Metabolism
Metabolism refers to the physical and chemical
processes that occur inside the cells of the body and
that maintain life. Metabolism consists of anabolism
(the constructive phase) and catabolism (the destruc-
tive phase, in which complex materials are broken
down). The transformation of themacronutrients,
carbohydrates,fats, and proteins in food to energy,
and other physiological processes are parts of the
metabolic process. ATP (adinosene triphosphate) is
the major form of energy used for cellular metabolism.Carbohydrate Metabolism
Carbohydrates made up of carbon, hydrogen, and
oxygen atoms are classified as mono-, di-, and poly-
saccharides, depending on the number of sugar units
they contain. The monosaccharides—glucose, galac-
tose, and fructose—obtained from the digestion of
food are transported from the intestinal mucosa via
the portal vein to the liver. They may be utilized
directly for energy by all tissues; temporarily stored
as glycogen in the liver or in muscle; or converted to
fat, amino acids, and other biological compounds.
Carbohydrate metabolism plays an important
role in both types ofdiabetes mellitus. The entry of
glucose into most tissues—including heart, muscle,
and adipose tissue—is dependent upon the presence
of the hormone insulin. Insulin controls the uptake
and metabolism of glucose in these cells and plays a
major role in regulating the blood glucose concentra-
tion. The reactions of carbohydrate metabolism can-
not take place without the presence of the Bvitamins,
which function as coenzymes. Phosphorous,magne-
sium,iron, copper,manganese,zinc, and chromium
are also necessary as cofactors.
Carbohydrate metabolism begins withglycolysis,
which releases energy from glucose or glycogen to form
two molecules of pyruvate, which enter the Krebs cycle
(or citric acid cycle), an oxygen-requiring process,
through which they are completely oxidized. Before
the Krebs cycle can begin, pyruvate loses a carbon
dioxide group to form acetyl coenzyme A (acetyl-
CoA). This reaction is irreversible and has important
metabolic consequences. The conversion of pyruvate to
acetyl-CoA requires the B vitamins.
The hydrogen in carbohydrate is carried to the
electron transport chain, where the energy is con-
served in ATP molecules. Metabolism of one molecule
of glucose yields thirty-one molecules of ATP. The
energy released from ATP through hydrolysis (a
chemical reaction withwater) can then be used for
biological work.Only a few cells, such as liver and kidney cells, can
produce their own glucose from amino acids, and only
liver and muscle cells store glucose in the form of
glycogen. Other body cells must obtain glucose from
the bloodstream.
Under anerobic conditions, lactate is formed from
pyruvate. This reaction is important in the muscle
when energy demands exceed oxygen supply. Glycol-
ysis occurs in the cytosol (fluid portion) of a cell and
has a dual role. It degrades monosaccharides to gen-
erate energy, and it provides glycerol for triglyceride
synthesis. The Krebs cycle and the electron transportKEY TERMS
Adipose tissue—Tissue containing fat deposits.
Anaerobic—Without air, or oxygen.
Deamination—Removal of an NH2 group from a
molecule.
Galactosemia—Inherited disorder preventing
digestion of milk sugar, galactose.
Glucose—A simple sugar; the most commonly
used fuel in cells.
Glycogen—Storage form of sugar.
Glycolysis—Cellular reaction that begins the
breakdown of sugars.
Ketones—Chemicals produced by fat breakdown;
molecule containing a double-bonded oxygen
linked to two carbons.
Ketosis—Build-up of ketone bodies in the blood,
due to fat breakdown.
Krebs cycle—Cellular reaction that breaks down
numerous nutrients and provides building blocks
for other molecules.
Lipoprotein—Blood protein that carries fats.
Mitochondria—Small bodies within a cell that har-
vest energy for use by the cell.
Oxidative—Related to chemical reaction with oxy-
gen or oxygen-containing compounds.
phenylketonuria—Inherited disease marked by the
inability to process the amino acid phenylalanine,
causing mental retardation.
Phospholipid—A type of fat used to build cell
membranes.
Sterol—Building blocks of steroid hormones; a
type of lipid.
Triglyceride—A type of fat.Metabolism