Maintenance of the Body
water, and the energy released during those reactions is har-
nessed to attach P
groups to ADP, forming ATP. As we noted
earlier, this type of phosphorylation process is called
. Let’s peek under the hood of a cell’s power
plant and see how this rather complicated process works.
Most components of the electron transport chain are proteins
that are bound to metal atoms (known as
). ±ese pro-
teins vary in composition and form multiprotein complexes that
are ﬁrmly embedded in the inner mitochondrial membrane as
Focus on Oxidative Phosphorylation
Electron Transport Chain and Oxidative Phosphorylation
Like glycolysis, none of the reactions of the Krebs cycle use
oxygen directly. ±is is the exclusive function of the
, which carries out the ﬁnal catabolic reactions
that occur on the mitochondrial cristae. However, because the
reduced coenzymes produced in the Krebs cycle are the sub-
strates for the electron transport chain, these two pathways are
coupled, and both are
, meaning they require oxygen.
In the electron transport chain, the hydrogens removed dur-
ing the oxidation of food fuels are combined with O
Pyruvic acid from glycolysis
Simpliﬁed version of the Krebs (citric acid) cycle.
each turn of the cycle, two carbon atoms are removed from the substrates
(decarboxylation reactions); four oxidations by removal of hydrogen
atoms occur, producing four molecules of reduced coenzymes (3 NADH
and 1 FADH
); and one ATP is synthesized by substrate-level phosphorylation.
An additional decarboxylation and an oxidation reaction occur in the transi-
tional phase (at top) that converts pyruvic acid, the product of glycolysis, to
acetyl CoA, the molecule that enters the Krebs cycle pathway.