.8 g of protein per kilogram of body
weight each day.
Vitamins
1. Many vitamins function as coenzymes or as parts of coenzymes.
2. Most vitamins are not produced by the body and must be obtained
in the diet.Some vitamins can be formed from provitamins.
3. Vitamins are classified as either fat-soluble or water-soluble.
4. Recommended Dietary Allowances (RDAs) are a guide for
estimating the nutritional needs ofgroups of people based on their
age,sex, and other factors.
Minerals
Minerals are necessary for normal metabolism,add mechanical strength to
bones and teeth,function as buffers, and are involved in osmotic balance.
DailyValues
1. Daily Values are dietary references that can be used to help plan a
healthy diet.
2. Daily Values for vitamins and minerals are based on Reference Daily
Intakes,which are generally the highest 1968 RDA value of an age
category.
3. Daily Values are based on Daily Reference Values.
• The Daily Reference Values for energy-producing nutrients
(carbohydrates,total fat, saturated fat, and proteins) and dietary
fiber are recommended percentages ofthe total kilocalories
ingested daily for each nutrient.
• The Daily Reference Values for total fats,saturated fats, cholesterol,
and sodium are the uppermost limit considered desirable because
oftheir link to diseases.
4. The % Daily Value is the percent ofthe recommended Daily Value of
a nutrient found in one serving ofa par ticular food.
Metabolism
(p. 920)
1. Metabolism consists ofanabolism and catabolism. Anabolism is the
building up ofmolecules and requires energy. Catabolism is the
breaking down ofmolecules and gives off energy.
2. The energy in carbohydrates,lipids, and proteins is used to produce
ATP through oxidation–reduction reactions.
Carbohydrate Metabolism
(p. 922)
Glycolysis
Glycolysis is the breakdown of glucose into two pyruvic acid molecules.
Also produced are two NADH molecules and two ATP molecules.
AnaerobicRespiration
1. Anaerobic respiration is the breakdown of glucose in the absence of
oxygen into two lactic acid and two ATP molecules.
2. Lactic acid can be converted to glucose (Cori cycle) using aerobically
produced ATP (oxygen debt).
AerobicRespiration
1. Aerobic respiration is the breakdown ofg lucose in the presence of
oxygen to produce carbon dioxide,water,and 38 (or 36) ATP
molecules.
2. The first phase is glycolysis,which produces two ATP,two NADH,
and two pyruvic acid molecules.
3. The second phase is the conversion ofthe two pyruvic acid
molecules into two molecules ofacetyl-CoA. These reactions also
produce two NADH and two carbon dioxide molecules.
4. The third phase is the citric acid cycle,which produces two ATP,six
NADH,two FADH2, and four carbon dioxide molecules.
5. The fourth phase is the electron-transport chain. The high-energy
electrons in NADH and FADH2 enter the electron-transport chain
and are used in the synthesis ofATP and water.
Lipid Metabolism
(p. 929)
1. Adipose triglycerides are broken down and released as free fatty acids.
2. Free fatty acids are taken up by cells and broken down by beta-
oxidation into acetyl-CoA.
• Acetyl-CoA can enter the citric acid cycle.
• Acetyl-CoA can be converted into ketone bodies.
Protein Metabolism
(p. 930)
1. New amino acids are formed by transamination,the transfer of an
amine group to a keto acid.
2. Amino acids are used to synthesize proteins.If used for energy,
ammonia is produced as a by-product ofoxidative deamination.
Ammonia is converted to urea and is excreted.
Interconversion ofNutrient Molecules
(p. 931)
1. Glycogenesis is the formation of glycogen from glucose.
2. Lipogenesis is the formation of lipids from glucose and amino acids.
3. Glycogenolysis is the breakdown ofg lycogen to glucose.
4. Gluconeogenesis is the formation of glucose from amino acids and
glycerol.
MetabolicStates
(p. 932)
1. In the absorptive state,nutrients are used as energy or stored.
2. In the postabsorptive state,stored nutrients are used for energy.
MetabolicRate
(p. 934)
Metabolic rate is the total energy expenditure per unit oftime, and it has
three components.
BasalMet abolic Rate
Basal metabolic rate is the energy used at rest.It is 60% of the metabolic rate.
ThermicEffect of Food
The thermic effect offood is the energy used to digest and absorb food. It
is 10% ofthe metabolic rate.
Muscular Activity
Muscular energy is used for muscle contraction.It is 30% of the metabolic
rate.
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
IV. Regulations and
Maintenance
25. Nutrition, Metabolism,
and Temperature
Regulation
© The McGraw−Hill
Companies, 2004
BodyTemperature Regulation
(p. 935)
1. Body temperature is a balance between heat gain and heat loss.
• Heat is produced through metabolism.
• Heat is exchanged through radiation,conduction, convection, and
evaporation.
Part4 Regulationsand Maintenance942
2. The greater the temperature difference between the body and the
environment,the greater the rate of heat exchange.
3. Body temperature is regulated by a “set point”in the hypothalamus.
1. Which ofthese statements concerning kilocalories is true?
a. A kilocalorie is the amount of energy required to raise the
temperature of1 g of water 1°C.
b. There are 9 kcal in a gram ofprotein.
c. There are 4 kcal in a gram of fat.
d. A pound ofbody fat contains 3500 kcal.
2. Complex carbohydrates include
a. sucrose.
b. milk sugar (lactose).
c. starch,an energ y storage molecule in plants.
d. all ofthe above.
3. What type of nutrient is recommended as the primary energy
source in the diet?
a. carbohydrates
b. fats
c. proteins
d. cellulose
4. A good source of monounsaturated fats is
a. fat associated with meat.
b. egg yolks.
c. whole milk.
d. fish oil.
e. olive oil.
5. A complete protein food
a. provides the daily amount (grams) of protein recommended in a
healthy diet.
b. can be used to synthesize the nonessential amino acids.
c. contains all 20 amino acids.
d. includes beans,peas, and leafy green vegetables.
6. Concerning vitamins,
a. most can be synthesized by the body.
b. they are normally broken down before they can be used by the
body.
c. A, D,E, and K are water-soluble vitamins.
d. many function as coenzymes.
7. Minerals
a. are inorganic nutrients.
b. compose about 4%–5% oftotal body weight.
c. act as buffers and osmotic regulators.
d. are components ofenzymes.
e. all of the above.
8. Glycolysis
a. is the breakdown of glucose to two pyruvic acid molecules.
b. requires the input oftwo ATP molecules.
c. produces two NADH molecules.
d. does not require oxygen.
e. all of the above.
9. Anaerobic respiration occurs in the ofoxygen and
produces energy (ATP) for the cell than aerobic
respiration.
a. absence,more
b. absence,less
c. presence,more
d. presence,less
10. Which ofthese reactions take place in both anaerobic and aerobic
respiration?
a. glycolysis
b. citric acid cycle
c. electron-transport chain
d. acetyl-CoA formation
e. all of the above
11. A molecule that moves electrons from the citric acid cycle to the
electron-transport chain is
a. tRNA.
b. mRNA.
c. ADP.
d. NADH.
e. pyruvic acid.
12. The production ofATP molecules by the electron-transport chain is
accompanied by the synthesis of
a. alcohol.
b. water.
c. oxygen.
d. lactic acid.
e. glucose.
13. The carbon dioxide you breathe out comes from
a. glycolysis.
b. the electron-transport chain.
c. anaerobic respiration.
d. the food you eat.
14. Lipids are
a. stored primarily as triglycerides.
b. synthesized by beta-oxidation.
c. broken down by oxidative deamination.
d. all ofthe above.
15. Amino acids
a. are classified as essential or nonessential.
b. can be synthesized in a transamination reaction.
c. can be used as a source of energy.
d. can be converted to keto acids.
e. all of the above.
16. Ammonia is
a. a by-product of lipid metabolism.
b. formed during ketogenesis.
c. converted into urea in the liver.
d. produced during lipogenesis.
e. converted to keto acids.
17. The conversion ofamino acids and glycerol into glucose is called
a. gluconeogenesis.
b. glycogenolysis.
c. glycogenesis.
d. ketogenesis.
18. Which ofthese e vents takes place during the absorptive state?
a. Glycogen is converted into glucose.
b. Glucose is converted into fats.
c. Ketones are produced.
d. Proteins are converted into glucose.
REVIEW AND COMPREHENSION
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
IV. Regulations and
Maintenance
25. Nutrition, Metabolism,
and Temperature
Regulation
© The McGraw−Hill
Companies, 2004
Chapter 25 Nutrition, Metabolism, and Temperature Regulation 943
19. The major use of energy by the body is in
a. basal metabolism.
b. physical activity.
c. the thermic effect of food.
20. The loss of heat resulting from the loss of water from the body’s
surface is
a. radiation.
b. evaporation.
c. conduction.
d. convection.
Answers in Appendix F
1. One serving of a food has 2 g of saturated fat. What % Daily Value
for saturated fat would appear on a food label for this food? (See
bottom offigure 25.2 for information needed to answer this
question.)
2. An active teenage boy has a daily intake of3000 kcal/day. What is the
maximum amount (weight) oftotal fats he should consume
according to the Daily Values?
3. If the teenager in question 2 eats a serving of food that has a total fat
content of10 g/serving, what is his % Daily Value for total fat?
4. Suppose the food in question 3 is in a package that lists a serving
size of1/2 cup with 4 serving s in the package.If the teenager eats
halfof the contents of the package (1 cup), how much of his %
Daily Value does he consume?
5. Why does a vegetarian usually have to be more careful about his or
her diet than a person who includes meat in the diet?
6. Explain why a person suffering from copper deficiency feels tired all
the time.
7. Some people claim that occasionally fasting for short periods can be
beneficial.How can fasts be damaging?
8. Why can some people lose weight on a 1200 kcal/day diet and others
cannot?
9. Lotta Bulk,a muscle builder, wanted to increase her muscle mass.
Knowing that proteins are the main components ofmuscle, she
began a high-protein diet in which most ofher daily kilocalories
were supplied by proteins.She also exercised regularly with heavy
weights.After 3 months of this diet and exercise program, Lotta
increased her muscle mass,but not any more than her friend, who
did the same exercises but did not have a high-protein diet.Explain
what happened.Was Lotta in positive or negative nitrogen balance?
10. On learning that sweat evaporation results in the loss of calories, an
anatomy and physiology student enters a sauna in an attempt to lose
weight.He reasons that a liter (about a quart) of water weighs
1000g, which is equivalent to 580,000 cal or 580 kcal of heat when
lost as sweat.Instead of reducing his diet by 580 kcal/day,if he loses
a liter ofsweat every day in the sauna, he believes he will lose about
a pound offat a week. Will this approach work? Explain.
Answers in Appendix G
CRITICAL THINKING
1. If vitamins were broken down during the process of digestion,their
structures would be destroyed,and, as a result, their ability to
function would be lost.
2. The Daily Value for carbohydrate is 300 g/day.One serving of food
with 30 g ofcarbohydrate has a % Daily Value of 10% (30/300 = .10,
or 10%).
3. On a 1800 kcal/day diet,the total percentage of Daily Values for
energy-producing nutrients should add up to no more than 90%,
because 1800/2000 0.9, or 90%.
4. If the electron of the electron-transport chain cannot be donated to
oxygen,the entire electron-transport chain stops, no ATP can be
produced aerobically,and the patient dies because too little energy is
available for the body to perform vital functions.Anaerobic
respiration is not adequate to provide all the energy needed to
maintain human life,except for a short time.
5. When muscles contract,they use ATP.As a result of the chemical
reactions necessary to synthesize ATP,heat is also produced.During
exercise the large amounts ofheat can raise body temperature, and
we feel warm.Shivering consists of small, rapid muscle contractions
that produce heat in an effort to prevent a decrease in body
temperature in the cold.
6. Vasoconstriction reduces blood flow to the skin,which reduces skin
temperature because less warm blood from the deeper parts ofthe
body reaches the skin.As the difference in temperature between the
skin and the environment decreases,less loss of heat occurs.
ANSWERS TO PREDICT QUESTIONS
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