46
UNIT 1
Organization of the Body
2
a solid. Fatty acids that contain one or more double bonds be-
tween carbon atoms are said to be
unsaturated
(
monounsatu-
rated
and
polyunsaturated
, respectively). Te double bonds
cause the fatty acid chains to kink so that they cannot be packed
closely enough to solidify. Hence, triglycerides with short fatty
acid chains or unsaturated fatty acids are oils (liquid at room
temperature) and are typical of plant lipids. Examples include
olive and peanut oils (rich in monounsaturated fats) and corn,
soybean, and sa±ower oils, which contain a high percentage of
polyunsaturated fatty acids. Longer fatty acid chains and more
saturated fatty acids are common in animal fats such as butterfat
and the fat of meats, which are solid at room temperature. Of
the two types of fatty acids, the unsaturated variety, especially
olive oil, is said to be more “heart healthy.”
most efficient and compact form of stored energy, and when
they are oxidized, they yield large amounts of energy.
²riglycerides are found mainly beneath the skin, where they
insulate the deeper body tissues from heat loss and protect them
from mechanical trauma. For example, women are usually more
successful English Channel swimmers than men. Teir success
is due partly to their thicker subcutaneous fatty layer, which
helps insulate them from the bitterly cold water of the Channel.
Te length of a triglyceride’s fatty acid chains and their degree
of
saturation
with H atoms determine how solid the molecule is
at a given temperature. Fatty acid chains with only single cova-
lent bonds between carbon atoms are referred to as
saturated
.
Teir fatty acid chains are straight and, at room temperature, the
molecules of a saturated fat are packed closely together, forming
Representative Lipids Found in the Body
LIPID TYPE
LOCATION/FUNCTION
Triglycerides (Neutral Fats)
 
Fat deposits (in subcutaneous tissue and around organs) protect and insulate body organs, and are the
major source of
stored
energy in the body.
Phospholipids
(phosphatidylcholine; cephalin; others)
 
Chief components of cell membranes. Participate in the transport of lipids in plasma. Prevalent in nervous
tissue.
Steroids
Cholesterol
The structural basis for manufacture of all body steroids. A component of cell membranes.
Bile salts
These breakdown products of cholesterol are released by the liver into the digestive tract, where they aid
fat digestion and absorption.
Vitamin D
A fat-soluble vitamin produced in the skin on exposure to UV radiation. Necessary for normal bone growth
and function.
Sex hormones
Estrogen and progesterone (female hormones) and testosterone (a male hormone) are produced in the
gonads. Necessary for normal reproductive function.
Adrenocortical hormones
Cortisol, a glucocorticoid, is a metabolic hormone necessary for maintaining normal blood glucose levels.
Aldosterone helps to regulate salt and water balance of the body by targeting the kidneys.
Other Lipoid Substances
Fat-soluble vitamins:
 
A
Ingested in orange-pigmented vegetables and fruits. Converted in the retina to retinal, a part of the
photoreceptor pigment involved in vision.
E
Ingested in plant products such as wheat germ and green leafy vegetables. Claims have been made (but
not proved in humans) that it promotes wound healing, contributes to fertility, and may help to neutralize
highly reactive particles called free radicals believed to be involved in triggering some types of cancer.
K
Made available to humans largely by the action of intestinal bacteria. Also prevalent in a wide variety of
foods. Necessary for proper clotting of blood.
Eicosanoids (prostaglandins;
leukotrienes; thromboxanes)
Group of molecules derived from fatty acids found in all cell membranes. The potent prostaglandins have
diverse effects, including stimulation of uterine contractions, regulation of blood pressure, control of
gastrointestinal tract motility, and secretory activity. Both prostaglandins and leukotrienes are involved in
inflammation. Thromboxanes are powerful vasoconstrictors.
Lipoproteins
Lipoid and protein-based substances that transport fatty acids and cholesterol in the bloodstream. Major
varieties are high-density lipoproteins (HDLs) and low-density lipoproteins (LDLs).
Table 2.2
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