■ The remnants of digestion (feces) contain dead cells, undi-
gested/unabsorbed material, the breakdown products of
bilirubin, and gases (carbon dioxide, methane). Feces are
excreted from the body through the anus in a coordinated
series of muscle contractions involving the rectum, rectal
sphincters, diaphragm, and abdominal muscles.
Nutrients Are Metabolized in a Number of
Ways 418
Your Diet Contains Many Nutrients 416
• Your diet consists of water and five types of nutrients: car-
bohydrates, lipids, proteins, minerals, and vitamins. Many
experts recommend that dietary calories be distributed as
follows: 50-60% from carbohydrates, 30% or less from fats,
and 12-15% from proteins. As shown, the MyPyramid guide
from the USDA represents a personal approach to diet and
encourages exercise.
T he U SD A M yPyram id nutritional g u id elin es
• Figure 1 4 .1 0
■ Metabolism refers to all of the chemical reactions in the body.
Most chemical reactions are catalyzed by enzymes, many of
which work together with coenzymes. Metabolic reactions
include synthesis reactions (anabolic reactions, or anabolism)
and decomposition reactions (catabolic reactions, or ca-
tabolism). In anabolic reactions, which usually require energy,
simple organic molecules are combined to make more com-
plex ones. In catabolic reactions, which usually release energy,
complex organic molecules are broken down into simpler
ones. Of the energy released in catabolic reactions, only 40%
captured as ATP. The rest is released as heat.
■ The main carbohydrate is glucose, which is oxidized into
carbon dioxide and water by glycolysis, the formation of
acetyl CoA, the Krebs cycle, and oxidative phosphorylation,
as shown below. These processes take place in the cytosol
and mitochondria and produce 36 ATPs per glucose molecule.
Glucose can also be stored as glycogen in the liver and skeletal
muscle or converted to triglycerides in adipose tissue. When
blood glucose is low, glucose can be mobilized from glycogen
(glycogenolysis) or made from other molecules by gluconeo-
• Absorbed amino acids are either made into proteins or
metabolized. The amino groups are removed (deamination),
and many become intermediates of the Krebs cycle and are
subsequently metabolized to make ATP.
S T Ü i L i r - ’
l i
i !
- i —*
t M * o r
m i <ti
2 Vi cup» rmry day
Cellular respiration • Figure 1 4 .1 2
in cytosol
■ Minerals have many important functions
in the structure of bone, regulation of
enzymatic functions, maintenance of
body fluids pH, regulation of osmosis,
and generation of action potentials in
nerves and muscles. Important minerals
include calcium, magnesium, phospho-
rus, potassium, sodium, iron, manga-
nese, copper, and zinc.
■ Vitamins are organic nutrients that are
required in small amounts to maintain
normal growth and metabolism. Most
vitamins are cofactors or coenzymes.
They are classified as fat-soluble
vitamins (vitamins A, D, E, K) or water-
soluble vitamins (vitamins B and C).
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