►CARBOHYDRATES
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
of plants.
► PROTEINS
Proteins are a class
of organic compounds consisting almost entirely of carbon, hydrogen, oxygen,
and
nitrogen. The protein
is actually a polymer composed of many subunits (monomers) known as amino
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
in nature.
► 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
temperatures.
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
body.
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