Page 43 - Anatomy and Physiology of Farm Animals, 8th Edition

28 / Anatomy and Physiology of Farm Animals

H
CH 3
VetBooks.ir CH 3 C CH 2 C
H

CH 2 CH CH 2 CH 2 CH 3
CH 3
C CH 2
CH 2
CH 3
CH 2 CH CH CH 2
C
CH 2 CH
CH CH 2
HO C
CH 2 CH
Figure 2-5. Cholesterol. Different biologic steroids are formed by modifying the cholesterol molecule,
but the four carbon rings remain intact.

of the steroids found in animals are derived lactose, or milk sugar (glucose + galactose);
from cholesterol (e.g., bile salts and various and maltose (glucose + glucose).
hormones, including several reproductive Multiple molecules of glucose can be
hormones). Cholesterol itself is an essen­ linked (polymerized) to form a polysaccha­
tial constituent of the cell membrane of all ride, glycogen. Two major sites of glycogen
animal cells. Cholesterol can be obtained synthesis are the liver and skeletal muscle.
from dietary sources, but it is also In the liver, the stored glycogen can be
synthesized in the liver of animals, broken down to glucose and metabolized
including humans. Inappropriate rates by liver cells or secreted as glucose into the
of cholesterol synthesis by the liver are blood. In skeletal muscle, glycogen stores
responsible for elevation in serum cho- are an immediate source of energy, but this
lesterol in humans in spite of reductions glycogen cannot be a source of glucose for
in dietary intake of cholesterol. release into the blood.
Waxes are a class of lipids. The waxes Glucose is a source of cell energy, and
synthesized in the animal occur mostly in the enzymatic pathway that metabolizes
the epithelial cells of the skin. Here the glucose to produce energy is glycolysis.
waxes form a protective coating on the This pathway can be completed within the
skin or hair as a water repellent and as a cytosol, resulting in the production of ATP
barrier against bacteria. Lanolin is wool and pyruvate. If oxygen is readily available,
fat, and cerumen is earwax. the pyruvate can enter the mitochondria to
be metabolized.
The sugar deoxyribose is found in com­
Carbohydrates bination with a base (purine or pyrimidine)
and a phosphate, forming DNA (deoxyri-
Like lipids, carbohydrates are composed of bonucleic acid). DNA is the carrier of all
carbon, oxygen, and hydrogen. Simple genetic information from generation to
sugars, or monosaccharides, are carbohy­ generation, and from cell to cell, and ulti­
drates containing three to seven carbon mately controls all functions of the cell.
atoms. Glucose, with six carbon atoms, is DNA is found almost exclusively in the
the most prevalent simple sugar in the nucleus of the cell. A related substance,
body. Two simple sugar molecules may be RNA (ribonucleic acid), includes the
combined to form a disaccharide. Some sugar ribose combined with a base and a
common and important disaccharides are: phosphate. RNA is transcribed from DNA
sucrose, or table sugar (glucose + fructose); within the nucleus of the cell and eventually