Chapter 24
Nutrition, Metabolism, and Body Temperature Regulation
913
24
7.
Which mineral is essential for thyroxine synthesis? For
making bones hard? For hemoglobin synthesis?
For answers, see Appendix H.
Overview of Metabolic
Reactions
Define metabolism. Explain how catabolism and anabolism
differ.
Define oxidation and reduction and indicate the
importance of these reactions in metabolism.
Indicate the role of coenzymes used in cellular oxidation
reactions.
Explain the difference between substrate-level
phosphorylation and oxidative phosphorylation.
Once inside body cells, nutrients become involved in an in-
credible variety of biochemical reactions known collectively
as
metabolism
(
metabol
5
change). During metabolism, sub-
stances are constantly built up and torn down. Cells use energy
to extract more energy from foods, and then use some of this
extracted energy to drive their activities. Even at rest, the body
uses energy on a grand scale.
Anabolism and Catabolism
Metabolic processes are either
anabolic
(synthetic, building up) or
catabolic
(degradative, tearing down).
Anabolism
(ah-nab
9
o-lizm)
is the general term for all reactions that build larger molecules
or structures from smaller ones, such as the bonding together of
amino acids to build proteins.
Catabolism
(kah-tab
9
o-lizm) refers
to all processes that break down complex structures to simpler
ones—for example, the hydrolysis of foods in the digestive tract.
In the group of catabolic reactions collectively called
cellular
respiration
, food fuels, particularly glucose, are broken down
in cells and some of the energy released is captured to form
ATP, the cells’ energy currency. ATP then serves as the “chemi-
cal drive shaF” that links energy-releasing catabolic reactions to
cellular work.
Recall from Chapter 2 that reactions driven by ATP are cou-
pled. ATP is never hydrolyzed directly. Instead enzymes shiF its
high-energy phosphate groups to other molecules, which are
then said to be
phosphorylated
(fos
0
for
9
ĭ-la-ted). Phosphor-
ylation primes a molecule to change in a way that increases its
activity, produces motion, or does work. ±or example, phos-
phorylation activates many regulatory enzymes that catalyze
key steps in metabolic pathways.
²ree major stages are involved in processing energy-
containing nutrients in the body
(Figure 24.3)
. (Note that
the blue arrows in the figure represent catabolic reactions and
the purple arrows represent anabolic reactions.)
Stage 1
is digestion in the gastrointestinal tract, which we
described in Chapter 23. ²e absorbed nutrients are then
transported in blood to the tissue cells.
still murky, but chemists propose that, much like a bucket
brigade, they pass the dangerous free electron from one mol-
ecule to the next, until a chemical such as glutathione finally
absorbs it and the body flushes it out in urine. Broccoli, cab-
bage, cauliflower, and brussels sprouts are all good sources of
vitamins A and C.
²e notion that megadoses of vitamin supplements are the
road to eternal youth and glowing health is useless at best—and
at worst, may cause serious health problems, particularly in the
case of fat-soluble vitamins.
Table 24.2
contains an overview of
the roles of vitamins in the body.
Minerals
List minerals essential for health.
Indicate important dietary sources of minerals and describe
how each is used.
²e body requires moderate amounts of seven
minerals
(calcium,
phosphorus, potassium, sulfur, sodium, chlorine, magnesium)
and trace amounts of about a dozen others
(Table 24.3)
. Miner-
als make up about 4% of the body by weight, with calcium and
phosphorus (as bone salts) accounting for about three-quarters of
this amount.
Minerals, like vitamins, are not used for fuel but work with
other nutrients to ensure a smoothly functioning body. Incor-
porating minerals into structures makes them stronger. ±or ex-
ample, calcium, phosphorus, and magnesium salts harden the
teeth and strengthen the skeleton.
Most minerals are ionized in body fluids or bound to or-
ganic compounds to form phospholipids, hormones, and vari-
ous functional proteins. ±or example, iron is essential to the
oxygen-binding heme of hemoglobin, and sodium and chloride
ions are the major electrolytes in blood. ²e amount of a partic-
ular mineral in the body gives very few clues to its importance
in body function. ±or example, just a few milligrams of iodine
(required for thyroid hormone synthesis) can make a critical
difference to health.
A fine balance between uptake and excretion is crucial for
retaining needed amounts of minerals while preventing toxic
overload. Sodium present in virtually all natural and mini-
mally processed foods poses little or no health risk. However,
the large amounts added to processed foods and sprinkled on
prior to eating may contribute to fluid retention and high blood
pressure. ²is is particularly true in American blacks, whose
kidneys have a greater tendency to retain salt than do those of
American whites.
±ats and sugars are practically devoid of minerals, and highly
refined cereals and grains are poor sources. ²e most mineral-
rich foods are vegetables, legumes, milk, and some meats.
Check Your Understanding
5.
Vitamins are not used for energy fuels. What are they
used for?
6.
Which B vitamin requires the help of a product made in the
stomach to be absorbed? What is that gastric product?
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