Chemical Composition of the Body
31
phosphate. In addition, a small polar or ionized nitrogen-con-
taining molecule is usually attached to this phosphate (
Figure
2–11b
). These groups constitute a polar (hydrophilic) region
at one end of the phospholipid, whereas the fatty acid chains
provide a nonpolar (hydrophobic) region at the opposite end.
Therefore, phospholipids are amphipathic. In water, they
become organized into clusters, with their polar ends attracted
to the water molecules. It is this property of phospholipids that
permits them to form the lipid bilayers of plasma and intracel-
lular membranes (Chapter 3).
Steroids
Steroids
have a distinctly different structure from those of the
other subclasses of lipid molecules. Four interconnected rings of
carbon atoms form the skeleton of every steroid (
Figure 2–12
).
A few hydroxyl groups, which are polar, may be attached to this
ring structure, but they are not numerous enough to make
a steroid water-soluble. Examples of steroids are cholesterol,
cortisol from the adrenal glands, and female (estrogen) and
male (testosterone) sex hormones secreted by the gonads.
Proteins
The term
protein
comes from the Greek
proteios
(“of the fi rst
rank”), which aptly describes their importance. Proteins account
for about 50 percent of the organic material in the body (17 per-
cent of the body weight), and they play critical roles in almost
every physiological process. Proteins are composed of carbon,
hydrogen, oxygen, nitrogen, and small amounts of other ele-
ments, notably sulfur. They are macromolecules, often contain-
ing thousands of atoms, and like most large molecules, they
are formed when a large number of small subunits (monomers)
bond together via dehydration reactions to create long chains.
Amino Acid Subunits
The subunits of proteins are
amino acids;
thus, proteins are
polymers of amino acids. Every amino acid except one (pro-
line) has an amino (—NH
2
) and a carboxyl (—COOH) group
bound to the terminal carbon in the molecule:
H
A
R—C—COOH
A
NH
2
The third bond of this terminal carbon is bonded to a hydro-
gen and the fourth to the remainder of the molecule, which
is known as the
amino acid side chain
(R in the formula).
These side chains are relatively small, ranging from a single
hydrogen to nine carbons with their associated hydrogens.
The proteins of all living organisms are composed of the
same set of 20 different amino acids, corresponding to 20 dif-
ferent side chains. The side chains may be nonpolar (8 amino
acids), polar (7 amino acids), or ionized (5 amino acids)
(
Figure 2–13
). The human body can synthesize many amino
acids, but several must be obtained in the diet; the latter are
known as essential amino acids.
Polypeptides
Amino acids are joined together by linking the carboxyl group
of one amino acid to the amino group of another. As in the
formation of glycogen and triglycerides, a molecule of water
is formed by dehydration (
Figure 2–14
). The bond formed
between the amino and carboxyl group is called a
peptide
bond.
Although peptide bonds are covalent, they can be broken
HO
CH
CH
3
CH
3
CH
3
CH
3
CH
2
CH
2
CH
2
CH
CH
CH
2
CH
2
CH
2
CH
2
CH
2
CH
CH
2
CH
2
CH
2
CH
CH
CH
CH
CH
2
CH
2
CH
Steroid ring structure
Cholesterol
CH
3
CH
2
(a)
(b)
Figure 2–12
(a) Steroid ring structure, shown with all the carbon and hydrogen atoms in the rings and again without these atoms to emphasize the overall
ring structure of this class of lipids. (b) Different steroids have different types and numbers of chemical groups attached at various locations on
the steroid ring, as shown by the structure of cholesterol.
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