918
UNIT 4
Maintenance of the Body
24
roles in glucose uptake
and
release. Te catabolic and anabolic
pathways for carbohydrates all begin with glucose-6-phosphate.
Oxidation of Glucose
Glucose is the pivotal fuel molecule in the oxidative (A±P-
producing) pathways. Glucose is catabolized via the reaction
C
6
H
12
O
6
1
6O
2
S
6H
2
O
1
6CO
2
1
32 A±P
1
heat
glucose
oxygen
water
carbon
dioxide
Tis equation gives few hints that glucose breakdown is com-
plex and involves three of the pathways featured in Figures 24.3
and 24.5:
1. Glycolysis (color-coded orange throughout the chapter)
2. Te Krebs cycle (color-coded green)
3. Te electron transport chain and oxidative phosphoryla-
tion (color-coded lavender)
Tese metabolic pathways occur in a definite order, and we will
consider them sequentially.
Glycolysis
Also called the
glycolytic pathway
,
glycolysis
(gli-kol
9
ĭ-sis; “sugar splitting”) occurs in the cytosol of cells.
Tis pathway, a series of ten chemical steps, converts glucose to
two
pyruvic acid
molecules. All steps are fully reversible except
the first, during which glucose entering the cell is phosphor-
ylated to glucose-6-phosphate.
Glycolysis is an
anaerobic process
(an-a
9
er-ōb-ik;
an
5
with-
out,
aero
5
air). Although this term is sometimes mistakenly
interpreted to mean the pathway occurs only in the absence of
oxygen, it actually means that glycolysis
does not use oxygen and
occurs whether or not oxygen is present.
Te three major phases of the glycolytic pathway shown in
Figure 24.6
are described below. Appendix D shows the com-
plete glycolytic pathway.
Phase 1. Sugar activation.
In phase 1, glucose is phosphor-
ylated and converted to fructose-6-phosphate, which is then
phosphorylated again. Tese three steps use two A±P mol-
ecules (which are recouped later) and yield fructose-1,6-
bisphosphate. Te two separate reactions of the sugar with
A±P provide the
activation energy
needed to prime the later
stages of the pathway, so phase 1 is sometimes called the
energy
investment phase
. (Recall the importance of activation energy
in preparing substances to react, as described in Chapter 2.)
Phase 2. Sugar cleavage.
During phase 2, fructose-1,6-
bisphosphate is split into two 3-carbon fragments that exist
(reversibly) as one of two isomers: glyceraldehyde (glis
0
er-
al
9
dĕ-hīd) 3-phosphate or dihydroxyacetone (dī
0
hi-drok
0
se-
as
9
ĕ-tōn) phosphate.
Phase 3. Sugar oxidation and ATP formation.
In phase 3, ac-
tually consisting of six steps, two major events happen. First,
the two 3-carbon fragments are oxidized by the removal of
hydrogen, which NAD
1
picks up. In this way, some of glu-
cose’s energy is transferred to NAD
1
. Second, inorganic phos-
phate groups (P
i
) are attached to each oxidized fragment by
high-energy bonds. Later, when these terminal phosphates
glucose-6-phosphate is a
different
molecule from simple glucose,
the reaction also keeps intracellular glucose levels low, maintaining
a concentration gradient for glucose entry. Only intestinal mucosa
cells, kidney tubule cells, and liver cells have the enzymes needed to
reverse this phosphorylation reaction, which reflects their central
Carbon atom
Phosphate
Glycolysis
ATP
ATP
ATP
Electron trans-
port chain
and oxidative
phosphorylation
Krebs
cycle
2 ATP
4 ATP
To Krebs
cycle
(aerobic
pathway)
P
P
P
P
P
i
2
2
2
Glucose
4 ADP
2 NAD
+
2 NAD
+
2 Lactic acid
NADH+H
+
NADH+H
+
2 Pyruvic acid
Phase 1
Sugar
activation:
Phosphorylation
activates glucose.
Glucose is
converted to
fructose-1,
6-bisphosphate.
Fructose-1,6-
bisphosphate
Dihydroxyacetone
phosphate
Glyceraldehyde
3-phosphate
Phase 2
Sugar
cleavage:
Fructose-1,
6-bisphosphate
is cleaved into
two 3-carbon
fragments.
Phase 3
Sugar oxidation
and ATP formation:
The 3-carbon
fragments are
oxidized (by
removing
hydrogen) and
4 ATP molecules
are formed.
2 ADP
O
2
O
2
Figure 24.6
The three major phases of glycolysis.
The fate of
pyruvic acid depends on whether or not molecular O
2
is available.
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