Cell Respiration and Metabolism 127Summary
D. The formation of glycogen from glucose is called
glycogenesis; the breakdown of glycogen is called
glycogenolysis.
1. Glycogenolysis yields glucose 6-phosphate, which can
enter the pathway of glycolysis.
2. The liver contains an enzyme (which skeletal muscles
do not) that can produce free glucose from glucose
6-phosphate. Thus, the liver can secrete glucose derived
from glycogen.
E. Carbohydrate metabolism is influenced by the availability of
oxygen and by a negative feedback effect of ATP on
glycolysis and the citric acid cycle.5.3 Interconversion of Glucose, Lactic Acid,
and Glycogen 117
A. Glycogen can be converted into free glucose by the liver
because the liver has glucose 6-phosphatase; this allows the
liver to secrete glucose into the blood.
B. Lactic acid produced by skeletal muscles during anaerobic
metabolism can travel through the blood to the liver, where it
is converted into glucose that can travel back to the muscles
to replenish their glycogen in the Cori cycle.5.4 Metabolism of Lipids and Proteins 119
A. In lipolysis, triglycerides yield glycerol and fatty acids.
1. Glycerol can be converted to phosphoglyceraldehyde
and used for energy.
2. In the process of b -oxidation of fatty acids, a number of
acetyl CoA molecules are produced.
3. Processes that operate in the reverse direction can
convert glucose to triglycerides.
B. Amino acids derived from the hydrolysis of proteins can
serve as sources of energy.
1. Through transamination, a particular amino acid and a
particular keto acid (pyruvic acid or one of the Krebs
cycle acids) can serve as substrates to form a new amino
acid and a new keto acid.
2. In oxidative deamination, amino acids are converted
into keto acids as their amino group is incorporated
into urea.
C. Each organ uses certain blood-borne energy carriers as its
preferred energy source.
1. The brain has an almost absolute requirement for blood
glucose as its energy source.
2. During exercise, the needs of skeletal muscles for
blood glucose can be met by glycogenolysis and by
gluconeogenesis in the liver.5.1 Glycolysis and the Lactic Acid Pathway 107
A. Glycolysis refers to the conversion of glucose to 2 molecules
of pyruvic acid.
- In the process, 2 molecules of ATP are consumed and
4 molecules of ATP are formed. Thus, there is a net gain
of 2 ATP. - In the steps of glycolysis, two pairs of hydrogens are
released. Electrons from these hydrogens reduce
2 molecules of NAD.
B. When metabolism is anaerobic, reduced NAD is oxidized
by pyruvic acid, which accepts two hydrogen atoms and is
thereby reduced to lactic acid. - Skeletal muscles produce lactic acid during exercise.
- Heart muscle undergoes lactic acid fermentation for just
a short time, under conditions of ischemia.
5.2 Aerobic Respiration 111
A. The citric acid cycle begins when coenzyme A donates ace-
tic acid to an enzyme that adds it to oxaloacetic acid to form
citric acid.
- Acetyl CoA is formed from pyruvic acid by the removal
of carbon dioxide and 2 hydrogens. - The formation of citric acid begins a cyclic pathway that
ultimately forms a new molecule of oxaloacetic acid. - As the citric acid cycle progresses, 1 molecule of ATP
is formed, and 3 molecules of NAD and 1 of FAD are
reduced by hydrogens from the citric acid cycle.
B. Reduced NAD and FAD donate their electrons to an
electron-transport chain of molecules located in the cristae. - The electrons from NAD and FAD are passed from one
cytochrome of the electron-transport chain to the next in
a series of coupled oxidation-reduction reactions. - As each cytochrome ion gains an electron, it
becomes reduced; as it passes the electron to the next
cytochrome, it becomes oxidized. - The last cytochrome becomes oxidized by donating its
electron to oxygen, which functions as the final electron
acceptor. - When 1 oxygen atom accepts 2 electrons and 2 protons,
it becomes reduced to form water. - The energy provided by electron transport is used to
form ATP from ADP and P i in the process known as
oxidative phosphorylation.
C. Thirty to 32 molecules of ATP are produced by the aerobic
respiration of 1 glucose molecule. Of these, 2 are produced
in the cytoplasm by glycolysis and the remainder are pro-
duced in the mitochondria.