Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
I. Organization of the
Human Body
2. The Chemical Basis of
Life
© The McGraw−Hill
Companies, 2004
Allthe structures of the body are com-
posed of chemicals, and allthe func-
tions of the body result from the
interactions of these chemicals with
one another. The generation ofnerve im-
pulsesand the physiologic processes of di-
gestion, muscle contraction, and metabolism
can be described in chemical terms. Likewise,
many illnessesand their treatment can be described
chemically. For example, Parkinson’sdisease, which causesuncontrollable shak-
ing movements, resultsfrom a shortage of a chemical called dopamine in certain
nerve cellsin the brain. It is treated by giving patients another chemical that is
converted to dopamine bybrain cells.
To understand anatomyand physiology, it is essential to have a basic
knowledge of chemistry
—
the scientific discipline concerned with the atomic
composition and structure ofsubstances and the reactions they undergo. This
chapter outlinesbasic chemistry (27), chemical reactionsand energy (34), inor-
ganicchemistry (39), and organicchemistry (43). It is nota comprehensive re-
view ofchemistry, but it does review some of the basic concepts. Refer back to
thischapter when chemical phenomena are discussed later in the text.
The Chemical
Basis of Life
Colorized scanning electron micrograph (SEM) of
bundlesof collagen fibers (brown) and elastic
fibers(blue). The chemical composition of these
fibersdetermines their functions within the body.
CHAPTER
2
Part 1 Organization ofthe Human Body
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
I. Organization of the
Human Body
2. The Chemical Basis of
Life
© The McGraw−Hill
Companies, 2004
Chapter 2 The ChemicalBasis of Life 27
Basic Chemistry
Objectives
■ Define the terms matter, mass, weight, element, and atom.
■ Describe the subatomic particles of an atom and explain
howthey determine atomic number, mass number,
isotopes, and atomicmass.
■ Describe the types of chemical bonding and contrast them
with intermolecularforces.
■ Distinguish between a molecule and a compound and
describe howeach dissolves in water.
Matter, Mass, and Weight
All living and nonliving things are composed of matter,which is
anything that occupies space and has mass.Mass is the amount of
matter in an object,and weight is the g ravitational force acting on
an object of a given mass.For example, the weight of an apple re-
sults from the force ofgravity “pulling” on the apple’s mass.
PREDICT
The difference between massand weight can be illustrated by
considering an astronaut. How doesan astronaut’s massand weight
in outer space compare to the astronaut’smass and weighton the
earth’ssurface?
The kilogram (kg), which is the mass of a platinum–irid-
ium cylinder kept at the International Bureau ofWeights and Mea-
surements in France,is the international unit for mass. The mass of
all other objects is compared to this cylinder. For example,a
2.2-pound lead weight or 1 liter (L) (1.06 qt) of water each has a
mass of approximately 1 kg.An object with 1/1000 the mass of a
kilogram is defined as having a mass of1 gram (g).
Chemists use a balance to determine the mass of objects.
Although we commonly refer to “weighing”an object on a bal-
ance, we are actually “massing”the object because the balance
compares objects of unknown mass to objects of known mass.
When the unknown and known masses are exactly balanced,the
gravitational pull ofthe ear th on both of them is the same. Thus,
the effect ofgravit y on the unknown mass is counteracted by the
effect of gravity on the known mass. A balance produces the
same results on a mountaintop as at sea level because it does not
matter ifthe g ravitational pull is strong or weak.It only matters
that the effect of gravity on both the unknown and known
masses is the same.
Elementsand Atoms
An element is the simplest type of matter with unique chemical
properties.To date, 112 elements are known.A list of the elements
commonly found in the human body is given in table 2.1.About
Table 2.1
Percent in Human
Atomic Mass Atomic Percent in Human Body by Number
Element Symbol Number Number Mass Body by Weight (%) of Atoms (%)
Hydrogen H 1 1 1.008 9.5 63.0
Carbon C 6 12 12.01 18.5 9.5
Nitrogen N 7 14 14.01 3.3 1.4
Oxygen O 8 16 16.00 65.0 25.5
Fluorine F 9 19 19.00 Trace Trace
Sodium Na 11 23 22.99 0.2 0.3
Magnesium Mg 12 24 24.31 0.1 0.1
Phosphorus P 15 31 30.97 1.0 0.22
Sulfur S 16 32 32.07 0.3 0.05
Chlorine Cl 17 35 35.45 0.2 0.03
Potassium K 19 39 39.10 0.4 0.06
Calcium Ca 20 40 40.08 1.5 0.31
Chromium Cr 24 52 51.00 Trace Trace
Manganese Mn 25 55 54.94 Trace Trace
Iron Fe 26 56 55.85 Trace Trace
Cobalt Co 27 59 58.93 Trace Trace
Copper Cu 29 63 63.55 Trace Trace
Zinc Zn 30 64 65.39 Trace Trace
Selenium Se 34 80 78.96 Trace Trace
Molybdenum Mo 42 98 95.94 Trace Trace
Iodine I 53 127 126.9 Trace Trace
Common Elements
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
I. Organization of the
Human Body
2. The Chemical Basis of
Life
© The McGraw−Hill
Companies, 2004
96% of the weight of the body results from the elements oxygen,
carbon,hydrogen, and nitrogen.
An atom is the smallest particle of an element that has the
chemical characteristics of that element.An element is composed
of atoms of only one kind. For example, the element carbon is
composed of only carbon atoms,and the element oxygen is com-
posed ofonly oxygen atoms.
An element,or an atom of that element, often is represented
by a symbol.Usually the first letter or letters of the element’s name
are used—for example,C for carbon, H for hydrogen, Ca for cal-
cium,and Cl for chlorine. Occasionally the symbol is taken from
the Latin,Greek, or Arabic name for the element—for example,Na
from the Latin word natriumis the symbol for sodium.
AtomicStructure
The characteristics of living and nonliving matter result from the
structure,organization, and behavior of atoms (figure 2.1). Atoms
are composed ofsubatomic particles, some of which have an elec-
tric charge.The three major types of subatomic particles are neu-
trons, protons,and elect rons.Neu trons have no electric charge,
protons have positive charges, and electrons have negative
charges.The positive charge of a proton is equal in magnitude to
the negative charge ofan electron. Because equal numbers of pro-
tons and electrons occur in an atom,the individual charges cancel
each other,and the atom is electrically neutral.
Protons and neutrons form the nucleus, and electrons are
moving around the nucleus (see figure 2.1).The nucleus accounts
for 99.97% of an atom’s mass but only 1 ten-trillionth ofits vol-
ume.Most of the volume of an atom is occupied by the electrons.
Although it is impossible to know precisely where any given elec-
tron is located at any particular moment, the region where it is
most likely to be found can be represented by an electron cloud.
The likelihood oflocating an electron at a specific point in a region
correlates with the darkness of that region in the diagram. The
darker the color,the greater the likelihood of finding the electron
there at any given moment.
AtomicNumber and Mass Number
Theatomic number of an element is equal to the number of pro-
tons in each atom,and because the number of electrons and pro-
tons is equal, the atomic number also indicates the number of
electrons.Each element is uniquely defined by the number of pro-
tons in the atoms of that element. For example, only hydrogen
atoms have one proton,only carbon atoms have six protons, and
only oxygen atoms have eight protons (figure 2.2;see table 2.1).
Scientists have been able to create new elements by changing
the number ofprotons in the nuclei of existing elements. Protons,
neutrons,or electrons from one atom are accelerated to very high
speeds and then smashed into the nucleus ofanother atom. The re-
sulting changes in the nucleus produce a new element with a new
atomic number. To date,20 elements with an atomic number
greater than 92 have been synthesized in this fashion.These artifi-
cially produced elements are usually unstable, and they quickly
convert back to more stable elements.
Protons and neutrons have about the same mass,and they
are responsible for most of the mass of atoms. Electrons, on the
other hand,have very little mass. The mass number of an element
is the number of protons plus the number of neutrons in each
atom. For example,the mass number for carbon is 12 because it
has six protons and six neutrons.
PREDICT
The atomicnumber of potassium is 19, and the mass number is 39.
Whatis the number of protons, neutrons, and electrons in an atom of
potassium?
Isotopesand Atomic Mass
Isotopes (ı¯so¯-to¯pz) are two or more forms of the same element
that have the same number ofprotons and electrons but a different
number of neutrons. Thus isotopes have the same atomic number
but different mass numbers.There are three isotopes of hydrogen:
hydrogen,deuterium, and tritium. All three isotopes have one pro-
ton and one electron,but hydrogen has no neutrons in its nucleus,
deuterium has one neutron, and tritium has two neutrons (figure
2.3).Isotopes can be denoted using the symbol of the element pre-
ceded by the mass number (number ofprotons and neutrons) of the
isotope.Thus hydrogen is
1
H,deuterium is
2
H,and tritium is
3
H.
Individual atoms have very little mass.A hydrogen atom
has a mass of1.67 10
24
g (see appendix B for an explanation
ofthe scientific notation of numbers).To avoid using such small
Part1 Organization of the Human Body28
Figure 2.1
Modelof an Atom
The tiny, dense nucleusconsists of positivelycharged protons and uncharged
neutrons. Mostof the volume of an atom is occupied byrapidly moving,
negativelycharged electrons, which can be represented asan electron cloud.
The probable location ofan electron is indicated by the color ofthe electron
cloud. The darker the color in each smallpartof the electron cloud, the more
likelythe electron is located there.
Proton
(positive charge)
Neutron
(no charge)
Nucleus
Atom
Electron cloud occupied by
negatively charged electrons
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
I. Organization of the
Human Body
2. The Chemical Basis of
Life
© The McGraw−Hill
Companies, 2004
Chapter 2 The ChemicalBasis of Life 29
numbers,a system of relative atomic mass is used. In this system,
aunified atomic mass unit (u), or dalton (D), is 1/12 the mass
of
12
C, a carbon atom with six protons and six neutrons.Thus
12
C has an atomic mass of exactly 12 u. A naturally occurring
sample of carbon,however, contains mostly
12
C but also a small
quantity ofother carbon isotopes such as
13
C,which has six pro-
tons and seven neutrons.The atomic mass of an element is the
average mass of its naturally occurring isotopes, taking into ac-
count the relative abundance of each isotope.For example, the
atomic mass of the element carbon is 12.01 u (see table 2.1),
which is slightly more than 12 u because of the additional mass
of the small amount of other carbon isotopes. Because the
atomic mass is an average,a sample of carbon can be treated as if
all the carbon atoms have an atomic mass of12.01 u.
1. Define matter. How is the mass and the matter of an object
different?
2. Define element and atom. What four elements are found in
the greatestabundance in humans?
3. For each subatomic particle of an atom, state its charge and
location. Which subatomicparticles are most responsible
forthe mass and volume of an atom? Which subatomic
particlesdetermine atomic number and mass number?
4. Define isotopes and give an example. Define atomic mass.
Whyis the atomic mass of most elements not exactlyequal
to the massnumber?
Electronsand Chemical Bonding
The outermost electrons ofan atom determine its chemical behav-
ior.When these outermost electrons are transferred or shared be-
tween atoms, chemical bonding occurs. Two major types of
chemical bonding are ionic and covalent bonding.
IonicBonding
An atom is electrically neutral because it has an equal number of
protons and electrons. If an atom loses or gains electrons, the
number of protons and electrons are no longer equal, and a
charged particle called an ion(ı¯on) is formed.After an atom loses
an electron, it has one more proton than it has electrons and is
positively charged.A sodium atom (Na) can lose an electron to
become a positively charged sodium ion (Na
) (figure 2.4a).After
an atom gains an electron, it has one more electron than it has
protons and is negatively charged.A chlorine atom (Cl) can accept
an electron to become a negatively charged chloride ion (Cl
).
Positively charged ions are called cations (katı¯-onz),and
negatively charged ions are called anions(anı¯-onz).Because op-
positely charged ions are attracted to each other,cations and an-
ions tend to remain close together,which is called ionic (ı¯-onik)
bonding.For example, sodium and chloride ions are held together
by ionic bonding to form an array ofions called sodium chlor ide,
or table salt (see figure 2.4band c). Some ions commonly found in
the body are listed in table 2.2.
Carbon
atom
6p
+
6n
0
6e
–
Oxygen
atom
8p
+
8n
0
8e
–
Hydrogen
atom
1e
–
1p
+
Figure 2.2
Hydrogen, Carbon, and Oxygen Atoms
Within the nucleus, the number ofpositively charged protons (p
) and uncharged neutrons(n
0
) isindicated. The negatively charged electrons(e
) are around the
nucleus. Atomsare electrically neutral because the number of protons and electronswithin an atom are equal.
1e
–
p
+
(a)Hydrogen (
1
H) (b)Deuterium (
2
H)
1e
–
p
+
n
0
1e
–
(c)Tritium (
3
H)
p
+
n
0
n
0
Figure 2.3
Isotopesof Hydrogen
(a) Hydrogen hasone proton and no neutrons in its nucleus. (b) Deuterium has one proton and one neutron in its nucleus. (c) Tritium hasone proton and two
neutronsin its nucleus.
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
I. Organization of the
Human Body
2. The Chemical Basis of
Life
© The McGraw−Hill
Companies, 2004
Table 2.2
Common
Ions Symbols Functions
Calcium Ca
2
Bones, teeth, blood clotting,
muscle contraction,
release of neurotransmitters
Sodium Na
Membrane potentials, water
balance
Potassium K
Membrane potentials
Hydrogen H
Acid–base balance
Hydroxide OH
Acid–base balance
Chloride Cl
Water balance
Bicarbonate HCO
3
Acid–base balance
Ammonium NH
4
Acid–base balance
Phosphate PO
4
3
Bone, teeth, energy exchange,
acid–base balance
Iron Fe
2
Red blood cell formation
Magnesium Mg
2
Necessary for enzymes
Iodide I
Present in thyroid hormones
Important Ions
Cl
–
Na
+
Chloride ion (Cl
–
)
18e
–
Sodium
chloride
11p
+
12n
0
11p
+
12n
0
L
o
s
e
s
e
l
e
c
t
r
o
n
G
a
i
n
s
e
l
e
c
t
r
o
n
17e
–
Chlorine atom (Cl)
17p
+
18n
0
Sodium atom (Na)
11e
–
17p
+
18n
0
10e
–
e
–
Sodium ion (Na
+
)
Figure 2.4
IonicBonding
(a) A sodium atom losesan electron to become a smaller-sized positively
charged ion, and a chlorine atom gainsan electron to become a larger-sized
negativelycharged ion. The attraction between the oppositelycharged ions
resultsin an ionic bond and the formation of sodium chloride. (b) The sodium
and chlorine ionsare organized to form a cube-shaped array. (c)
Microphotograph ofsalt crystals reflects the cubic arrangement of the ions.
(a)
(b)
(c)
CovalentBonding
Covalent bonding results when atoms share one or more pairs of
electrons.The resulting combination of atoms is called a molecule.
An example is the covalent bond between two hydrogen atoms to
form a hydrogen molecule (figure 2.5).Each hydrogen atom has one
electron.As the two hydrogen atoms get closer together, the posi-
tively charged nucleus ofeach atom begins to attract the electron of
the other atom.At an optimal distance, the two nuclei mutually at-
tract the two electrons,and each electron is shared by both nuclei.
The two hydrogen atoms are now held together by a covalent bond.
When an electron pair is shared between two atoms,a single
covalent bondresults. A single covalent bond is represented by a
single line between the symbols of the atoms involved (e.g.,
HOH). A double covalent bond results when two atoms share
four electrons, two from each atom.When a carbon atom com-
bines with two oxygen atoms to form carbon dioxide,two double
covalent bonds are formed.Double covalent bonds are indicated
by a double line between the atoms (OPCPO).
When electrons are shared equally between atoms,as in a hy-
drogen molecule,the bonds are called nonpolar covalent bonds.
Atoms bound to one another by a covalent bond do not always
share their electrons equally,however, because the nucleus of one
atom attracts the electrons more strongly than does the nucleus of
Part1 Organization of the Human Body30
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
I. Organization of the
Human Body
2. The Chemical Basis of
Life
© The McGraw−Hill
Companies, 2004
Chapter 2 The ChemicalBasis of Life 31
Many molecules are compounds, however.Most covalent sub-
stances consist of molecules because their atoms form distinct
units as a result ofthe joining of the atoms to each other by a pair
of shared electrons. For example,a water molecule is a covalent
compound.
On the other hand, ionic compounds are not molecules
because the ions are held together by the force ofattraction be-
tween opposite charges. A piece of sodium chloride does not
consist of sodium chloride molecules positioned next to each
other. Instead,table salt is an organized array of sodium and
chloride ions in which each charged ion is surrounded by several
ions of the opposite charge (see figure 2.4b).Sodium chloride is
an example of a substance that is a compound but is not a
molecule.
The kinds and numbers of atoms (or ions) in a molecule or
compound can be represented by a formula consisting ofthe sym-
bols ofthe atoms (or ions) plus subscripts denoting the number of
each type of atom (or ion). The formula for glucose (a sugar) is
C
6
H
12
O
6
,indicating that glucose has 6 carbon, 12 hydrogen, and 6
oxygen atoms (table 2.3).
the other atom.Bonds of this type are called polar covalent bonds
and are common in both living and nonliving matter.
Polar covalent bonds can result in polar molecules,which
are electrically asymmetric. For example, oxygen atoms attract
electrons more strongly than do hydrogen atoms.When covalent
bonding between an oxygen atom and two hydrogen atoms
forms a water molecule,the electrons are located in the vicinity
of the oxygen nucleus more than in the vicinity of the hydrogen
nuclei.Because electrons have a negative charge, the oxygen side
of the molecule is slightly more negative than the hydrogen side
(figure 2.6).
Molecules and Compounds
A molecule is formed when two or more atoms chemically com-
bine to form a structure that behaves as an independent unit.The
atoms that combine to form a molecule can be of the same type,
such as two hydrogen atoms combining to form a hydrogen
molecule.More typically, a molecule consists of two or more dif-
ferent types of atoms,such as two hydrogen atoms and an oxygen
atom forming water.Thus, a glass of water consists of a collection
ofindividual water molecules positioned next to one another.
Acompound is a substance composed of two or more differ-
enttypes of atoms that are chemically combined. Not all molecules
are compounds.For example, a hydrogen molecule is not a com-
pound because it does not consist of different types of atoms.
H
O
H
H
H
O
+
–
δ
δ
Figure 2.6
Polar CovalentBonds
(a) A water molecule formswhen two hydrogen atoms form covalent bonds
with an oxygen atom. (b) Electron pairs(indicated by dots) are shared
between the hydrogen atomsand oxygen. The electrons are shared unequally,
asshown by the electron cloud (yellow) not coinciding with the dashed
outline. Consequently, the oxygen side ofthe molecule hasa slight negative
charge (indicated by
δ
) and the hydrogen side ofthe molecule has a slight
positive charge (indicated by
δ
).
No interaction between the two hydrogen atoms because they are too far
apart.
The positively charged nucleus of each hydrogen atom begins to attract
the electron of the other.
Acovalent bond is formed when the electrons are shared between the
nuclei because the electrons are equally attracted to each nucleus.
e
–
p
+
e
–
p
+
e
–
p
+
e
–
p
+
e
–
p
+
e
–
p
+
Figure 2.5
CovalentBonding
(a)
(b)
Seeley−Stephens−Tate:
Anatomy and Physiology,
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I. Organization of the
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2. The Chemical Basis of
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Themolecular mass of a molecule or compound can be de-
termined by adding up the atomic masses of its atoms (or ions).
The term molecular mass is used for convenience for ionic com-
pounds, even though they are not molecules. For example,the
atomic mass ofsodium is 22.99 and chloride is 35.45. The molecu-
lar mass ofNaCl is therefore 58.44 (22.99 35.45).
5. Describe how ionic bonding occurs. What is a cation and an
anion?
6. Describe how covalent bonding occurs. What is the
difference between polarand nonpolar covalent bonds?
7. Distinguish between a molecule and a compound. Are all
moleculescompounds? Are all compounds molecules?
8. Define molecular mass.
PREDICT
Whatis the molecular mass of a molecule of glucose? (Use table 2.1.)
Intermolecular Forces
Intermolecular forces result from the weak electrostatic attrac-
tions between the oppositely charged parts of molecules, or be-
tween ions and molecules.Intermolecular forces are much weaker
than the forces producing chemical bonding.
Hydrogen Bonds
Molecules with polar covalent bonds have positive and negative
“ends.”Intermolecular force results from the attraction of the
positive end ofone polar molecule to the negative end of another
Clinical Focus Radioactive Isotopesand X Rays
Protons, neutrons, and electrons are re-
sponsible for the chemical properties of
atoms. Theyalso have other properties that
can be usefulin a clinical setting. For exam-
ple, theyhave been used to develop meth-
ods for examining the inside of the body.
Radioactive isotopes are commonly used
byclinicians and researchers because sen-
sitive measuring devicescan detecttheir ra-
dioactivity, even when they are presentin
verysmall amounts.
Radioactive isotopeshave unstable nu-
clei thatspontaneously change to form more
stable nuclei. Asa result, either new isotopes
or new elements are produced. In this
process of nuclear change, alpha particles,
beta particles, and gamma raysare emitted
from the nuclei ofradioactive isotopes. Alpha
(α) particles are positively charged helium
ions(He
2
), which consistoftwo protons and
two neutrons. Beta (β) particlesare electrons
formed asneutrons change into protons. An
electron isejected from the neutron, and the
proton that is produced remainsin the nu-
cleus. Gamma (γ) raysare a form of electro-
magnetic radiation (high-energy photons)
released from nuclei asthey lose energy.
All isotopes of an element have the
same atomic number, and their chemical
behavior isvery similar. For example,
3
H (tri-
tium) can substitute for
1
H (hydrogen), and
either
125
iodine or
131
iodine can substitute
for
126
iodine in chemicalreactions.
Several procedures that are used to
determine the concentration of sub-
stancessuch as hormones depend on the
incorporation of small amounts of ra-
dioactive isotopes, such as
125
iodine, into
the substances being measured. Clini-
ciansusing these procedures can more ac-
curatelydiagnose disorders of the thyroid
gland, the adrenal gland, and the repro-
ductive organs.
Radioactive isotopes are also used to
treatcancer. Some of the particles released
from isotopeshave a very high energy con-
tentand can penetrate and destroy tissues.
Thus radioactive isotopescan be used to
destroytumors because rapidly growing tis-
sues such astumors are more sensitive to
radiation than healthycells. Radiation can
also be used to sterilize materialsthat can-
notbe exposed to high temperatures (e.g.,
some fabric and plasticitems used during
surgical procedures). In addition, radioac-
tive emissionscan be used to sterilize food
and other items.
X rays are electromagnetic radiations
with a much shorter wavelength than visible
light. When electriccurrent is used to heata
filamentto very high temperatures, energy
of the electrons becomes so great that
some electronsare emitted from the hot fil-
ament. When these electronsstrike a posi-
tive electrode athigh speeds, they release
some oftheir energy in the form of x rays.
Xrays do not penetrate dense material
asreadily as they penetrate less dense ma-
terial, and xrays can expose photographic
film. Consequently, an x-raybeam can pass
through a person and onto photographic
film. Dense tissuesof the body absorb the
xrays, and in these areas the film is underex-
posed and so appearswhite or light in color
on the developed film. On the other hand,
the x rays readily passthrough less dense
tissue, and the film in these areasis overex-
posed and appearsblack or darkin color. In
an x-ray film of the skeletal system the
dense bonesare white, and the less dense
softtissues are dark, often so dark that no
detailscan be seen. Because the dense bone
materialis clearly visible, xrays can be used
to determine whether bones are broken or
have other abnormalities.
Soft tissues can be photographed by
using low-energyx rays. Mammograms are
low-energyx rays of the breast that can be
used to detecttumors, because tumors are
slightlydenser than normal tissue.
Radiopaque substancesare dense ma-
terials that absorb xrays. If a radiopaque
liquid is given to a patient, the liquid as-
sumesthe shape of the organ into which itis
placed. For example, ifa barium solution is
swallowed, the outline of the upper diges-
tive tractcan be photographed using x rays
to detectsuch abnormalities as ulcers.
Part1 Organization of the Human Body32
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
I. Organization of the
Human Body
2. The Chemical Basis of
Life
© The McGraw−Hill
Companies, 2004
Chapter 2 The ChemicalBasis of Life 33
polar molecule.When hydrogen forms a covalent bond with oxy-
gen,nitrogen, or fluorine, the resulting molecule becomes very po-
larized. If the positively charged hydrogen of one molecule is
attracted to the negatively charged oxygen,nitrogen, or fluorine of
another molecule, a hydrogen bond is formed. For example, the
positively charged hydrogen atoms ofa water molecule form hy-
drogen bonds with the negatively charged oxygen atoms ofother
water molecules (figure 2.7).
Hydrogen bonds play an important role in determining the
shape of complex molecules because the hydrogen bonds between
different polar parts of the molecule hold the molecule in its nor-
mal three-dimensional shape (see the sections “Proteins”and “Nu-
cleic Acids:DNA and RNA”later in this chapter).
Table 2.4 summarizes the important characteristics ofchem-
ical bonding (ionic and covalent) and intermolecular forces
(hydrogen bonds).
Solubilityand Dissociation
Solubilityis the ability of one substance to dissolve in another,for
example,when sugar dissolves in water. Charged substances such
as sodium chloride,and polar substances such as glucose, dissolve
Table 2.3
Representation Hydrogen Carbon Dioxide Glucose
Picturing Molecules
Chemical Formula
Shows the kind H
2
CO
2
C
6
H
12
O
6
and number of
atoms present.
Electron-Dot Formula
The bonding H.
.
HO.
.
.
.
C.
.
.
.
O Not used for
electrons are complex molecules
shown as dots Single covalent bond Double covalent bond
between the symbols
of the atoms.
Bond-Line Formula
The bonding HOHOPCPO
electrons are
shown as lines Single covalent bond Double covalent bond
between the
symbols of the atoms.
Models
Atoms are shown
asdifferent-sized
and different-colored spheres.
OH
HO
OH
OH
CH
2
OH
O
Hydrogen
atom
Carbon
atom
Oxygen
atom
Hydrogen bond
Hydrogen
Oxygen
Water molecule
Figure 2.7
Hydrogen Bonds
The positive hydrogen partof one water molecule forms a hydrogen bond (red
dotted line) with the negative oxygen partof another water molecule. As a
result, hydrogen bondshold the water molecules together.
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in water readily,whereas nonpolar substances such as oils do not.
We all have seen how oil floats on water.Substances dissolve in wa-
ter when they become surrounded by water molecules.If the posi-
tive and negative ends ofthe water molecules are attracted more to
the charged ends ofother molecules than they are to each other, the
hydrogen bonds between the ends ofthe water molecules are bro-
ken,and the water molecules surround the other molecules, which
become dissolved in the water.
When ionic compounds dissolve in water,their ions dissoci-
ate,or separate, from one another because the cations are attracted
to the negative ends ofthe water molecules, and the anions are at-
tracted to the positive ends ofthe water molecules. When sodium
chloride dissociates in water,the sodium and chloride ions sepa-
rate, and water molecules surround and isolate the ions,thereby
keeping them in solution (figure 2.8).
When molecules (covalent compounds) dissolve in water,
they usually remain intact even though they are surrounded by wa-
ter molecules. Thus,in a glucose solution, glucose molecules are
surrounded by water molecules.
Cations and anions that dissociate in water are sometimes
called electrolytes (e¯-lektro¯-lı¯tz) because they have the capacity
to conduct an electric current,which is the flow of charged parti-
cles. An electrocardiogram (ECG) is a recording of electric cur-
rents produced by the heart. These currents can be detected by
electrodes on the surface ofthe body because the ions in the body
fluids conduct electric currents. Molecules that do not dissociate
form solutions that do not conduct electricity and are called
nonelectrolytes.
9. Define hydrogen bond, and explain how hydrogen bonds
hold polarmolecules, such as water, together. Howdo
hydrogen bondsaffect the shape of a molecule?
Part1 Organization of the Human Body34
Table 2.4
Definition Charge Distribution Example
Ionic Bond
Complete transfer of electrons Separate positively charged and Na
Cl
between two atoms negatively charged ions Sodium chloride
Polar Covalent Bond
Unequal sharing of electrons Slight positive charge (
) on one side
between two atoms of the molecule and slight negative
charge (
) on the other side of
the molecule Water
Nonpolar Covalent Bond
Equal sharing of electrons Charge evenly distributed among the
between two atoms atoms of the molecule
Methane
Hydrogen Bond
Attraction of oppositely charged Charge distribution within the polar
ends of one polar molecule molecules results from polar
to another polar molecule covalent bonds
Water molecules
Comparison of Bonds
H
HOCOH
H
OO
H
O
H
O
O
H
O
.....
HOO
H H
O
O
O
10. Define solubility. How do ionic and covalent compounds
typicallydissolve in water?
11. Distinguish between electrolytes and nonelectrolytes.
Chemical Reactionsand Energy
Objectives
■ Describe and give examples of the types of chemical
reactionsoccurring in the body.
■ Define potential and kinetic energy. Describe mechanical,
chemical, and heatenergy as they relate to the human
body.
■ List the factors that affect the speed of a chemical reaction.
In a chemical reaction,atoms, ions, molecules, or compounds in-
teract either to form or to break chemical bonds.The substances
that enter into a chemical reaction are called the reactants,and the
substances that result from the chemical reaction are called
theproducts.
For our purposes,three important points can be made about
chemical reactions.First, in some reactions, less complex reactants
are combined to form a larger,more complex product.An example
is the synthesis ofthe complex molecules of the human body from
basic “building blocks”obtained in food (figure 2.9a). Second, in
other reactions, a reactant can be broken down,or decomposed,
into simpler,less complex products. An example is the breakdown
of food molecules into basic building blocks.(figure 2.9b). Third,
atoms are generally associated with other atoms through chemical
bonding or intermolecular forces; therefore, to synthesize new
products or break down reactants it is necessary to change the rela-
tionship between atoms.
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Chapter 2 The ChemicalBasis of Life 35
SynthesisReactions
When two or more reactants chemically combine to form a new
and larger product,the process is called a synthesis reaction. An
example of a synthesis reaction is the combination of two amino
acids to form a dipeptide (figure 2.10a).In this particular synthesis
reaction,water is removed from the amino acids as they are bound
together.Synthesis reactions in which water is a product are called
dehydration(water out) reactions. Note that old chemical bonds
are broken and new chemical bonds are formed as the atoms re-
arrange as a result ofa synthesis reaction.
Another example of a synthesis reaction in the body is the
formation of adenosine triphosphate (ATP).In ATP,A stands for
adenosine, T stands for tri- or three, and P stands for phosphate
group (PO
4
3
).Thus, ATP consists of adenosine and three phos-
phate groups (see p.53 for the details of the structure of ATP).ATP
is synthesized from adenosine diphosphate (ADP),which has two
phosphate groups,and an inorganic phosphate (H
2
PO
4
),which
is often symbolized as P
i
.
A-P-P P
i
n A-P-P-P
(ADP) (Inorganic (ATP)
phosphate)
Salt crystal
Salt
Water
molecules
Na
+
Cl
–
Cl
–
Na
+
Figure 2.8
Dissociation
Sodium chloride (table salt) dissociating in water. The positivelycharged sodium ions(Na
) are attracted to the negative oxygen (red) end ofthe water molecule,
and the negativelycharged chlorine ions(Cl
) are attracted to the positivelycharged hydrogen (blue) end of the water molecule.
Synthesis
reaction
Amino acids
(a)
(b)
Protein molecule
Decomposition
reaction
Carbohydrate molecule
Glucose molecules
Figure 2.9
Synthesisand Decomposition Reactions
(a) Synthesisreaction in which amino acids, the basic“building blocks” of
proteins, combine to form a protein molecule. (b) Decomposition reaction in
which a complexcarbohydrate breaks down into smaller glucose molecules,
which are the “building blocks” ofcarbohydrates.
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Synthesis reactions produce the molecules characteristic oflife,
such as ATP,proteins,carbohydrates, lipids, and nucleic acids. All of
the synthesis reactions that occur within the body are referred to col-
lectively as anabolism(a˘-nabo¯ -lizm). The growth, maintenance, and
repair ofthe body could not take place without anabolic reactions.
Decomposition Reactions
The term decompose means to break down into smaller parts.A
decomposition reactionis the reverse of a synthesis reaction—
a larger reactant is chemically broken down into two or more
smaller products. The breakdown of a disaccharide (a type of
carbohydrate) into glucose molecules (figure 2.10b) is an exam-
ple. Note that this particular reaction requires that water be
split into two parts and that each part be contributed to one
of the new glucose molecules. Reactions that use water in this
manner are called hydrolysis (hı¯-droli-sis;water dissolution)
reactions.
The breakdown ofATP to ADP and an inorganic phosphate
is another example ofa decomposition reaction.
A-P-P-P n A-P-P P
i
(ATP) (ADP) (Inorganic
phosphate)
The decomposition reactions that occur in the body are col-
lectively called catabolism(ka˘-tab-o¯-lizm).They include the di-
gestion of food molecules in the intestine and within cells, the
breakdown offat stores, and the breakdown of foreign matter and
microorganisms in certain blood cells that function to protect the
Part1 Organization of the Human Body36
body.All of the anabolic and catabolic reactions in the body are
collectively defined as metabolism.
Reversible Reactions
Areversible reaction is a chemical reaction in which the reaction
can proceed from reactants to products or from products to reac-
tants.When the rate of product formation is equal to the rate of the
reverse reaction,the reaction system is said to be at equilibrium.
At equilibrium the amount of reactants relative to the amount of
products remains constant.
The following analogy may help to clarify the concept ofre-
versible reactions and equilibrium. Imagine a trough containing
water.The trough is divided into two compartments by a partition,
but the partition contains holes that allow water to move freely be-
tween the compartments.Because water can move in either direc-
tion, this is like a reversible reaction. Let the water in the left
compartment be the reactant and the water in the right compart-
ment be the product.At equilibrium, the amount of reactant rela-
tive to the amount of product in each compartment is always the
same because the partition allows water to pass between the two
compartments until the level ofwater is the same in both compart-
ments.If additional water is added to the reactant compartment,
water flows from it through the partition to the product compart-
ment until the level of water is the same in both compartments.
Likewise, if additional reactants are added to a reaction system,
some will form product until equilibrium is reestablished.Unlike
this analogy,however,the amount of the reactants compared to the
amount ofproducts of most reversible reactions is not one to one.
C
H
Amino acid
Synthesis (dehydration) reaction
Decomposition (hydrolysis) reaction
Amino acid Dipeptide Water (H
2
O)
Disaccharide
Glucose Glucose
Water (H
2
O)
NC
O
OHH
H
R
1
H++OH
O
OH
HO
OH
CH
2
OH
O
OH
OH
CH
2
OH
C
H
NC
O
OHH++
H
R
2
C
H
NC
O
H
H
R
1
C
H
NCOH
O
R
2
HOH
OO
OH
OH
HO
OH
CH
2
OH
O
OH
OH
HO
OH
CH
2
OH
O
Figure 2.10
Synthesis(Dehydration) and Decomposition (Hydrolysis) Reactions
(a) Synthesisreaction in which two amino acidscombine to form a dipeptide. This reaction is also a dehydration reaction because it results in the removal of a
water molecule from the amino acids. (b) Decomposition reaction in which a disaccharide breaksapartto form glucose molecules. This reaction is also a hydrolysis
reaction because itinvolves the splitting of a water molecule.
(a)
(b)
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Chapter 2 The ChemicalBasis of Life 37
Depending on the specific reversible reaction,one part reactant to
two parts product,two parts reactant to one part product, or many
other possibilities can occur.
An important reversible reaction in the human body involves
carbon dioxide and hydrogen ions.The reaction between carbon
dioxide (CO
2
) and water (H
2
O) to form carbonic acid (H
2
CO
3
) is
reversible.Carbonic acid then separates by a reversible reaction to
form hydrogen ions (H
) and bicarbonate ions (HCO
3
).
CO
2
H
2
O H
2
CO
3
H
HCO
3
If CO
2
is added to H
2
O,additional H
2
CO
3
forms,which, in
turn,causes more H
and HCO
3
to form.The amount of H
and
HCO
3
relative to CO
2
therefore remains constant.Maintaining a
constant level of H
is necessary for proper functioning of the
nervous system.This can be achieved, in part, by regulating blood
CO
2
levels.For example, slowing down the respiration rate causes
blood carbon dioxide levels to increase.
PREDICT
Ifthe respiration rate increases, CO
2
iseliminated from the blood.
Whateffect does this change have on blood H
ⴙ
ion levels?
Oxidation–Reduction Reactions
Chemical reactions that result from the exchange ofelectrons be-
tween the reactants are called oxidation–reduction reactions.
When sodium and chlorine react to form sodium chloride, the
sodium atom loses an electron, and the chlorine atom gains an
electron.The loss of an electron by an atom is called oxidation,
and the gain ofan electron is called reduction. The transfer of the
electron can be complete,resulting in an ionic bond, or it can be a
partial transfer,resulting in a covalent bond.Because the complete
or partial loss of an electron by one atom is accompanied by the
gain of that electron by another atom, these reactions are called
oxidation–reduction reactions. Synthesis and decomposition
reactions can be oxidation–reduction reactions.Thus, it is possi-
ble for a chemical reaction to be described in more than one way.
12. Define a chemical reaction and compare synthesis and
decomposition reactions. Howdo anabolism, catabolism,
and metabolism relate to synthesisand decomposition
reactions?
13. Describe a dehydration and a hydrolysis reaction.
14. Describe reversible reactions. What is meant by the
equilibrium condition in reversible reactions?
15. What is an oxidation–reduction reaction?
PREDICT
When hydrogen gascombines with oxygen gasto form water, is the
hydrogen reduced or oxidized? Explain.
Energy
Energy,unlike matter, does not occupy space, and it has no mass.
Energyis defined as the capacity to do work , that is, to move mat-
ter.Energy can be subdivided into potential energy and kinetic en-
ergy.Potential energy is stored energy that could do work but is
n
m
n
m
not doing so.Kinetic (ki-netik) energy is the form of energy that
actually does work and moves matter.A ball held at arm’s length
above the floor has potential energy.No energy is expended as long
as the ball does not move.If the ball is released and falls toward the
floor,however,it has kinetic energ y.
According to the conservation ofenergy principle, energy is
neither created nor destroyed. Potential energy,however, can be
converted into kinetic energy,and kinetic energy can be converted
into potential energy.For example, the potential energy in the ball
is converted into kinetic energy as the ball falls toward the floor.
Conversely,the kinetic energy required to raise the ball from the
floor is converted into potential energy.
Potential and kinetic energy can be found in many different
forms.Mechanical energy is energy resulting from the position or
movement of objects. Many of the activities of the human body,
such as moving a limb,breathing, or circulating blood involve me-
chanical energy.Other forms of energ y are chemical energy,heat
energy,electric energy, and electromagnetic (radiant) energy.
Chemical Energy
Thechemical energy of a substance is a form ofstored (potential)
energy within its chemical bonds.In any given chemical reaction,
the potential energy contained in the chemical bonds of the reac-
tants can be compared to the potential energy in the chemical
bonds of the products. If the potential energy in the chemical
bonds ofthe reactants is less than that of the products, then energy
must be supplied for the reaction to occur.For example, the syn-
thesis ofATP from ADP.
ADP H
2
PO
4
Energy n ATP H
2
O
(Less potential (More potential
energy in reactants) energy in products)
For simplicity,the H
2
O is often not shown in this reaction,
and P
i
is used to represent inorganic phosphate (H
2
PO
4
).For this
reaction to occur,bonds in H
2
PO
4
are broken and bonds are
formed in ATP and H
2
O.As a result of the breaking of existing
bonds, the formation of new bonds, and the input of energy,
these products have more potential energy than the reactants
(figure2.11a).
Ifthe potential energy in the chemical bonds of the reactants
is greater than that ofthe products, energy is released by the reac-
tion.For example, the chemical bonds of food molecules contain
more potential energy than the waste products that are produced
when food molecules are decomposed.The energy released from
the chemical bonds of food molecules is used by living systems to
synthesize ATP.Once ATP is produced,the breakdown of ATP to
ADP results in the release ofenergy.
ATP H
2
O n ADP H
2
PO
4
Energy
(More potential (Less potential
energy in reactants) energy in products)
For this reaction to occur,the bonds in ATP and H
2
O are
broken and bonds in H
2
PO
4
are formed.As a result of breaking
the existing bonds and forming new bonds, these products have
less potential energy than the reactants, and energy is released
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(figure 2.11b).Note that the energy released does not come from
breaking the phosphate bond ofATP,because breaking a chemical
bond requires the input ofenergy. It is commonly stated, however,
that the breakdown ofATP results in the release of energy, which
is true when the overall reaction is considered. The energy re-
leased when ATP is broken down can be used in the synthesis of
other molecules; to do work, such as muscle contraction; or to
produce heat.
HeatEnergy
Heat is the energy that flows between objects that are at different
temperatures. For example,when you touch someone who has a
fever,you can feel the increased heat from the person’s body.Tem-
perature is a measure of how hot or cold a substance is relative to
another substance.Heat is always transferred from a hotter object
to a cooler object,such as from a hot stove top to a finger.
All other forms of energy can be converted into heat energy.
For example,when a moving object comes to rest,its kinetic energy
is converted into heat energy by friction.Some of the potential en-
ergy of chemical bonds is released as heat energy during chemical
reactions.The body temperature of humans is maintained by heat
produced in this fashion.
16. How is energy different from matter? How are potential and
kineticenergy different from each other?
17. Define mechanical energy, chemical energy, and heat
energy. Howis chemical energy converted to mechanical
energyand heat energy in the body?
Part1 Organization of the Human Body38
18. Use ATP and ADP to illustrate the release or input of energy
in chemical reactions.
PREDICT
Energyfrom the breakdown of ATP providesthe kinetic energy for
muscle movement. Whydoes body temperature increase during
exercise?
Speed of Chemical Reactions
Molecules are constantly in motion and therefore have kinetic en-
ergy.A chemical reaction occurs only when molecules with suffi-
cient kinetic energy collide with each other.As two molecules move
closer together,the negatively charged electron cloud of one mole-
cule repels the negatively charged electron cloud ofthe other mol-
ecule. If the molecules have sufficient kinetic energy, they
overcome this repulsion and come together.The nuclei in some
atoms attract the electrons of other atoms,resulting in the break-
ing and formation of new chemical bonds.The activation energy
is the minimum energy that the reactants must have to start a
chemical reaction (figure 2.12a).Even reactions that result in a re-
lease ofenergy must overcome the activation energy barrier for the
reaction to proceed.For example, heat in the form of a spark is re-
quired to start the reaction between oxygen and gasoline vapor.
Once some oxygen molecules react with gasoline,the energ y re-
leased can start additional reactions.
Given any population ofmolecules, some of them have more
kinetic energy and move about faster than others.Even so, at nor-
mal body temperatures,most of the chemical reactions necessary
P
ATP
P
PRODUCTS
ADP
P
ATP
P
P
P
P
REACTANTS
ADP
P
P
ATP
ATP
P
P
i
P
REACTANT
PRODUCT
Energy
released
Energy
input
Less potential
energy
More potential
energy
Less potential
energy
More potential
energy
i
ADP + P
i
+ Energy
ADP + P
i
+ Energy
Figure 2.11
Energyand Chemical Reactions
In each figure the upper shelfrepresents a higher energy level, and the lower shelfrepresents a lower energy level. (a) Reaction in which the input of energy is
required for the synthesisof ATP. (b) Reaction in which energyis released as a result ofthe breakdown of ATP.
(a)
(b)
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Chapter 2 The ChemicalBasis of Life 39
for life proceed too slowly to support life because few molecules
have enough energy to start a chemical reaction. Catalysts
(kata˘-listz) are substances that increase the rate ofchemical reac-
tions without being permanently changed or depleted. Enzymes
(enzı¯mz),which are discussed in greater detail on p. 49, are pro-
tein catalysts.Enzy mes increase the rate of chemical reactions by
lowering the activation energy necessary for the reaction to begin
(figure 2.12b).As a result, more molecules have sufficient energy to
undergo chemical reactions.With an enzyme, the rate of a chemi-
cal reaction can take place more than a million times faster than
without the enzyme.
Temperature can also affect the speed ofchemical reactions.
As temperature increases, reactants have more kinetic energy,
move at faster speeds, and collide with one another more fre-
quently and with greater force,thereby increasing the likelihood of
a chemical reaction.When a person has a fever of only a few de-
grees,reactions occur throughout the body at an accelerated rate,
resulting in increased activity in the organ systems such as in-
creased heart and respiratory rates.When body temperature drops,
various metabolic processes slow.In cold weather, the fingers are
less agile largely because of the reduced rate of chemical reactions
in cold muscle tissue.
Within limits,the greater the concentration of the reactants,
the greater the rate at which a given chemical reaction proceeds.
Effect of
enzyme
ATP
ATP
ADP + P + Energy
Enzyme
i
ATP
ADP + P + Energy
i
ADP
Activation
energy
with
enzyme
Activation
energy
ATP
ADP
Less potential
energy
Less potential
energy
More
potential
energy
More
potential
energy
P
P
P
P
i
P
P
P
i
P
P
P
P
P
Figure 2.12
Activation Energyand Enzymes
(a) Activation energyis needed to change ATP to ADP. The upper shelfrepresents a higher energy level, and the lower shelf represents a lower energy level. The
“wall” extending above the upper shelfrepresents the activation energy. Even though energy isgiven up moving from the upper to the lower shelf, the activation
energy“wall” must be overcome before the reaction can proceed. (b) The enzyme lowersthe activation energy, making it easier for the reaction to proceed.
This occurs because, as the concentration of reactants increases,
they are more likely to come into contact with one another.For ex-
ample, the normal concentration of oxygen inside cells enables
oxygen to come into contact with other molecules and produce the
chemical reactions necessary for life. If the oxygen concentration
decreases, the rate of chemical reactions decreases.This decrease
can impair cell function and even result in death.
19. Define activation energy, catalysts, and enzymes. How do
enzymesincrease the rate of chemical reactions?
20. What effect does increasing temperature or increasing
concentration of the reactantshave on the rate of a
chemical reaction?
Inorganic Chemistry
Objectives
■ Describe the properties of water that make it important for
living organisms.
■ Discuss mixtures.
■ Define acids, bases, salts, and buffers, and describe the pH
scale.
■ Explain the importance of oxygen and carbon dioxide to
living organisms.
(a)
(b)
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It was once believed that inorganic substances were those that came
from nonliving sources and organic substances were those extracted
from living organisms.As the science of chemistry developed, how-
ever,it became apparent that organic substances could be manufac-
tured in the laboratory.As defined currently, inorganic chemistry
generally deals with those substances that do not contain carbon,
whereas organic chemistry is the study of carbon-containing sub-
stances.These definitions have a few exceptions. For example, car-
bon monoxide (CO),car bon dioxide (CO
2
), and bicarbonate ion
(HCO
3
) are classified as inorganic molecules.
Water
A molecule ofwater is composed ofone atom of oxygen joined to
two atoms ofhydrogen by covalent bonds.Water molecules are po-
lar, with a partial positive charge associated with the hydrogen
atoms and a partial negative charge associated with the oxygen
atom. Hydrogen bonds form between the positively charged hy-
drogen atoms of one water molecule and the negatively charged
oxygen atoms of another water molecule. These hydrogen bonds
organize the water molecules into a lattice that holds the water
molecules together (see figures 2.6 and 2.7).
Water accounts for approximately 50% ofthe weight of a
young adult female and 60% ofa young adult male. Females have a
lower percentage of water than males because they typically have
more body fat,which is relatively free of water. Plasma, the liquid
portion of blood, is 92% water.Water has physical and chemical
properties well suited for its many functions in living organisms.
These properties are outlined in the following sections.
Stabilizing BodyTemperature
Water has a high specific heat, meaning that a relatively large
amount of heat is required to raise its temperature; therefore, it
tends to resist large temperature fluctuations.When water evapo-
rates,it changes from a liquid to a gas, and because heat is required
for that process,the evaporation of water from the surface of the
body rids the body ofexcess heat.
Protection
Water is an effective lubricant that provides protection against dam-
age resulting from friction.For example, tears protect the surface of
the eye from the rubbing of the eyelids. Water also forms a fluid
cushion around organs that helps to protect them from trauma.The
cerebrospinal fluid that surrounds the brain is an example.
Chemical Reactions
Many ofthe chemical reactions necessary for life do not take place
unless the reacting molecules are dissolved in water.For example,
sodium chloride must dissociate in water into sodium and chloride
ions before they can react with other ions.Water also directly par-
ticipates in many chemical reactions.As previously mentioned, a
dehydration reaction is a synthesis reaction in which water is pro-
duced,and a hydrolysis reaction is a decomposition reaction that
requires a water molecule (see figure 2.10).
Part1 Organization of the Human Body40
Mixing Medium
Amixture is a combination of two or more substances physically
blended together,but not chemically combined. A solution is any
liquid,gas, or solid in which the substances are uniformly distrib-
uted with no clear boundary between the substances.For example,
a salt solution consists of salt dissolved in water,air is a solution
containing a variety of gases,and wax is a solid solution of several
fatty substances. Solutions are often described in terms of one
substance dissolving in another:the solute (solu¯t) dissolves in the
solvent.In a salt solution, water is the solvent and the dissolved salt
is the solute.Sweat is a salt solution in which sodium chloride and
other solutes are dissolved in water.
Asuspension is a mixture containing materials that separate
from each other unless they are continually,physically blended to-
gether.Blood is a suspension containing red blood cells suspended
in a liquid called plasma.As long as the red blood cells and plasma
are mixed together as they pass through blood vessels, the red
blood cells remain suspended in the plasma.If the blood is allowed
to sit in a container,however,the red blood cells and plasma sepa-
rate from each other.
Acolloid (koloyd) is a mixture in which a dispersed (solute-
like) substance is distributed throughout a dispersing (solventlike)
substance.The dispersed par ticles are larger than a simple mole-
cule but small enough that they remain dispersed and do not settle
out.Proteins, which are large molecules, and water form colloids.
For instance,the plasma portion of blood and the liquid interior of
cells are colloids containing many important proteins.
In living organisms the complex fluids inside and outside
cells consist ofsolutions, suspensions, and colloids. Blood is an ex-
ample ofall of these mixtures.It is a solution containing dissolved
nutrients such as sugar,a suspension holding red blood cells, and a
colloid containing proteins.
The ability of water to mix with other substances enables it
to act as a medium for transport,moving substances from one part
ofthe body to another. Body fluids such as plasma transport nutri-
ents, gases, waste products,and a variet y of molecules involved
with regulating body functions.
Solution Concentrations
The concentration of solute particles dissolved in solvents can be
expressed in several ways.One common way is to indicate the per-
cent ofsolute by weight per volume of solution. A 10% solution of
sodium chloride can be made by dissolving 10 g of sodium chlo-
ride into enough water to make 100 mL ofsolution.
Physiologists often determine concentrations in osmoles
(osmo¯lz),which express the number of particles in a solution. A
particle can be an atom,ion, or molecule.An osmole (osm) is 6.022
10
23
particles of a substance in 1 kilogram (kg) of water.Just as
a grocer sells eggs in lots of12 (a dozen), a chemist groups atoms in
lots of6.022 10
23
.Theosmolality(os-mo¯-lali-te¯) ofa solution
is a reflection ofthe number, not the type, of particles in a solution.
For example,a 1 osm glucose solution and a 1 osm sodium chlo-
ride solution both contain 6.022 10
23
particles per kg water.The
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Chapter 2 The ChemicalBasis of Life 41
glucose solution,however, has 6.022 10
23
molecules of glucose,
whereas the sodium chloride dissociates into 3.011 10
23
sodium
ions and 3.011 10
23
chloride ions.
Because the concentration of particles in body fluids is so
low,the measurement milliosmole (mOsm), 1/1000 of an osmole,
is used.Most body fluids have a concentration of about 300 mOsm
and contain many different ions and molecules.The concentration
of body fluids is important because it influences the movement of
water into or out of cells (see chapter 3). Appendix C contains
more information on calculating concentrations.
21. Define inorganic and organic chemistry.
22. List four functions that water performs in living organisms
and give an example of each.
23. Describe solutions, suspensions, and colloids, and give an
example of each. Define solventand solute.
24. How is the osmolality of a solution determined? What is a
milliosmole?
Acidsand Bases
Many molecules and compounds are classified as acids or bases.
For most purposes an acidis defined as a proton donor. Because a
hydrogen atom without its electron is a proton (H
), any sub-
stance that releases hydrogen ions is an acid.Hydrochloric acid
(HCl) forms hydrogen ions (H
) and chloride ions (Cl
) in solu-
tion and therefore is an acid.
HCln H
Cl
A base is defined as a proton acceptor,and any substance
that binds to (accepts) H
ions is a base.Many bases function as
proton acceptors by releasing hydroxide ions (OH
) when they
dissociate.The base sodium hydroxide (NaOH) dissociates to form
Na
and OH
ions.
NaOHn Na
OH
The OH
ions are proton acceptors that combine with H
ions to
form water.
OH
H
nH
2
O
Acids and bases are classified as strong or weak.Strong acids
or bases dissociate almost completely when dissolved in water.
Consequently,they release almost all of their hydrogen or hydrox-
ide ions. The more completely the acid or base dissociates,the
stronger it is. For example,HCl is a strong acid because it com-
pletely dissociates in water.
HCln H
Cl
Not freely reversible
Weak acids or bases only partially dissociate in water.Conse-
quently,they release only some of their H
or OH
ions.For ex-
ample,when acetic acid (CH
3
COOH) is dissolved in water,some
ofit dissociates, but some of it remains in the undissociated form.
An equilibrium is established between the ions and the undissoci-
ated weak acid.
0 Hydrochloric acid (HCl)
1 Stomach acid
2 Lemon juice
3 Vinegar, cola, beer
4 Tomatoes
5 Black coffee
6 Urine
7 Distilled water
8 Seawater
9 Baking soda
10 Great Salt Lake
11 Household ammonia
12 Soda ash
13 Oven cleaner
14 Sodium hydroxide (NaOH)
10
0
10
–1
10
–2
10
–3
10
–4
10
–5
10
–6
10
–7
10
–8
10
–9
10
–10
10
–11
10
–12
10
–13
10
–14
10
–14
10
–13
10
–12
10
–11
10
–10
10
–9
10
–8
10
–7
10
–6
10
–5
10
–4
10
–3
10
–2
10
–1
10
0
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
Saliva (6.5)
Neutral
Blood (7.4)
Concentration in moles/liter
[OH
–
][H
+
]
pH Examples
Increasing acidity
Increasing alkalinity (basicity)
Figure 2.13
The pH Scale
A pH of7 is considered neutral. Valuesless than 7 are acidic (the lower the
number, the more acidic). Valuesgreater than 7 are basic(the higher the
number, the more basic). Representative fluidsand their approximate pH
valuesare listed.
CH
3
COOH CH
3
COO
H
Freely reversible
For a given weak acid or base,the amount of the dissociated
ions relative to the weak acid or base is a constant.
The pH Scale
ThepH scale is a means of referring to the hydrogen ion concen-
tration in a solution (figure 2.13). Pure water is defined as a
neutral solution and has a pH of 7. A neutral solution has equal
concentrations of hydrogen and hydroxide ions.Solutions with a
pH less than 7 are acidic and have a greater concentration of
hydrogen ions than hydroxide ions.Alkaline (alka˘-lı¯n),or basic,
n
m
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Part1 Organization of the Human Body42
solutions have a pH greater than 7 and have fewer hydrogen ions
than hydroxide ions.
The symbol pH stands for power (p) of hydrogen ion (H
)
concentration. The power is a factor of 10, which means that a
change in the pH of a solution by 1 pH unit represents a 10-fold
change in the hydrogen ion concentration.For example, a solution
of pH 6 has a hydrogen ion concentration 10 times greater than a
solution ofpH 7 and 100 times greater than a solution of pH 8.As
the pH value becomes smaller,the solution has more hydrogen
ions and is more acidic,and as the pH value becomes larger,the so-
lution has fewer hydrogen ions and is more basic.Appendix D con-
siders pH in greater detail.
Acidosisand Alkalosis
The normalpH range for human blood is 7.35 to 7.45. Acidosis results if
blood pH dropsbelow 7.35, in which case the nervous system becomes
depressed, and the individualcan become disoriented and possibly
comatose. Alkalosisresults if blood pH rises above 7.45. Then the nervous
system becomesoverexcitable, and the individualcan be extremely
nervousor have convulsions. Both acidosis and alkalosiscan be fatal.
Salts
Asalt is a compound consisting of a cation other than a hydrogen
ion and an anion other than a hydroxide ion.Salts are formed by the
interaction ofan acid and a base in which the hydrogen ions of the
acid are replaced by the positive ions ofthe base. For example, in a
solution when hydrochloric acid (HCl) reacts with the base sodium
hydroxide (NaOH),the salt, sodium chloride (NaCl),is formed.
HCl NaOH n NaCl H
2
O
(Acid) (Base) (Salt) (Water)
Typically,when salts such as sodium chloride dissociate in wa-
ter,they form positively and negatively charged ions (see figure 2.8).
Buffers
The chemical behavior ofmany molecules changes as the pH of the
solution in which they are dissolved changes.For example, many
enzymes work best within narrow ranges ofpH. The survival of an
organism depends on its ability to regulate body fluid pH within a
narrow range. Deviations from the normal pH range for human
blood are life-threatening.
One way body fluid pH is regulated involves the action of
buffers,which resist changes in solution pH when either acids or
bases are added.A buffer is a solution of a conjugate acid–base
pair in which the acid component and the base component occur
in similar concentrations.A conjugate base is everything that re-
mains of an acid after the hydrogen ion (proton) is lost.A conju-
gate acid is formed when a hydrogen ion is transferred to the
conjugate base.Two substances related in this way are a conjugate
acid–base pair. For example, carbonic acid (H
2
CO
3
) and bicar-
bonate ion (HCO
3
),formed by the dissociation of carbonic acid,
are a conjugate acid–base pair.
H
2
CO
3
H
HCO
3
In the forward reaction,carbonic acid loses a hydrogen ion to
produce bicarbonate ion,which is a conjugate base. In the reverse
n
m
reaction,a hydrogen ion is transferred to the bicarbonate ion (con-
jugate base) to produce carbonic acid,which is a conjugate acid.
For a given condition,this reversible reaction results in an
equilibrium,in which the amounts of carbonic acid relative to the
amounts of hydrogen ion and bicarbonate ions remains constant.
The conjugate acid–base pair can resist changes in pH because of
this equilibrium.If an acid is added to a buffer, the hydrogen ions
from the added acid can combine with the base component ofthe
conjugate acid–base pair.As a result, the concentration of hydro-
gen ions does not increase as much as it would without this
reaction. If hydrogen ions are added to a carbonic acid solution,
many ofthe hydrogen ions combine with bicarbonate ions to form
carbonic acid.
On the other hand,if a base is added to a buffered solution,
the conjugate acid can release hydrogen ions to counteract the ef-
fects ofthe added base. For example, if hydroxide ions are added to
a carbonic acid solution,the hydroxide ions combine with hydro-
gen ions to form water.As the hydrogen ions are incorporated into
water,carbonic acid dissociates to form hydrogen and bicarbonate
ions, thereby maintaining the hydrogen ion concentration (pH)
within a normal range.
The greater the buffer concentration,the more effective it is
in resisting a change in pH, but buffers cannot entirely prevent
some change in the pH ofa solution. For example, when an acid is
added to a buffered solution,the pH decreases but not to the extent
it would have without the buffer.Several very important buffers are
found in living systems and include bicarbonate, phosphates,
amino acids,and proteins as components.
25. Define acid and base, and describe the pH scale. What is
the difference between a strong acid orbase and a weak
acid orbase?
26. Define acidosis and alkalosis, and describe the symptoms
of each.
27. What is a salt? What is a buffer, and why are buffers
importantto organisms?
PREDICT
Dihydrogen phosphate ion (H
2
PO
4
) and monohydrogen phosphate
ion (HPO
4
2
) form the phosphate buffer system.
H
2
PO
4
H
HPO
4
2
Identifythe conjugate acid and conjugate base in the phosphate
buffer system. Explain how theyfunction as a buffer when either
hydrogen or hydroxide ionsare added to the solution.
Oxygen
Oxygen(O
2
) is an inorganic molecule consisting oftwo oxygen atoms
bound together by a double covalent bond.About 21% of the gas in
the atmosphere is oxygen,and it is essential for most animals. Oxygen
is required by humans in the final step ofa series of reactions in which
energy is extracted from food molecules (see chapters 3 and 25).
Carbon Dioxide
Carbon dioxide (CO
2
) consists of one carbon atom bound by
double covalent bonds to two oxygen atoms.Carbon dioxide is
produced when organic molecules such as glucose are metabolized
n
m
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Chapter 2 The ChemicalBasis of Life 43
The monosaccharides most important to humans include both
five- and six-carbon sugars.Common six-carbon sugars, such as glu-
cose,fructose, and galactose,are isomers (ı¯so¯-merz),which are mol-
ecules that have the same number and types of atoms but differ in
their three-dimensional arrangement (figure 2.14).Glucose, or blood
sugar,is the major carbohydrate found in the blood and is a major nu-
trient for most cells of the body.Fructose and galactose are also im-
portant dietary nutrients. Important five-carbon sugars include
ribose and deoxyribose (see figure 2.24),which are components of ri-
bonucleic acid (RNA) and deoxyribonucleic acid (DNA),respectively.
Disaccharides
Disaccharides (dı¯-saka˘-rı¯dz; di- means two) are composed of
two simple sugars bound together through a dehydration reaction.
Glucose and fructose,for example, combine to form a disaccharide
calledsucrose (table sugar) plus a molecule of water (figure 2.15a).
Several disaccharides are important to humans,including sucrose,
lactose, and maltose.Lactose, or milk sugar, is glucose combined
with galactose;and maltose, or malt sugar,is two glucose molecules
joined together.
Polysaccharides
Polysaccharides (pol-e¯-saka˘-rı¯dz;poly- means many) consist of
many monosaccharides bound together to form long chains that are
either straight or branched.Glycogen, or animal starch,is a polysac-
charide composed ofmany glucose molecules (figure 2.15b). Because
glucose can be metabolized rapidly and the resulting energy can be
used by cells, glycogen is an important energy-storage molecule.A
substantial amount ofthe glucose that is metabolized to produce en-
ergy for muscle contraction during exercise is stored in the form of
glycogen in the cells ofthe liver and skeletal muscles.
Starch and cellulose are two important polysaccharides
found in plants,and both are composed of long chains of glucose.
Plants use starch as an energy storage molecule in the same way that
animals use glycogen,and cellulose is an important structural com-
ponent ofplant cell walls. When humans ingest plants,the starch can
be broken down and used as an energy source.Humans,however, do
not have the digestive enzymes necessary to break down cellulose.
The cellulose is eliminated in the feces,where it provides bulk.
Table 2.5
Role Example
Structure Ribose forms part of RNA and ATP molecules, and
deoxyribose forms part of DNA.
Energy Monosaccharides (glucose, fructose, galactose)
can be used as energy sources. Disaccharides
(sucrose, lactose, maltose) and polysaccharides
(starch, glycogen) must be broken down to
monosaccharides before they can be used for
energy. Glycogen is an important energy-storage
molecule in muscles and in the liver.
Bulk Cellulose forms bulk in the feces.
Role of Carbohydrates in the Body
within the cells of the body (see chapters 3 and 25). Much ofthe
energy stored in the covalent bonds of glucose is transferred to
other organic molecules when glucose is broken down,and carbon
dioxide is released.Once carbon dioxide is produced, it is elimi-
nated from the cell as a metabolic by-product,transferred to the
lungs by blood,and exhaled during respiration. If carbon dioxide is
allowed to accumulate within cells,it becomes toxic.
28. What are the functions of oxygen and carbon dioxide in
living systems?
Organic Chemistry
Objectives
■ Describe the building blocks and functions of
carbohydrates, lipids, proteins, and nucleicacids in
thebody.
■ Explain the function of ATP in the body.
The ability of carbon to form covalent bonds with other atoms
makes possible the formation of the large, diverse, complicated
molecules necessary for life. A series of carbon atoms bound to-
gether by covalent bonds constitutes the “backbone”of many large
molecules. Variation in the length ofthe carbon chains and the
combination of atoms bound to the carbon backbone allows for
the formation of a wide variety of molecules. For example, some
protein molecules have thousands ofcarbon atoms bound by co-
valent bonds to one another or to other atoms,such as nitrogen,
sulfur,hydrogen, and oxygen.
The four major groups oforganic molecules essential to living
organisms are carbohydrates,lipids,proteins, and nucleic acids. Each
ofthese groups has specific structural and functional characteristics.
Carbohydrates
Carbohydratesare composed primarily of carbon, hydrogen, and
oxygen atoms and range in size from small to very large.In most
carbohydrates,for each carbon atom there are approximately two
hydrogen atoms and one oxygen atom.Note that the ratio of hy-
drogen atoms to oxygen atoms is two to one,the same as in water.
They are called carbohydrates because carbon (carbo) atoms are
combined with the same atoms that form a water molecule (hy-
drated). The large number of oxygen atoms in carbohydrates
makes them relatively polar molecules.Consequently,the y are sol-
uble in polar solvents such as water.
Carbohydrates are important parts of other organic mole-
cules,and they can be broken down to provide the energy necessary
for life.Undigested carbohydrates also provide bulk in feces, which
helps to maintain the normal function and health of the digestive
tract.Table 2.5 summarizes the roles of carbohydrates in the body.
Monosaccharides
Large carbohydrates are composed ofnumerous, relatively simple
building blocks called monosaccharides (mon-o¯-saka˘-rı¯dz;the
prefix mono- means one;the term saccharide means sugar), or sim-
ple sugars. Monosaccharides commonly contain three carbons
(trioses), four carbons (tetroses), five carbons (pentoses),or six
carbons (hexoses).
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Lipids
Lipidsare a second major group of organic molecules common to
living systems.Like carbohydrates, they are composed principally
of carbon, hydrogen, and oxygen; but other elements, such as
phosphorus and nitrogen,are minor components of some lipids.
Lipids contain a lower ratio of oxygen to carbon than do
carbohydrates,which makes them less polar. Consequently, lipids
can be dissolved in nonpolar organic solvents,such as alcohol or
acetone,but they are relatively insoluble in water.
Lipids have many important functions in the body.They
provide protection and insulation,help to regulate many physio-
logic processes, and form plasma membranes.In addition, lipids
are a major energy storage molecule and can be broken down and
used as a source ofenergy. Table 2.6 summarizes the many roles of
lipids in the body.Several different kinds of molecules, such as fats,
phospholipids,steroids, and prostaglandins, are classified as lipids.
Fatsare a major type of lipid. Like carbohydrates, fats are in-
gested and broken down by hydrolysis reactions in cells to release
energy for use by those cells.Conversely,if intake exceeds need,ex-
cess chemical energy from any source can be stored in the body as
fat for later use as energy is needed.Fats also provide protection by
surrounding and padding organs,and under-the-skin fats act as an
insulator to prevent heat loss.
Part1 Organization of the Human Body44
H
H
C
O
Glucose
OH
C
H
OH
C
H
H
OH
C
H
OH
C
HO
H
C
H
C
O
Fructose
H
OH
C
H
OH
C
H
H
OH
C
H
OH
C
HO
H
C
OH
HO
OH
OH
CH
2
OH
O
H
H
C
O
Galactose
OH
C
C
H
H
OH
C
H
OH
C
HO
H
HO
H
C
OH
HO
OH
OH
CH
2
OH
O
CH
2
OH
CH
2
OH
OH
HO
HO
O
Structural isomer Stereoisomer
Figure 2.14
Monosaccharides
These monosaccharidesalmost always form a ring-shaped molecule. Theyare represented as linear models to more readily illustrate the relationships between the
atomsof the molecules. Fructose is a structural isomer of glucose because ithas identical chemical groups bonded in a different arrangement in the molecule
(indicated byred shading). Galactose is a stereoisomer ofglucose because it has exactly the same groups bonded to each carbon atom but located in a different
three-dimensionalorientation (indicated by yellowshading).
Table 2.6
Role Example
Role of Lipids in the Body
Protection Fat surrounds and pads organs.
Insulation Fat under the skin prevents heat loss. Myelin
surrounds nerve cells and electrically insulates
the cells from one another.
Regulation Steroid hormones regulate many physiologic
processes. For example, estrogen and
testosterone are sex hormones responsible for
many of the differences between males and
females. Prostaglandins help regulate tissue
inflammation and repair.
Vitamins Fat-soluble vitamins perform a variety of functions.
Vitamin A forms retinol, which is necessary for
seeing in the dark; active vitamin D promotes
calcium uptake by the small intestine; vitamin E
promotes wound healing; and vitamin K is
necessary for the synthesis of proteins
responsible for blood clotting.
Structure Phospholipids and cholesterol are important
components of plasma membranes.
Energy Lipids can be stored and broken down later for
energy; per unit of weight, they yield more
energy than carbohydrates or proteins.
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Chapter 2 The ChemicalBasis of Life 45
Trig ly c e r id e s (trı¯-gliser-ı¯dz) constitute 95% ofthe fats in
the human body.Triglycerides,which are sometimes called triacyl-
glycerols (tri-asil-gliser-olz),consist of two different types of
building blocks: one glycerol and three fatty acids. Glycerolis a
three-carbon molecule with a hydroxyl group attached to each car-
bon atom, and fatty acids consist of a straight chain of carbon
atoms with a carboxyl group attached at one end (figure 2.16).A
carboxyl(kar-boksil) group (OCOOH) consists of both an oxy-
gen atom and a hydroxyl group attached to a carbon atom.The
carboxyl group is responsible for the acidic nature ofthe molecule
because it releases hydrogen ions into solution.Glycerides can be
described according to the number and kinds of fatty acids that
combine with glycerol through dehydration reactions.Monoglyc-
erides have one fatty acid, diglycerides have two fatty acids,and
triglycerides have three fatty acids bound to glycerol.
Fatty acids differ from one another according to the length
and the degree ofsatur ation of their carbon chains. Most naturally
occurring fatty acids contain an even number ofcarbon atoms, with
14- to 18-carbon chains being the most common.A fatty acid is sat-
urated (figure 2.17) if it contains only single covalent bonds be-
tween the carbon atoms. Sources of saturated fats include beef,
pork,whole milk, cheese, butter,eggs, coconut oil, and palm oil.The
carbon chain is unsaturatedif it has one or more double covalent
bonds between carbon atoms.Because the double covalent bonds
CH
2
OH
H
2
O
OH
O
O
+
HO
HO
HOOH
OH
OH
Glucose
(a)
(b)
Fructose Sucrose
CH
2
OH
CH
2
OH
CH
2
OH
OH
O
O
HO
HO
O
OH
OH
CH
2
OH
CH
2
OH
O
O
OH
OH
CH
2
OH
O
C
OH
OH
O
O
Branch
O
OH
OH
CH
2
OH
O
OH
OH
CH
2
OH
O
O
OH
OH
CH
2
OH
O
O
O
OH
OH
CH
2
OH
O
O
OH
OH
CH
2
OH
O
O
Glycogen main chain
Figure 2.15
Disaccharide and Polysaccharide
(a) Formation ofsucrose, a disaccharide, bya dehydration reaction involving glucose and fructose (monosaccharides). (b) Glycogen is a polysaccharide formed by
combining manyglucose molecules. The photo shows glycogen granulesin a liver cell.
Nucleus
Glycogen
granules
LM 2000x
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can occur anywhere along the carbon chain,many types of unsatu-
rated fatty acids with an equal degree of unsaturation are possible.
Monounsaturatedfats, such as olive and peanut oils,have one dou-
ble covalent bond between carbon atoms. Polyunsaturated fats,
such as safflower,sunflower, corn, or fish oils, have two or more
double covalent bonds between carbon atoms.Unsaturated fats are
the best type of fats in the diet because unlike saturated fats they do
not contribute to the development ofcardiovascular disease.
Phospholipids are similar to triglycerides, except that one of
the fatty acids bound to the glycerol is replaced by a molecule contain-
ing phosphate and,usually,nitrogen (figure 2.18). They are polar at the
end ofthe molecule to which the phosphate is bound and nonpolar at
the other end.The polar end of the molecule is attracted to water, and
Part1 Organization of the Human Body46
the nonpolar end is repelled by water.Phospholipids are important
structural components ofplasma membranes (see chapter 3).
The eicosanoids (ı¯k¯o-s ˘a-noydz) are a group of important
chemicals derived from fatty acids.They include prostag landins
(prosta˘-glandinz), thromboxanes (thrombok-za¯nz), and
leukotrienes(loo-ko¯-trı¯e¯nz).Eicosanoids are made in most cells
and are important regulatory molecules. Among their numerous
effects is their role in the response of tissues to injuries.
Prostaglandins have been implicated in regulating the secretion of
some hormones,blood clotting, some reproductive functions, and
many other processes.Many of the therapeutic effects of aspirin
and other anti-inflammatory drugs result from their ability to in-
hibit prostaglandin synthesis.
Fatty acids
Triglyceride molecule
3 H
2
O
Enzymes
C–C–C–C–C–C–HH– C– O H HO–
O H H H H H
H
H
H H H H H
Glycerol
C–C–C–C–C–C–HH– C– O H HO–
O H H H H H
H H H H H
C–C–C–C–C–C–HH– C– O H HO–
O H H H H H
H H H H H
C–C–C–C–C–C–H
O H H H H H
H H H H H
C–C–C–C–C–C–H
O H H H H H
H H H H H
C–C–C–C–C–C–H
O H H H H H
H H H H H
H–C–O
H
H
H–C–O
H–C–O
Figure 2.16
Triglyceride
Production ofa triglyceride from one glycerol molecule and three fatty acids.
OHHHHHHH HH HH HHH
C—C—C—C—C—C—C—C—C
—
C—C—C
—
C—C—C
—
C—C—C
HHHHHHHH HH HH HH
——
——
—
—
——
——
——
——
——
—
—
——
—
—
——
—
—
—
—
—
—
HO— —H
Linolenic acid (unsaturated)
———
OHHHH
H
HHHH
HH
HHHH
C—C—C—C—C—C—C—C—C—C—C—C—C—C—C—C
HHHHHHHHHHHHHHH
HO—
—
—
——
——
——
——
——
——
——
——
——
——
——
——
——
——
——
Palmitic acid (saturated)
—H
Figure 2.17
FattyAcids
(a) Palmiticacid (saturated with no double bonds between the carbons). (b) Linolenicacid (unsaturated with three double bonds between the carbons).
(a)
(b)
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Chapter 2 The ChemicalBasis of Life 47
Steroidsdiffer in chemical structure from other lipid mole-
cules, but their solubility characteristics are similar.All steroid
molecules are composed ofcarbon atoms bound together into four
ringlike structures (figure 2.19). Important steroid molecules
include cholesterol, bile salts,estrogen, progesterone, and testos-
terone. Cholesterol is an important steroid because other mole-
cules are synthesized from it.For example,bile salts, which increase
fat absorption in the intestines,are derived from cholesterol, as are
the reproductive hormones estrogen, progesterone,and testos-
terone. In addition, cholesterol is an important component of
plasma membranes. Although high levels of cholesterol in the
blood increase the risk ofcardiovascular disease, a certain amount
ofcholesterol is vital for normal function.
Another class of lipids is the fat-soluble vitamins. Their
structures are not closely related to one another,but they are
nonpolar molecules essential for many normal functions of
thebody.
Polar (hydrophilic) region
(phosphate-
containing region)
Nitrogen
Phosphorus
Oxygen
Carbon
Hydrogen
Nonpolar (hydrophobic) region
(fatty acids)
Figure 2.18
Phospholipids
(a) Molecular modelof a phospholipid. (b) Simplified way in which phospholipids are often depicted.
Cholesterol
Estrogen (estradiol)
HO
CH
3
CH
3
CH
3
CH
3
CH
2
CH
2
CH
2
CH
CH
3
CH
HO
CH
3
OH
Bile salt (glycocholate)
HO
OH
OH
CH
3
CH
3
O
O
O
–
CH
2
CH
2
CH
2
CH
3
CH C CNH
Testosterone
O
CH
3
CH
3
OH
Figure 2.19
Steroids
Steroidsare four-ringed molecules that differ from one another according to the groupsattached to the rings. Cholesterol, the most common steroid, can be
modified to produce other steroids.
(a)
(b)
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Proteins
All proteins contain carbon, hydrogen, oxygen, and nitrogen
bound together by covalent bonds, and most proteins contain
some sulfur.In addition, some proteins contain small amounts of
phosphorus,iron, and iodine. The molecular mass of proteins can
be very large.For the purpose of comparison, the molecular mass
ofwater is approximately 18, sodium chloride 58, and glucose 180;
but the molecular mass of proteins ranges from approximately
1000 to several million.
Proteins regulate bodily processes,act as a transpor tation
system in the body,provide protection, help muscles contract,and
provide structure and energy.Table 2.7 summarizes the functions
ofproteins in the body.
Protein Structure
The basic building blocks for proteins are the 20 amino(a˘-me¯n¯o)
acid molecules. Each amino acid has an amine (a˘-me¯n) group
(ONH
2
), a carboxyl group (OCOOH),a hydrogen atom, and a
side chain designated by the symbol Rattached to the same carbon
atom.The side chain can be a variety of chemical structures, and
the differences in the side chains make the amino acids different
from one another (figure 2.20).
Covalent bonds formed between amino acid molecules during
protein synthesis are called peptide bonds(figure 2.21). A dipeptide
is two amino acids bound together by a peptide bond,a tripeptide is
three amino acids bound together by peptide bonds,and a polypep-
tide is many amino acids bound together by peptide bonds.Proteins
are polypeptides composed ofhundreds of amino acids.
Theprimary st ructure of a protein is determined by the se-
quence ofthe amino acids bound by peptide bonds (figure 2.22a).
Part1 Organization of the Human Body48
The potential number of different protein molecules is enormous
because 20 different amino acids exist and each amino acid can be
located at any position along a polypeptide chain.The characteris-
tics ofthe amino acids in a protein ultimately determine the three-
dimensional shape of the protein, and the shape of the protein
determines its function.A change in one, or a few, amino acids in
the primary structure can alter protein function, usually making
the protein less or even nonfunctional.
The secondary structure results from the folding or bend-
ing of the polypeptide chain caused by the hydrogen bonds be-
tween amino acids (figure 2.22b).Two common shapes that result
are helices or pleated sheets.If the hydrogen bonds that maintain
the shape ofthe protein are broken, the protein becomes nonfunc-
tional.This change in shape is called denaturation, and it can be
Table 2.7
Role Example
Role of Proteins in the Body
Regulation Enzymes control chemical reactions.
Hormones regulate many physiologic
processes; for example, insulin affects
glucose transport into cells.
Transport Hemoglobin transports oxygen and
carbon dioxide in the blood. Plasma
proteins transport many substances
in the blood. Proteins in plasma
membranes control the movement
of materials into and out of the cell.
Protection Antibodies and complement protect
against microorganisms and other
foreign substances.
Contraction Actin and myosin in muscle are
responsible for muscle contraction.
Structure Collagen fibersform a structural
framework in many parts of the
body. Keratin adds strength to skin,
hair, and nails.
Energy Proteins can be broken down for energy;
per unit of weight, they yield as
much energy as carbohydrates.
H
2
N
C
R
H
Amine
group
The general structure of an amino acid
showing the amine group ( NH
2
),
carboxyl group ( COOH), and hydrogen
atom highlighted in yellow. The R side
chain is the part of an amino acid that
makes it different from other amino acids.
Glycine is the simplest amino acid. The
side chain is a hydrogen atom.
Tyrosine, which has a more complicated
side chain, is an important
component of thyroid hormones.
Improper metabolism of
phenylalanine in the genetic disease
phenylketonuria (PKU) can cause
mental retardation.
Aspartic acid combined with
phenylalanine forms the artificial
sweetener aspartame (Nutrasweet
TM
and
Equal
TM
).
O
C OH
H
2
N
C
H
H
Glycine
Tyrosine
O
C OH
H
2
N
CH
2
C
H
OH
O
C OH
Carboxyl
group
Phenylalanine
H
2
N
CH
2
C
H
O
C OH
Aspartic
acid
H
2
N
CH
2
C
C
H
O
O
C OH
OH
Figure 2.20
Amino Acids
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Chapter 2 The ChemicalBasis of Life 49
C
N
CH
H
R
1
O
OH
H
C
N
CH
H
R
1
O
H
C
OH
N
CH
R
3
O
H
C
N
CH
H
R
3
O
OH
H
O
N
C
C
H
H
R
2
2 H
2
O
H
HO
O
N
C
C
H
R
2
H
Figure 2.21
Peptide Bonds
Dehydration reaction between three amino acids(left) to form a tripeptide
(right). One water molecule (H
2
O) isgiven off for each peptide bond formed.
caused by abnormally high temperatures or changes in the pH of
body fluids.An everyday example of denaturation is the change in
the proteins ofegg whites when they are cooked.
Thetertiary str ucture results from the folding of the helices
or pleated sheets (figure 2.22c).Some amino acids are quite polar
and therefore form hydrogen bonds with water.The polar portions
of proteins tend to remain unfolded, maximizing their contact
with water,whereas the less polar regions tend to fold into a glob-
ular shape,minimizing their contact with water. The formation of
covalent bonds between sulfur atoms ofone amino acid and sulfur
atoms in another amino acid located at a different place in the se-
quence ofamino acids can also contribute to the tertiary str ucture
of proteins. The tertiary structure determines the shape of a do-
main,which is a folded sequence of 100–200 amino acids within a
protein.The functions of proteins occur at one or more domains.
Therefore,changes in the primary or secondary st ructure that af-
fect the shape ofthe domain can change protein function.
If two or more proteins associate to form a functional unit,
the individual proteins are called subunits. The quaternary
structurerefers to the spatial relationships between the individual
subunits (figure 2.22d).
Enzymes
Proteins perform many roles in the body,including acting as en-
zymes. An enzyme is a protein catalyst that increases the rate at
which a chemical reaction proceeds without the enzyme being per-
manently changed. The three-dimensional shape of enzymes is
critical for their normal function because it determines the struc-
ture of the enzyme’s active site. According to the lock-and-key
model of enzyme action, a reaction occurs when the reactants
(key) bind to the active site (lock) on the enzyme.This view of en-
zymes and reactants as rigid structures fitting together has been
modified by the induced fit model,in which the enzyme is able to
slightly change shape and better fit the reactants. The enzyme is
like a glove that does not achieve its functional shape until the
hand (reactants) moves into place.
At the active site,reactants are brought into close proximity
(figure 2.23).After the reactants combine, they are released from
the active site,and the enzyme is capable of catalyzing additional
reactions.The activation energy required for a chemical reaction to
occur is lowered by enzymes (see figure 2.12) because they orient
the reactants toward each other in such a way that it is more likely
a chemical reaction will occur.
Slight changes in the structure of an enzyme can destroy the
ability of the active site to function.Enzymes are very sensitive to
changes in temperature or pH, which can break the hydrogen
bonds within them. As a result,the relationship between amino
acids changes,thereby producing a change in shape that prevents
the enzyme from functioning normally.
To be functional,some enzymes require additional, nonpro-
tein substances called cofactors.The cofactor can be an ion, such
as magnesium or zinc,or an organic molecule. Cofactors that are
organic molecules,such as certain vitamins, may be referred to as
coenzymes. Cofactors normally form part of the enzyme’s active
site and are required to make the enzyme functional.
Enzymes are highly specific because their active site can
bind only to certain reactants. Each enzyme catalyzes a specific
chemical reaction and no others. Many different enzymes are
therefore needed to catalyze the many chemical reactions ofthe
body.Enzymes often are named by adding the suffix -ase to the
name of the molecules on which they act. For example, an en-
zyme that catalyzes the breakdown of lipids is a lipase (lipa¯s,
lı¯pa¯s),and an enzyme that breaks down proteins is called a pro-
tease(pr o¯te¯-a¯s).
Enzymes control the rate at which most chemical reactions
proceed in living systems.Consequently,they control essentially all
cellular activities. At the same time, the activity of enzymes
themselves is regulated by several mechanisms that exist within the
cells.Some mechanisms control the enzyme concentration by in-
fluencing the rate at which the enzymes are synthesized,and others
alter the activity of existing enzymes. Much of what is known
about the regulation ofcellular activity involves knowledge of how
enzyme activity is controlled.
Nucleic Acids: DNA and RNA
Deoxyribonucleic (de¯-okse¯-rı¯bo¯-noo-kle¯ik)acid (DNA) is
the genetic material of cells, and copies of DNA are transferred
from one generation of cells to the next generation. DNA con-
tains the information that determines the structure of proteins.
Ribonucleic(rı¯bo¯-noo-kle¯ik)acid (RNA) is structurally related
to DNA,and three types of RNA also play important roles in pro-
tein synthesis.In chapter 3 the means by which DNA and RNA di-
rect the functions ofthe cell are described.
The nucleic (noo-kle¯ik,noo-kla¯ik)acids are large mole-
cules composed ofcar bon,hydrogen, oxygen, nitrogen, and phos-
phorus.Both DNA and RNA consist of basic building blocks called
nucleotides (nookle¯-o¯-tı¯dz).Each nucleotide is composed of a
monosaccharide to which a nitrogenous organic base and a
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Part1 Organization of the Human Body50
C
C
N
N
C
N
NH
H
HOC
CC
O
O
CC
C
H OC
C
C
N
C
NHO
C
C
H OC
C
N
N
N
NH
H
HOC
O
O
CC
C
H OC
N
N
N
N
N
H
H
H
H
H
C
C
C
C
O
O
O
C
C
C
C
C
C
O
C
O
N
N
H
H
C
O
C
C
O
C
H
N
N
H
O
O
C
C
C
C
OHO
N
HH
Pleated sheet
Alpha helix
(b)Secondary structure with folding
as a result of hydrogen bonding
(
dotted red lines
)
(c)Tertiary structure with secondary folding
caused by interactions within the polypeptide
and its immediate environment
(d)Quaternary str ucture—the relationships
between individual subunits
(a)Primary structure —the amino acid
sequence
Amino acids
Peptide bond
Figure 2.22
Protein Structure
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Chapter 2 The ChemicalBasis of Life 51
Molecule A Molecule B
Enzyme
New molecule AB
Figure 2.23
Enzyme Action
The enzyme bringsthe two reacting molecules together. This is possible
because the reacting molecules“fit” the shape of the enzyme (lock-and-key
model). After the reaction, the unaltered enzyme can be used again.
phosphate group are attached (figure 2.24).The monosaccharide is
deoxyribose for DNA,and ribose for RNA. The organic bases are
thymine (thı¯me¯n, thı¯min),cytosine (sı¯to¯-se¯n), and uracil (u¯ra˘-
sil),which are single-ringed pyrimidines (pı¯-rimi-de¯nz);and ade-
nine (ade˘-ne¯n) and guanine (gwahne¯n),which are double-ringed
purines (pu¯re¯nz) (figure 2.25).
DNA has two strands ofnucleotides joined together to form
a twisted ladderlike structure called a double helix (figure 2.26).
The uprights of the ladder are formed by covalent bonds between
the deoxyribose molecules and phosphate groups of adjacent nu-
cleotides. The rungs of the ladder are formed by the bases of the
nucleotides ofone uprig ht connected to the bases ofthe other up-
right by hydrogen bonds.Each nucleotide of DNA contains one of
the organic bases:adenine, thymine, cytosine, or guanine. Adenine
binds only to thymine because the structure ofthese organic bases
allows two hydrogen bonds to form between them.Cytosine binds
only to guanine because the structure ofthese organic bases allows
three hydrogen bonds to form between them.
The sequence of organic bases in DNA molecules stores ge-
netic information. Each DNA molecule consists of millions of
organic bases, and their sequence ultimately determines the type
and sequence ofamino acids found in protein molecules. Because
enzymes are proteins,DNA structure determines the rate and type
O
H
H
OH
H
OH
HH
HOCH
2
O
H
H
OH
OH
HH
HOCH
2
OH
O
CH
2
OH
O
P
O
–
O
–
O
Deoxyribose(a) (b)
Ribose
Deoxyribose
Nitrogen
base
Phosphate
group
Deoxyribonucleotide(c)
Figure 2.24
Componentsof Nucleotides
(a) Deoxyribose sugar, which formsnucleotides used in DNA production.
(b)Ribose sugar, which forms nucleotides used in RNA production. Note that
deoxyribose isribose minus an oxygen atom. (c) Deoxyribonucleotide
consisting ofdeoxyribose, a nitrogen base, and a phosphate group.
C
C
C
N
N
C
N
N
C
O
H
H
H
HH
H
N
C
C
C
N
N
C
N
N
C
N
H
H
C
N
C
N
C
C
H
H
H
O
C
N
H
C
N
C
C
O
H
H
3
C
H
O
C
N
H
C
N
C
C
O
H
H
H
O
H
H
H
H
N
Cytosine
(DNA and RNA)
Guanine
(DNA and RNA)
Thymine
(DNA only)
Adenine
(DNA and RNA)
Uracil
(RNA only)
Pyrimidines Purines
Figure 2.25
NitrogenousOrganic Bases
The organicbases found in nucleic acids are separated into two groups. Purines
are double-ringed molecules, and pyrimidinesare single-ringed molecules.
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of chemical reactions that occur in cells by controlling enzyme
structure.The information contained in DNA therefore ultimately
defines all cellular activities.Other proteins, such as collagen, that
are coded by DNA determine many of the structural features of
humans.
Part1 Organization of the Human Body52
RNA has a structure similar to a single strand of DNA.Like
DNA,four different nucleotides make up the RNA molecule, and
the organic bases are the same,except that thymine is replaced with
uracil (see figure 2.25).Uracil can bind only to adenine.
O
O
O
O
–
O
P
O
O
O
–
O
P
O
O
O
–
O
P
O
O
O
–
O
P
O
O
O
O
O
O
O
O
O
O
O
O
O
–
O
P
O
–
P
O
–
P
O
–
P
H
H
H
H
C
C
C
C
O
N
N
N
H
H
H
H
C
C
C
C
C
O
N
N
N
N
N
CH
3
H
H
C
C
C
C
O
O
N
N
H
H
H
H
C
C
C
C
C
N
N
N
N
N
N
H
H
H
H
C
C
C
C
C
O
N
N
N
N
H
H
H
H
C
C
C
C
O
N
N
N
CH
3
H
H
H
H
H
H
O
O
C
C
C
C
C
C
C
C
C
N
N
N
N
N
N
N
O
H
H
H
H
H
CH
2
O
H
H
H
H
H
CH
2
O
H
H
H
H
H
CH
2
O
H
H
H
H
H
CH
2
O
H
H
H
H
H
O
H
H
H
H
H
O
H
H
H
H
H
O
H
H
H
H
H
CH
2
CH
2
CH
2
CH
2
C
C
G
G
A
A
T
T
C
TA
A
G
G
T
C
1
2
3
4
A DNA molecule is two strands of
nucleotides joined together to form a
double-stranded helix.
Cytosine (C)
Thymine (T)
Guanine (G)
Adenine (A)
The strands are uncoiled and enlarged.
The deoxyribose molecules and
phosphate groups of each strand
are joined by covalent bonds.
The strands are held together by
hydrogen bonds (
dotted red lines
)
between the bases of the nucleotides.
1.
2.
3.
4.
Figure 2.26
Structure ofDNA
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Chapter 2 The ChemicalBasis of Life 53
Adenosine Triphosphate
Adenosine triphosphate(a˘-deno¯-se¯n trı¯-fosfa¯t) (ATP)is an es-
pecially important organic molecule found in all living organisms.
It consists ofadenosine and three phosphate groups (figure 2.27).
Adenosine is the sugar ribose with the organic base adenine.The
potential energy stored in the covalent bond between the second
and third phosphate groups is important to living organisms
because it provides the energy used in nearly all ofthe chemical re-
actions within cells.
The catabolism ofglucose and other nutrient molecules results
in chemical reactions that release energy.Some of that energy is used
to synthesize ATP from ADP and an inorganic phosphate group (P
i
):
ADP P
i
Energy (from catabolism) n ATP
The transfer of energy from nutrient molecules to ATP in-
volves a series of oxidation–reduction reactions in which a high-
energy electron is transferred from one molecule to the next
molecule in the series.In chapter 25 the oxidation–reduction reac-
tions ofmetabolism are considered in greater detail.
Once produced, ATP is used to provide energy for other
chemical reactions (anabolism) or to drive cell processes such as
muscle contraction.In the process, ATP is converted back to ADP
and an inorganic phosphate group.
ATP n ADP P
i
Energy (for anabolism and other cell processes)
ATP is often called the energy currency ofcells because it is capable
ofboth storing and providing energy. The concentration of ATP is
maintained within a narrow range of values, and essentially all
energy-requiring chemical reactions stop when there is an inade-
quate quantity ofATP.
29. List the four types of organic molecules important to life.
30. Name the basic building blocks of carbohydrates, fats,
proteins, and nucleicacids.
31. List three types of carbohydrates, and explain the role of
each in the body.
32. Distinguish between fats, phospholipids, and steroids, and
give an example of each. Whatis a saturated fat?
33. Define a peptide bond. What makes proteins different from
one another?
34. What determines the primary, secondary, tertiary, and
quaternarystructures of proteins? Define denaturation and
name two thingsthat can cause it to occur.
35. Compare the lock-and-key model and the induced fit model
of enzyme activity. Define cofactorand coenzyme.
36. What are the structural and functional differences between
DNA and RNA?
37. Describe the structure of ATP. Whatrole does this molecule
playin energy exchange?
P
O
H
H
H
H
OH
OH
H
H
CH
2
Ribose
Adenine
Adenosine
Adenosine diphosphate (ADP)
Phosphate groups
Adenosine triphosphate (ATP)
C
C
C
C
C
N
N
N
N
NH
2
OO
O
O
–
OP
O
O
–
O
O
–
PO
–
Figure 2.27
Adenosine Triphosphate (ATP) Molecule
Chemistry is the study of the composition, structure, and properties of
substances and the reactions they undergo.Much of the structure and
function ofhealthy or diseased organisms can be understood at the chem-
ical level.
BasicChemistry
(p. 27)
Matter, Mass, and Weight
1. Matter is anything that occupies space.
2. Mass is the amount of matter in an object.
3. Weight results from the force exerted by earth’s gravity on matter.
Elementsand Atoms
1. An element is the simplest type of matter with unique chemical and
physical properties.
2. An atom is the smallest particle of an element that has the chemical
characteristics ofthat element. An element is composed of only one
kind ofatom.
SUMMARY
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3. Atoms consist of protons,neutrons, and electrons.
• Protons are positively charged,electrons are negatively charged,
and neutrons have no charge.
• Protons and neutrons are found in the nucleus,and electrons,
which are located around the nucleus,can be represented by an
electron cloud.
4. The atomic number is the unique number of protons in an atom.
The mass number is the sum ofthe protons and the neutrons.
5. Isotopes are atoms that have the same atomic number but different
mass numbers.
6. The atomic mass of an element is the average mass of its naturally
occurring isotopes weighted according to their abundance.
Electronsand Chemical Bonding
1. The chemical behavior of atoms is determined mainly by their
outermost electrons.A chemical bond occurs when atoms share or
transfer electrons.
2. Ions are atoms that have gained or lost electrons.
• An atom that loses one or more electrons becomes positively
charged and is called a cation.An anion is an atom that becomes
negatively charged after accepting one or more electrons.
• Ionic bonding is the attraction of the oppositely charged cation
and anion to each other.
3. A covalent bond is the sharing of electron pairs between atoms.A
polar covalent bond results when the sharing ofelectrons is unequal
and can produce a polar molecule that is electrically asymmetric.
Moleculesand Compounds
1. A molecule is two or more atoms chemically combined to form a
structure that behaves as an independent unit.A compound is two
or more differenttypes of atoms chemically combined.
2. The kinds and numbers of atoms (or ions) in a molecule or
compound can be represented by a formula consisting ofthe
symbols ofthe atoms (or ions) plus subscripts denoting the number
ofeach type of atom (or ion).
3. The molecular mass of a molecule or compound can be determined
by adding up the atomic masses ofits atoms (or ions).
Intermolecular Forces
1. A hydrogen bond is the weak attraction that occurs between the
oppositely charged regions ofpolar molecules. Hydrogen bonds are
important in determining the three-dimensional structure oflarge
molecules.
2. Solubility is the ability of one substance to dissolve in another.Ionic
substances that dissolve in water by dissociation are electrolytes.
Molecules that do not dissociate are nonelectrolytes.
ChemicalReactions and Energy
(p. 34)
SynthesisReactions
1. Synthesis reactions are the chemical combination of two or more
substances to form a new or larger substance.
2. Dehydration reactions are synthesis reactions in which water is
produced.
3. Anabolism is the sum of all the synthesis reactions in the body.
Decomposition Reactions
1. Decomposition reactions are the chemical breakdown of a larger
substance to two or more different smaller substances.
2. Hydrolysis reactions are decomposition reactions in which water is
depleted.
3. All of the decomposition reactions in the body are called
catabolism.
Reversible Reactions
Reversible reactions produce an equilibrium condition in which the
amount ofreactants relative to the amount of products remains constant.
Oxidation
–
Reduction Reactions
Oxidation–reduction reactions involve the complete or partial transfer of
electrons between atoms.
Energy
Energy is the ability to do work.Potential energy is stored energy, and ki-
netic energy is energy resulting from movement ofan object.
ChemicalEnergy
1. Chemical bonds are a form of potential energy.
2. Chemical reactions in which the products contain more potential
energy than the reactants require the input ofenergy.
3. Chemical reactions in which the products have less potential energy
than the reactants release energy.
HeatEnergy
1. Heat energy is energy that flows between objects that are at different
temperatures.
2. Heat energy is released in chemical reactions and is responsible for
body temperature.
Speed ofChemical Reactions
1. Activation energy is the minimum energy that the reactants must
have to start a chemical reaction.
2. Enzymes are specialized protein catalysts that lower the activation
energy for chemical reactions.Enzymes speed up chemical reactions
but are not consumed or altered in the process.
3. Increased temperature and concentration of reactants can increase
the rate ofchemical reactions.
InorganicChemistry
(p. 39)
Inorganic chemistry is mostly concerned with noncarbon-containing sub-
stances but does include some carbon-containing substances,such as car-
bon dioxide and carbon monoxide.
Water
1. Water is a polar molecule composed ofone atom of oxygen and two
atoms ofhydrogen.
2. Water stabilizes body temperature,protects against friction and
trauma,makes chemical reactions possible, directly participates in
chemical reactions (e.g.,dehydration and hydrolysis reactions),and
is a mixing medium (e.g.,solutions, suspensions, and colloids).
3. A mixture is a combination of two or more substances physically
blended together,but not chemically combined.
4. A solution is any liquid,gas, or solid in which the substances are
uniformly distributed with no clear boundary between the
substances.
5. A solute dissolves in the solvent.
6. A suspension is a mixture containing materials that separate from
each other unless they are continually,physically blended together.
7. A colloid is a mixture in which a dispersed (solutelike) substance is
distributed throughout a dispersing (solventlike) substance.
Particles do not settle out ofa colloid.
Solution Concentrations
1. One way to describe solution concentration is an osmole,which
contains 6.022 10
23
ofparticles (i.e., atoms, ions, or molecules) in
1 kilogram water.
2. A milliosmole is 1/1000 of an osmole.
Acidsand Bases
1. Acids are proton (i.e.,hydrogen ion) donors, and bases (e.g.,
hydroxide ion) are proton acceptors.
2. A strong acid or base almost completely dissociates in water.A weak
acid or base partially dissociates.
Part1 Organization of the Human Body54
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
I. Organization of the
Human Body
2. The Chemical Basis of
Life
© The McGraw−Hill
Companies, 2004
Chapter 2 The ChemicalBasis of Life 55
3. The pH scale refers to the hydrogen ion concentration in a solution.
• A neutral solution has an equal number of hydrogen ions and
hydroxide ions and is assigned a pH of7.
• Acid solutions, in which the number of hydrogen ions is greater
than the number ofhydroxide ions, have pH values less than 7.
• Basic, or alkaline, solutions have more hydroxide ions than
hydrogen ions and a pH greater than 7.
4. A salt is a molecule consisting of a cation other than hydrogen and
an anion other than hydroxide.Salts are formed when acids react
with bases.
5. A buffer is a solution of a conjugate acid–base pair that resists
changes in pH when acids or bases are added to the solution.
Oxygen
Oxygen is necessary in the reactions that extract energy from food mole-
cules in living organisms.
Carbon Dioxide
During metabolism when the organic molecules are broken down,carbon
dioxide and energy are released.
OrganicChemistry
(p. 43)
Organic molecules contain carbon atoms bound together by covalent
bonds.
Carbohydrates
1. Monosaccharides are the basic building blocks of other carbohydrates.
They,especially glucose, are important sources of energy.Examples
are ribose,deoxyribose, glucose, fructose, and galactose.
2. Disaccharide molecules are formed by dehydration reactions
between two monosaccharides.They are broken apart into
monosaccharides by hydrolysis reactions.Examples of disaccharides
are sucrose,lactose, and maltose.
3. Polysaccharides are many monosaccharides bound together to form
long chains.Examples include cellulose, starch, and glycogen.
Lipids
1. Triglycerides are composed ofglycerol and fatty acids. One, two, or
three fatty acids can attach to the glycerol molecule.
• Fatty acids are straight chains of carbon molecules with a carboxyl
group.Fatty acids can be saturated (only single covalent bonds
between carbon atoms) or unsaturated (one or more double
covalent bonds between carbon atoms).
• Energy is stored in fats.
2. Phospholipids are lipids in which a fatty acid is replaced by a
phosphate-containing molecule.Phospholipids are a major
structural component ofplasma membranes.
3. Steroids are lipids composed of four interconnected ring molecules.
Examples include cholesterol,bile salts, and sex hormones.
4. Other lipids include fat-soluble vitamins, prostaglandins,
thromboxanes,and leukotrienes.
Proteins
1. The building blocks of protein are amino acids,which are joined by
peptide bonds.
2. The number,kind, and arrangement of amino acids determine the
primary structure ofa protein. Hydrogen bonds between amino
acids determine secondary structure,and hydrogen bonds between
amino acids and water determine tertiary structure.Interactions
between different protein subunits determine quaternary structure.
3. Enzymes are protein catalysts that speed up chemical reactions by
lowering their activation energy.
4. The active sites of enzymes bind only to specific reactants.
5. Cofactors are ions or organic molecules such as vitamins that are
required for some enzymes to function.
NucleicAcids: DNA and RNA
1. The basic unit of nucleic acids is the nucleotide, which is a
monosaccharide with an attached phosphate and organic base.
2. DNA nucleotides contain the monosaccharide deoxyribose and the
organic bases adenine,thymine, guanine, or cytosine. DNA occurs
as a double strand ofjoined nucleotides and is the genetic material
ofcells.
3. RNA nucleotides are composed ofthe monosaccharide r ibose.The
organic bases are the same as for DNA,except that thymine is
replaced with uracil.
Adenosine Triphosphate
ATP stores energy derived from catabolism.The energy is released from
ATP and is used in anabolism and other cell processes.
1. The smallest particle of an element that still has the chemical
characteristics ofthat element is a (an)
a. electron.
b. molecule.
c. neutron
d. proton.
e. atom.
2. The number of electrons in an atom is equal to the
a. atomic number.
b. mass number.
c. number of neutrons.
d. isotope number.
e. molecular mass.
3.
12
C and
14
C are
a. atoms of different elements.
b. isotopes.
c. atoms with different atomic numbers.
d. atoms with different numbers ofprotons.
e. compounds.
4. A cation is a (an)
a. uncharged atom.
b. positively charged atom.
c. negatively charged atom.
d. atom that has gained an electron.
e. both c and d.
5. A polar covalent bond between two atoms occurs when
a. one atom attracts shared electrons more strongly than another
atom.
b. atoms attract electrons equally.
c. an electron from one atom is completely transferred to another
atom.
d. the molecule becomes ionized.
e. a hydrogen atom is shared between two different atoms.
6. Table salt (NaCl) is
a. an atom.
b. organic.
c. a molecule.
d. a compound.
e. a cation.
REVIEW AND COMPREHENSION
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
I. Organization of the
Human Body
2. The Chemical Basis of
Life
© The McGraw−Hill
Companies, 2004
Part1 Organization of the Human Body56
7. The weak attractive force between two water molecules forms a (an)
a. covalent bond.
b. hydrogen bond.
c. ionic bond.
d. compound.
e. isotope.
8. Electrolytes are
a. nonpolar molecules.
b. covalent compounds.
c. substances that usually don’t dissolve in water.
d. found in solutions that do not conduct electricity.
e. cations and anions that dissociate in water.
9. In a decomposition reaction
a. anabolism occurs.
b. proteins are formed from amino acids.
c. large molecules are broken down to form small molecules.
d. a dehydration reaction may occur.
e. all of the above.
10. Oxidation–reduction reactions
a. can be synthesis or decomposition reactions.
b. have one reactant gaining electrons.
c. have one reactant losing electrons.
d. can create ionic or covalent bonds.
e. all of the above.
11. Potential energy
a. is energy caused by movement of an object.
b. is the form of energy that is actually doing work.
c. includes energy within chemical bonds.
d. can never be converted to kinetic energy.
e. all of the above.
12. Which of these descriptions of heat energy is not correct?
a. Heat energy flows between objects that are at different temperatures.
b. Heat energy can be produced from all other forms ofenergy.
c. Heat energy can be released during chemical reactions.
d. Heat energy must be added to break apart ATP molecules.
e. Heat energy is always transferred from a hotter object to a cooler
object.
13. A decrease in the speed of a chemical reaction occurs if
a. the activation energy requirement is increased.
b. catalysts are increased.
c. temperature increases.
d. the concentration ofthe reactants increases.
e. all of the above.
14. Which of these statements concerning enzymes is correct?
a. Enzymes increase the rate of reactions but are permanently
changed as a result.
b. Enzymes are proteins that function as catalysts.
c. Enzymes increase the activation energy requirement for a
reaction to occur.
d. Enzymes usually can only double the rate of a chemical reaction.
e. Enzymes increase the kinetic energy of the reactants.
15. Water
a. is composed of two oxygen atoms and one hydrogen atom.
b. has a low specific heat.
c. is composed of polar molecules into which ionic substances
dissociate.
d. is produced in a hydrolysis reaction.
e. is a very small organic molecule.
16. When sugar is dissolved in water,the water is called the
a. solute.
b. solution.
c. solvent.
17. Which of these is an example of a suspension?
a. sweat
b. water and proteins inside cells
c. sugar dissolved in water
d. red blood cells in plasma
18. A solution with a pH of 5 is and contains
hydrogen ions than a neutral solution.
a. a base, more
b. a base,less
c. an acid, more
d. an acid,less
e. neutral, the same number of
19. A buffer
a. slows down chemical reactions.
b. speeds up chemical reactions.
c. increases the pH of a solution.
d. maintains a relatively constant pH.
e. works by forming salts.
20. A conjugate acid–base pair
a. acts as a buffer.
b. can combine with hydrogen ions in a solution.
c. can release hydrogen ions to combine with hydroxide ions.
d. describes carbonic acid (H
2
CO
3
) and bicarbonate ion (HCO
3
)
e. all of the above.
21. Carbon dioxide
a. consists of two oxygen atoms ionically bonded to carbon.
b. becomes toxic ifallowed to accumulate within cells.
c. is mostly eliminated by the kidneys.
d. is combined with fats to produce glucose during metabolism
within cells.
e. is taken into cells during metabolism.
22. Which of these is an example of a carbohydrate?
a. glycogen
b. prostaglandin
c. steroid
d. DNA
e. triglyceride
23. The polysaccharide used for energy storage in the human body is
a. cellulose.
b. glycogen.
c. lactose.
d. sucrose.
e. starch.
24. The basic units or building blocks of triglycerides are
a. simple sugars (monosaccharides).
b. double sugars (disaccharides).
c. amino acids.
d. glycerol and fatty acids.
e. nucleotides.
25. A fatty acid has one double covalent bond
between carbon atoms.
a. cholesterol
b. monounsaturated
c. phospholipid
d. polyunsaturated
e. saturated
26. A peptide bond joins together
a. amino acids.
b. fatty acids and glycerol.
c. monosaccharides.
d. disaccharides.
e. nucleotides.
27. The structure ofa protein results from the folding
ofthe helices or pleated sheets.
a. primary
b. secondary
c. tertiary
d. quaternary
Seeley−Stephens−Tate:
Anatomy and Physiology,
Sixth Edition
I. Organization of the
Human Body
2. The Chemical Basis of
Life
© The McGraw−Hill
Companies, 2004
Chapter 2 The ChemicalBasis of Life 57
28. According to the lock-and-key model ofenzyme action,
a. reactants must first be heated.
b. enzyme shape is not important.
c. each enzyme can catalyze many types of reactions.
d. reactants must bind to an active site on the enzyme.
e. enzymes control only a small number of reactions in the cell.
29. DNA molecules
a. are the genetic material of cells.
b. contain a single strand of nucleotides.
c. contain the nucleotide uracil.
d. have three different types that have roles in protein synthesis.
e. contain up to 100 organic bases.
30. ATP
a. is formed by the addition of a phosphate group to ADP.
b. is formed with energy released during catabolism reactions.
c. provides the energy for anabolism reactions.
d. contains three phosphate groups.
e. all of the above.
Answers in Appendix F
1. Iron has an atomic number of 26 and a mass number of 56. How
many protons,neutrons, and electrons are in an atom of iron? If an
atom ofiron lost three electrons, what would the charge of the
resulting ion be? Write the correct symbol for this ion.
2. Which of the following pairs of terms applies to the reaction that
results in the formation offatty acids and glycerol from a
triglyceride molecule?
a. Decomposition or synthesis reaction
b. Anabolism or catabolism
c. Dehydration or hydrolysis reaction
3. A mixture of chemicals is warmed slightly.As a consequence,
although no more heat is added,the solution becomes very hot.
Explain what occurred to make the solution so hot.
4. Two solutions,when mixed together at room temperature, produce
a chemical reaction.When the solutions are boiled and allowed to
cool to room temperature before mixing,however,no chemical
reaction takes place.Explain.
5. In terms of the potential energy in the food, explain why eating food
is necessary for increasing muscle mass.
6. Solution A has a pH of 2, and solution B has a pH of 8. If equal
amounts ofsolutions A and B are mixed, is the resulting solution
acidic or basic?
7. Given a buffered solution that is based on the following equilibrium:
CO
2
+ H
2
O H
2
CO
3
H
HCO
3
what happens to the pH ofthe solution if NaHCO
3
is added?
8. An enzyme E catalyzes the following reaction:
The product C,however,binds to the active site of the enzyme in a
reversible fashion and keeps the enzyme from functioning.What
happens ifA and B are continually added to a solution that contains
a fixed amount ofthe enzyme?
9. Given the materials commonly found in a kitchen,explain how one
could distinguish between a protein and a lipid.
10. A student is given two unlabeled substances:one a ty pical
phospholipid and one a typical protein.She is asked to determine
which substance is the protein and which is the phospholipid.The
available techniques allow her to determine the elements in each
sample.How can she identify each substance?
Answers in Appendix G
E
A B n C
n
m
n
m
CRITICAL THINKING
1. The mass (amount of matter) of the astronaut on the surface of the
earth and in outer space does not change.In outer space, where the
force ofgravity from the earth is very small, the astronaut is
“weightless”compared to his or her weight on the earth’s surface.
2. Potassium has 19 protons (the atomic number),20 neutrons (the
mass number minus the atomic number),and 19 electrons (because
the number ofelectrons equals the number of protons).
3. The molecular formula for glucose is C
6
H
12
O
6
.The atomic mass of
carbon is 12.01,hydrogen is 1.008, and oxygen is 16.00.The
molecular mass ofglucose is therefore (6 12.01) (12 1.008)
(6 16.00), or 180.2.
4. A decrease in blood CO
2
decreases the amount ofH
2
CO
3
and
therefore the blood H
level.Because CO
2
and H
2
O are in
equilibrium with H
and HCO
3
,with H
2
CO
3
as an intermediate,a
decrease in CO
2
causes some H
and HCO
3
to join together to
form H
2
CO
3
,which then forms CO
2
and H
2
O.Consequently,the
H
concentration decreases.
5. When two hydrogen atoms combine with an oxygen atom to form
water,a polar covalent bond forms between each hydrogen atom
and the oxygen atom.Unequal sharing of electrons occurs, and the
electrons are associated with the oxygen atom more than with the
hydrogen atoms.In this sense, the hydrogen atoms lose their
electrons,and the oxygen atom gains electrons. The hydrogen atoms
are therefore oxidized,and the oxygen atom is reduced.
6. During exercise,muscle contractions increase, which requires energy.
This energy is obtained from the energy in the chemical bonds of
ATP.As ATP is broken down,energy is released.Some of the energy
is used to drive muscle contractions,and some becomes heat.
Because the rate ofthese reactions increases during exercise, more
heat is produced than when at rest,and body temperature increases.
7. Monohydrogen phosphate ion (HPO
4
2
) is the conjugate base
formed when the conjugate acid,dihydrogen phosphate ion
(H
2
PO
4
) loses a hydrogen ion.If hydrogen ions are added to the
solution,they combine with the conjugate base, monohydrogen
phosphate ions,to form dihydrogen phosphate ions, which helps to
prevent an increase in hydrogen ion concentration.If hydroxide ions
are added to the solution,they combine with hydrogen ions to form
water.Then the conjugate acid, dihydrogen phosphate ions,
dissociate to replace the hydrogen ions,which helps to prevent a
decrease in hydrogen ion concentration.
ANSWERS TO PREDICT QUESTIONS
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