Chemical Composition of the Body
27
For example, if the acidity of a solution containing lactate is
increased by adding hydrochloric acid, the concentration of
lactic acid will increase and that of lactate will decrease.
If the electric charge on a molecule is altered, its inter-
action with other molecules or with other regions within
the same molecule changes, and thus its functional char-
acteristics change. In the extracellular fl
uid,
hydrogen ion
concentrations beyond the 10-fold pH range of 7.8 to 6.8 are
incompatible with life if maintained for more than a brief
period of time.
Even small changes in the hydrogen ion con-
centration can produce large changes in molecular inter-
action. For example, many enzymes in the body operate
effi ciently within very narrow ranges of pH. Should pH vary
from the normal homeostatic range due to disease, these
enzymes work at reduced levels, creating an even worse
pathological situation.
This concludes our overview of atomic and molecular
structure, water, and pH. We turn now to a description of the
molecules essential for life in all living organisms, including
humans. These are the carbon-based molecules required for
forming the building blocks of cells, tissues, and organs; pro-
viding energy; and forming the genetic blueprints of all life.
Classes of Organic Molecules
Because most naturally occurring carbon-containing molecules
are found in living organisms, the study of these compounds
became known as organic chemistry. (Inorganic chemistry is
the study of noncarbon-containing molecules.) However, the
chemistry of living organisms, biochemistry, now forms only a
portion of the broad fi eld of organic chemistry.
One of the properties of the carbon atom that makes life
possible is its ability to form four covalent bonds with other
atoms, including with other carbon atoms. Because carbon
atoms can also combine with hydrogen, oxygen, nitrogen,
and sulfur atoms, a vast number of compounds can form from
relatively few chemical elements. Some of these molecules
are extremely large
(macromolecules),
composed of thou-
sands of atoms. Such large molecules form when many smaller
molecules, or subunits, link together. These large molecules
are known as
polymers
(literally “many small parts”). The
structure of macromolecules depends upon the structure of
the subunits (monomers), the number of subunits bonded
together, and the three-dimensional way in which the sub-
units are linked.
Most of the organic molecules in the body can be clas-
sifi ed into one of four groups: carbohydrates, lipids, proteins,
and nucleic acids (
Table 2–5
).
Carbohydrates
Although carbohydrates account for only about 1 percent of
body weight, they play a central role in the chemical reactions
that provide cells with energy. Carbohydrates are composed of
carbon, hydrogen, and oxygen atoms in the proportions repre-
sented by the general formula C
n
(H
2
O)
n
, where
n
is any whole
number. It is from this formula that the class of molecules gets
its name,
carbohydrate
—water-containing (hydrated) carbon
atoms. Linked to most of the carbon atoms in a carbohydrate
are a hydrogen atom and a hydroxyl group:
A
H—C—OH
A
Table 2–5
Major Categories of Organic Molecules in the Body
Category
Percent of
Body Weight
Predominant
Atoms
Subclass
Subunits
Carbohydrates
1
C, H, O
Polysaccharides
(and disaccharides)
Monosaccharides
Lipids
15
C, H
Triglycerides
3 fatty acids + glycerol
Phospholipids
2 fatty acids + glycerol + phosphate +
small charged nitrogen molecule
Steroids
Proteins
17
C, H, O, N
Peptides and polypeptides
Amino acids
Nucleic acids
2
C, H, O, N
DNA
Nucleotides containing the bases
adenine, cytosine, guanine, thymine,
the sugar deoxyribose, and phosphate
RNA
Nucleotides containing the bases
adenine, cytosine, guanine, uracil,
the sugar ribose, and phosphate
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