CHAPTER
13
Carbohydrate Metabolism I:
Glycolysis and TCA Cycle
Carbohydrates are metabolized by several metabolic path-
ways, each with different functions. Although these path-
ways usually begin with glucose, other sugars may en-
ter a pathway via appropriate intermediates. Glucose can
be stored as glycogen (glycogenesis), which, in turn, can
be broken down to glucose (glycogenolysis), synthesized
from noncarbohydrate sources (gluconeogenesis), con-
verted to nonessential amino acids, used in the formation
of other carbohydrates or their derivatives (e.g., pentoses,
hexoses, uronic acids) and other noncarbohydrate metabo-
lites, converted to fatty acids (the reverse process does not
occur in humans) and stored as triacylglycerols, used in the
biosynthesis of glycoconjugates (e.g., glycoproteins, gly-
colipids, proteoglycans), or catabolized to provide energy
for cellular function (glycolysis, tricarboxylic acid cycle,
and electron transport and oxidative phosphorylation).
13.1
Glycolysis
Glycolysis is common to most organisms and in humans
occurs in virtually all tissues. Ten reactions culminate in
formation of two pyruvate molecules from each glucose
molecule. All 10 reactions occur in the cytoplasm and
are anaerobic. In cells that lack mitochondria (e.g., ery-
throcytes) and in cells that contain mitochondria but un-
der limiting conditions of oxygen (e.g., skeletal muscle
during heavy exercise), the end product is lactate. Under
aerobic conditions in cells that contain mitochondria (i.e.,
most cells of the body), pyruvate enters the mitochon-
dria, where it is oxidized to acetyl-coezyme A (acetyl-
CoA), which is then oxidized through the tricarboxylic
acid (TCA) cycle. Thus, the pathway is the same in the
presence or absence of oxygen, except for the end product
formed.
Source and Entry of Glucose into Cells
Glucose can be derived from exogenous sources by as-
similation and transport of dietary glucose (Chapter 12)
or from endogenous sources by glycogenolysis or gluco-
neogenesis (Chapter 13). The blood circulation transports
glucose between tissues (e.g., from intestine to liver, from
liver to muscle). Control and integration of this transport
are discussed in Chapter 22. Glucose transport across cell
membranes can occur by carrier-mediated active transport
or by a concentration gradient-dependent facilitated trans-
port that requires a specific carrier. The latter type can
be either insulin-independent or insulin-dependent. The
active transport system is Na+-dependent and occurs in
intestinal epithelial cells (Chapter 12) and epithelial cells
of the renal tubule (Chapter 39).
The properties of glucose transporter proteins (GLUT)
consist of tissue specificity and differences in func-
tional properties reflected in their glucose metabolism.
Five transporter proteins, Glut 1-5, have been identified
225
