Chapter 24
Nutrition, Metabolism, and Body Temperature Regulation
917
24
10.
What is the energy source for the proton pumps of oxidative
phosphorylation?
For answers, see Appendix H.
Metabolism of Major Nutrients
Carbohydrate Metabolism
Summarize important events and products of glycolysis, the
Krebs cycle, and electron transport.
Define glycogenesis, glycogenolysis, and gluconeogenesis.
Te story of carbohydrate metabolism is really a tale of glu-
cose metabolism because all food carbohydrates are eventually
transformed to glucose. Glucose enters tissue cells by facilitated
diffusion, a process that is greatly enhanced by insulin. Imme-
diately aFer entering a cell, glucose is phosphorylated to
glucose-
6-phosphate
by transfer of a phosphate group to its sixth carbon
during a coupled reaction with A±P:
Glucose
1
A±P
S
glucose-6-PO
4
1
ADP
Most body cells lack the enzymes needed to reverse this re-
action, so it effectively traps glucose inside the cells. Because
substrate-level phosphorylations are located in both the cytosol
(where glycolysis occurs) and in the watery matrix inside the mi-
tochondria (where the Krebs cycle takes place)
(Figure 24.5)
.
Oxidative phosphorylation
is more complicated, but it also re-
leases most of the energy that is eventually captured in A±P bonds
during cellular respiration. Oxidative phosphorylation, which is
carried out by electron transport proteins forming part of the in-
ner mitochondrial membranes, is an example of a chemiosmotic
process.
Chemiosmotic processes
couple the movement of sub-
stances across membranes to chemical reactions.
In this case, some of the energy released during the oxida-
tion of food fuels (the “chemi” part of chemiosmotic) is used to
pump (
osmo
5
push) protons (H
1
) across the inner mitochon-
drial membrane into the intermembrane space (²igure 24.4b).
Tis creates a steep concentration gradient for protons across
the membrane. Ten, when H
1
flows back across the membrane
(through a membrane channel protein called
ATP synthase
),
some of this gradient energy is captured and used to attach phos-
phate groups to ADP.
Check Your Understanding
8.
What is a redox reaction?
9.
How are anabolism and catabolism linked by ATP?
Via oxidative
phosphorylation
Via substrate-level
phosphorylation
Mitochondrion
Inner
mitochondrial
membrane (cristae)
Cytosol
Krebs
cycle
Glucose
Glycolysis
Pyruvic
acid
Electron transport
chain and oxidative
phosphorylation
Chemical energy (high-energy electrons)
Chemical energy
ATP
ATP
ATP
1
Glycolysis, in the cytosol, breaks
down each glucose molecule into
two molecules of pyruvic acid.
2
The pyruvic acid then enters
the mitochondrial matrix, where
the Krebs cycle decomposes it to
CO
2
.
During glycolysis and the
Krebs cycle, substrate-level
phosphorylation forms small
amounts of ATP.
3
Energy-rich electrons picked up
by coenzymes are transferred to
the electron transport chain, built
into the cristae membrane. The
electron transport chain carries out
oxidative phosphorylation, which
accounts for most of the ATP
generated by cellular respiration.
Figure 24.5
During cellular respiration, ATP is formed in the cytosol and in the
mitochondria.
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