930
UNIT 4
Maintenance of the Body
24
Tese pools are interconvertible because their pathways are linked
by key intermediates. Te liver, adipose tissue, and skeletal muscles
are the primary effector organs or tissues determining the amounts
and direction of the conversions shown in the figure.
Te
amino acid pool
is the body’s total supply of free amino
acids. Our bodies lose small amounts of amino acids and pro-
teins daily in urine and in sloughed hairs and skin cells. ±ypi-
cally, we replace these lost molecules via our diet. Otherwise,
amino acids arising from tissue breakdown return to the pool.
Tis pool is the source of amino acids used to synthesize pro-
teins and form amino acid derivatives. In addition, as we de-
scribed above, gluconeogenesis can convert deaminated amino
acids to glucose. Not all events of amino acid metabolism occur
in all cells. For example,
only
the liver forms urea. Nonetheless,
the concept of a common amino acid pool is valid because all
cells are connected by the blood.
Because carbohydrates are easily and frequently converted to
fats, the
carbohydrate
and
fat pools
are usually considered to-
gether. Tere are two major differences between the carbohydrate-
fat pool and the amino acid pool:
Fats and carbohydrates are oxidized directly to produce cel-
lular energy, whereas amino acids can be used to supply en-
ergy
only afer being converted to a carbohydrate intermediate
(a keto acid).
Excess carbohydrate and fat can be stored as such, whereas
excess amino acids are
not
stored as protein. Instead, they are
oxidized for energy or converted to fat or glycogen for storage.
Check Your Understanding
19.
What does the liver use as its substrates when it synthesizes
nonessential amino acids?
20.
What happens to the ammonia removed from amino acids
when they are used for energy fuel?
For answers, see Appendix H.
Metabolic States of the Body
Explain the concept of amino acid or carbohydrate-fat
pools, and describe pathways by which substances in these
pools can be interconverted.
Summarize important events of the absorptive and
postabsorptive states, and explain how these events are
regulated.
Catabolic-Anabolic Steady State
of the Body
Now that we have examined metabolism at cellular levels, let’s
step back to look at the body as a system that deploys metabolic
processes to provide fuel. Your body exists in a
dynamic catabolic-
anabolic state
, that is, its organic molecules are continuously bro-
ken down and rebuilt—frequently at a head-spinning rate.
Te body draws on its
nutrient pools
—stores of amino acids,
carbohydrates, and fats—to meet its varying needs
(Figure 24.17)
.
Proteins
Proteins
Carbohydrates
Fats
Excreted in urine
Glycogen
Glucose
Glucose-6-phosphate
Glyceraldehyde 3-phosphate
Pyruvic acid
Acetyl CoA
Amino acids
Keto acids
Triglycerides (neutral fats)
Lactic acid
Ketone
bodies
Glycerol and fatty acids
NH
3
Krebs
cycle
Urea
Figure 24.17
Interconversion of carbohydrates, fats, and proteins.
The liver, adipose
tissue, and skeletal muscles are the primary effectors determining the amounts and direction of
the conversions shown from one nutrient pool to another.
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