Maintenance of the Body
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.
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,
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
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-
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
stored as protein. Instead, they are
oxidized for energy or converted to fat or glycogen for storage.
Check Your Understanding
What does the liver use as its substrates when it synthesizes
nonessential amino acids?
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
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
, that is, its organic molecules are continuously bro-
ken down and rebuilt—frequently at a head-spinning rate.
Te body draws on its
—stores of amino acids,
carbohydrates, and fats—to meet its varying needs
Excreted in urine
Triglycerides (neutral fats)
Glycerol and fatty acids
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.