Nutrition, Metabolism, and Body Temperature Regulation
What is the energy source for the proton pumps of oxidative
For answers, see Appendix H.
Metabolism of Major Nutrients
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
by transfer of a phosphate group to its sixth carbon
during a coupled reaction with A±P:
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)
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
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
push) protons (H
) 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
ﬂows back across the membrane
(through a membrane channel protein called
some of this gradient energy is captured and used to attach phos-
phate groups to ADP.
Check Your Understanding
What is a redox reaction?
How are anabolism and catabolism linked by ATP?
chain and oxidative
Chemical energy (high-energy electrons)
Glycolysis, in the cytosol, breaks
down each glucose molecule into
two molecules of pyruvic acid.
The pyruvic acid then enters
the mitochondrial matrix, where
the Krebs cycle decomposes it to
During glycolysis and the
Krebs cycle, substrate-level
phosphorylation forms small
amounts of ATP.
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.
During cellular respiration, ATP is formed in the cytosol and in the