Essentials of Anatomy and Physiology

which forms the myelin sheath around nerve cells and
provides electrical insulation for nerve impulse trans-
mission.
The structure of steroidsis very different from
that of the other lipids. Cholesterolis an important
steroid; it is made of four rings of carbon and hydro-
gen (not fatty acids and glycerol) and is shown in Fig.
2–7. The liver synthesizes cholesterol, in addition to
the cholesterol we eat in food as part of our diet.
Cholesterol is another component of cell membranes
and is the precursor (raw material) for the synthesis of
other steroids. In the ovaries or testes, cholesterol is
used to synthesize the steroid hormones estrogen or
testosterone, respectively. A form of cholesterol in the
skin is changed to vitamin D on exposure to sunlight.
Liver cells use cholesterol for the synthesis of bile
salts, which emulsify fats in digestion. Despite its link
to coronary artery disease and heart attacks, choles-
terol is an essential substance for human beings.
The structure and functions of lipids are summa-
rized in Table 2–4.

PROTEINS

Proteinsare made of smaller subunits or building
blocks called amino acids, which all contain the ele-
ments carbon, hydrogen, oxygen, and nitrogen. Some
amino acids contain sulfur, which permits the forma-
tion of disulfide bonds. There are about 20 amino
acids that make up human proteins. The structure of

amino acids is shown in Fig. 2–8. Each amino acid has

a central carbon atom covalently bonded to an atom of

hydrogen, an amino group (NH 2 ), and a carboxyl

group (COOH). At the fourth bond of the central car-

bon is the variable portion of the amino acid, repre-

sented by R. The R group may be a single hydrogen

atom, or a CH 3 group, or a more complex configura-

tion of carbon and hydrogen. This gives each of the 20

amino acids a slightly different physical shape. A bond

between two amino acids is called a peptide bond,

and a short chain of amino acids linked by peptide

bonds is a polypeptide.

A protein may consist of from 50 to thousands of

amino acids. The sequence of the amino acids is

specific and unique for each protein, and is called

its primary structure. This unique sequence, and the

hydrogen bonds and disulfide bonds formed within

the amino acid chain, determines how the protein will

be folded to complete its synthesis. The folding may

be simple, a helix (coil) or pleated sheet, called the sec-

ondary structure, or a more complex folding may

occur to form a globular protein, called the tertiary

structure. Myoglobin, found in muscles, is a globular

protein (Fig. 2–8). When complete, each protein has a

characteristic three-dimensional shape, which in turn

determines its function. Some proteins consist of

more than one amino acid chain (quaternary struc-

ture). Hemoglobin, for example, has four amino acid

chains (see Box 3–2). Notice that myoglobin contains

an atom of iron (a hemoglobin molecule has four iron

Some Basic Chemistry 37

Table 2–4 LIPIDS

Name Structure Function

True fats

Phospholipids

Steroids (cholesterol)

A triglyceride consists of three

fatty acid molecules bonded to

a glycerol molecule (some are

monoglycerides or diglycerides)

Diglycerides with a phosphate

group bonded to the glycerol

molecule

Four carbon–hydrogen rings

• Storage form for excess food molecules in
  subcutaneous tissue


• Cushion organs such as the eyes and kidneys


• Part of cell membranes (lecithin)


• Form the myelin sheath to provide electrical
  insulation for neurons


• Part of cell membranes


• Converted to vitamin D in the skin on exposure to
  UV rays of the sun


• Converted by the liver to bile salts, which emulsify
  fats during digestion


• Precursor for the steroid hormones such as estrogen
  in women (ovaries) or testosterone in men (testes)