Chapter 67 – Metabolism of Carbohydrates and
Formation of Adenosine Triphosphate
Most of the chemical reactions in the cell are aimed at making the energy in food available to
the various physiologic systems of the cell. E.g. energy is needed for muscle activity,
secretion by the glands, maintenance of membrane potentials by the nerve and absorption of
foods from the gastrointestinal tract. To provide the energy, the chemical reactions must be
“coupled” with the systems responsible for these physiologic functions. This coupling is
accomplished by special cellular enzyme and energy transfer systems. The amount of energy
liberated by complete oxidation of a food is called the free energy of oxidation of the food
(ΔG). Adenosine triphosphate (ATP) is an essential link between energy-utilizing and energy-
producing functions of the body. Energy derived from the oxidation of carbohydrates, fats and
proteins is used to convert adenosine diphosphate (ADP) to ATP, which is then consumed by
the various reactions of the body that are necessary for (1) active transport of molecules
across cell membranes, (2) contraction of muscles and performance of mechanical work, (3)
various synthetic reactions that create hormones, cell membranes and many other essential
molecules of the body, (4) conduction of nerve impulses, (5) cell division and growth, (6)
many other physiologic functions that are necessary to maintain and propagate life. ATP is a
combination of adenine, ribose and three phosphate radicals. The last two radicals are
connected with the remainder of the molecule by high-energy bonds. After loss of one
phosphate radical the compound becomes ADP, and after loss of the second one it becomes
adenosine monophosphate (AMP). ATP is present everywhere in the cytoplasm and
nucleoplasm of all cells, and essentially all the physiologic mechanisms that require energy
for operation obtain it directly from ATP. Glucose is the final common pathway for the
transport of almost all carbohydrates to the tissue cells (fructose and galactose are rapidly
converted to glucose in the liver as well).
Before glucose can be used by the body’s tissue cells, it must be transported through the tissue
cell membrane into the cellular cytoplasm. However, glucose cannot easily diffuse through
the pores of the cell membrane because of the maximum molecular weight. Does part with
facilitated diffusion: protein carrier molecules (glucose carrier proteins) bind with glucose
through the lipid matrix of the cell membrane. Glucose will be transported from the high-
concentrated area to the lower concentrated one. It transports through the gastrointestinal
membrane or the epithelium of the renal tubulus through active sodium-glucose co-transport;
the active transport of sodium provides energy for absorbing glucose against a concentration
difference.
The rate of glucose transport as well as transport of some other monosaccharides is greatly
increased by insulin. Large rates: 10x more glucose transport. Immediately on entry into cells,
glucose combines with a phosphate radical in accordance with the following reaction: glucose
+ ATP + glucokinase or hexokinase glucose-6-phosphate. This phosphorylation is
promoted mainly by the enzyme glucokinase in the liver and hexokinase in most other cells. Is
used to “capture” the cell; with phosphate binding it cannot leave the cell. Only in the liver,
renal tubular epithelial cells and intestinal epithelial cells this process is reversible with the
enzyme glucose phosphatase.
Formation of Adenosine Triphosphate
Most of the chemical reactions in the cell are aimed at making the energy in food available to
the various physiologic systems of the cell. E.g. energy is needed for muscle activity,
secretion by the glands, maintenance of membrane potentials by the nerve and absorption of
foods from the gastrointestinal tract. To provide the energy, the chemical reactions must be
“coupled” with the systems responsible for these physiologic functions. This coupling is
accomplished by special cellular enzyme and energy transfer systems. The amount of energy
liberated by complete oxidation of a food is called the free energy of oxidation of the food
(ΔG). Adenosine triphosphate (ATP) is an essential link between energy-utilizing and energy-
producing functions of the body. Energy derived from the oxidation of carbohydrates, fats and
proteins is used to convert adenosine diphosphate (ADP) to ATP, which is then consumed by
the various reactions of the body that are necessary for (1) active transport of molecules
across cell membranes, (2) contraction of muscles and performance of mechanical work, (3)
various synthetic reactions that create hormones, cell membranes and many other essential
molecules of the body, (4) conduction of nerve impulses, (5) cell division and growth, (6)
many other physiologic functions that are necessary to maintain and propagate life. ATP is a
combination of adenine, ribose and three phosphate radicals. The last two radicals are
connected with the remainder of the molecule by high-energy bonds. After loss of one
phosphate radical the compound becomes ADP, and after loss of the second one it becomes
adenosine monophosphate (AMP). ATP is present everywhere in the cytoplasm and
nucleoplasm of all cells, and essentially all the physiologic mechanisms that require energy
for operation obtain it directly from ATP. Glucose is the final common pathway for the
transport of almost all carbohydrates to the tissue cells (fructose and galactose are rapidly
converted to glucose in the liver as well).
Before glucose can be used by the body’s tissue cells, it must be transported through the tissue
cell membrane into the cellular cytoplasm. However, glucose cannot easily diffuse through
the pores of the cell membrane because of the maximum molecular weight. Does part with
facilitated diffusion: protein carrier molecules (glucose carrier proteins) bind with glucose
through the lipid matrix of the cell membrane. Glucose will be transported from the high-
concentrated area to the lower concentrated one. It transports through the gastrointestinal
membrane or the epithelium of the renal tubulus through active sodium-glucose co-transport;
the active transport of sodium provides energy for absorbing glucose against a concentration
difference.
The rate of glucose transport as well as transport of some other monosaccharides is greatly
increased by insulin. Large rates: 10x more glucose transport. Immediately on entry into cells,
glucose combines with a phosphate radical in accordance with the following reaction: glucose
+ ATP + glucokinase or hexokinase glucose-6-phosphate. This phosphorylation is
promoted mainly by the enzyme glucokinase in the liver and hexokinase in most other cells. Is
used to “capture” the cell; with phosphate binding it cannot leave the cell. Only in the liver,
renal tubular epithelial cells and intestinal epithelial cells this process is reversible with the
enzyme glucose phosphatase.
