914
UNIT 4
Maintenance of the Body
24
Stage 2
occurs in the tissue cells. Newly delivered nutrients are
either built into lipids, proteins, and glycogen by anabolic path-
ways or broken down by catabolic pathways to
pyruvic acid
(pi-
roo
9
vik) and
acetyl CoA
(as
9
ĕ-til ko-a
9
) in the cell cytoplasm.
Notice in Figure 24.3 that a major catabolic pathway of stage 2
is
glycolysis
, which we will discuss later in this chapter.
Stage 3
, which occurs in the mitochondria, is almost entirely
catabolic. It requires oxygen, and completes the breakdown
of foods, producing carbon dioxide and water and harvesting
large amounts of ATP. As Figure 24.3 shows, the
Krebs cycle
and
oxidative phosphorylation
are key pathways in stage 3.
We will discuss both later.
±e primary function of
cellular respiration
, which consists
of the glycolysis of stage 2 and all events of stage 3, is to generate
ATP, which traps some of the chemical energy of the original
food molecules in its own high-energy bonds. ±e body can
ATP
ATP
ATP
Stage 1
Digestion in GI tract
lumen to absorbable forms.
Transport via blood to
tissue cells.
Stage 2
Anabolism (incorporation
into molecules) and catabolism of
nutrients to form intermediates
within tissue cells.
Stage 3
Oxidative breakdown of
stage 2 products occurs in mitochondria
of tissue cells. CO
2
is liberated, and
H atoms removed are ultimately
delivered to molecular oxygen, forming
water. Some energy released is
used to form ATP.
Catabolic reactions
Anabolic reactions
Glycogen
PROTEINS
Proteins
Fats
CARBOHYDRATES
Glucose
FATS
Amino acids
Glucose and other sugars
Glycerol
Fatty acids
Pyruvic acid
Glycolysis
Acetyl CoA
Infrequent
CO
2
NH
3
H
Krebs
cycle
Oxidative phosphorylation
(in electron transport chain)
O
2
H
2
O
Figure 24.3
Three stages of metabolism of energy-containing nutrients.
also store energy in fuels, such as glycogen and fats, and mobi-
lize these stores later to produce ATP for cellular use.
You do not need to memorize Figure 24.3, but you may want
to refer to it o²en as a cohesive summary of nutrient processing
and metabolism in the body.
Oxidation-Reduction Reactions
and the Role of Coenzymes
Many of the reactions that take place within cells are
oxidation
reactions
.
Oxidation
was originally defined as the combina-
tion of oxygen with other elements, seen in the rusting of iron
(the slow formation of iron oxide) and the burning of wood. In
burning, oxygen combines rapidly with carbon, producing car-
bon dioxide, water, and an enormous amount of energy, which
is liberated as heat and light.
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