Carbohydrates are hydrates of carbon with a general empirical formula of C,( H,O),.. Carbohydrates include sugars. The basic sugar unit
is a monosaccharide, or simple sugar. It may contain from three to seven or more carbon atoms, but the most common monosaccharides
contain six carbon atoms and are known as hexoses.
Glycogen is the major polysaccharide associated with higher animal species. The principal polysac- charides of plants are starch and
cellulose. All these polysaccharides are composed of glucose units as the basic monomer. Glycogen is a highly branched chain of glucose
units that serves as a calorie storage molecule in animals, principally in liver and muscle.
In plants, starch is the primary storage form of glucose. It occurs in two forms: a-amylose, which consists of long, unbranched chains, and
amylopectin, a branched form with 1 + 6 linkages forming the branches. The primary structural component in plants is cellulose, a waterinsoluble
polysaccharide that forms long, unbranched chains of 1 -+4 linkages. These chains are cemented together to form the cell walls
Proteins are a class of organic compounds consisting almost entirely of carbon, hydrogen, oxygen, and
acids. The amino acids usually found in proteins show the following structure: The COOH (carboxyl)
group is characteristic of all organic acids and is attached to the same carbon as the NH2 group. This
carbon is designated the a-carbon atom; the entire amino acid is known as an alpha-amino acid. The R
is a general designation for a variety of side groups that differentiate the 20 different amino acids found
► STRUCTURE AND FUNCTION OF LIPIDS
Lipids are a class of organic compounds that tend to be insoluble in water or other polar solvents but soluble in organic solvents
such as toluene or ether. They consist largely of carbon, hydrogen, and oxygen, but they may contain other elements as well. Triglycerides
and other lipids have much more energy associated with their bonding structure than do the carbohydrates or proteins. One gram of most
carbohydrates yields approximately 4.3 kcal upon oxidation, 1g of protein yields 4.6 kcal, while the oxidation of 1g of triglyceride
produces more than 9kcal. Fats as energy storage media also take up much less room and involve less weight than carbohydrates do. This
is because carbohydrates incorporate water during their storage, while fats do not require water in their final storage form or in the
intermediate conversions that produce storage molecules.
Besides serving as media of energy storage, certain kinds of lipids cushion and protect the internal organs of the body, while
others, in the form of a layer of fat just below the skin in many mammals, provide insulation against possible low environmental
Lipids are more difficult to categorize than the carbohydrates or proteins, since there is such diversity in the lipid group. Among
the major classes of lipids functioning within living organisms are the neutral fats (triglycerides), the phospholipids, and the steroids.
Waxes are found as protective layers on the surfaces of many plants and animals.
The neutral fats, or triglycerides, are the most common and familiar of the lipids. They are composed of three fatty acids
joined to each of the three hydroxyl groups of the triple alcohol glycerol (see Fig. 3.4). Since the union of an acid and an alcohol yields an
ester, triglycerides are also known as triesters.
Phospholipids are similar in chemical makeup to the triglycerides. The first two hydroxyl groups of glycerol are joined in ester
linkage to two fatty acids, but the third position is occupied by a phosphate group. Most phospholipids also contain another charged group
attached to the phosphate portion. Note the charges occurring in the phosphate portion. It is these charges that give phospholipids their
unique properties-one end is polar and soluble in water, while the bulk of the molecule is nonpolar and insoluble in water. Phospholipids
play an important role in the cell membrane in maintaining the polar-nonpolar layering structure. Phospholipids are also useful in the
transport of lipid material within such aqueous media as blood.
STEROIDS The steroids are markedly different in structure from the neutral fats and phospholipids. They are classified as lipids because
of their insolubility in water. They consist of four interconnecting rings of carbon atoms, three of which are six-membered rings and one of
which is a five-membered ring.
EXAMPLE 4 Cholesterol is typical of the structure of a steroid (see Fig. 3.6). Although cholesterol is associated with the advent of
arteriosclerosis in humans, it is actually a vital structural component of the cell membrane and plays a key role in the proper function of such
diverse animal tissues as nerve and blood. Cholesterol is not found in plants. In addition to cholesterol, steroids include such fat-soluble vitamins
as vitamin D; the sex hormones and the hormones of the adrenal cortex are steroids that seem to be derived from cholesterol produced within the