Cambridge International A Level Biology Revision Guide

Chapter 12: Energy and respiration
great deal of chemical potential energy. When free oxygen
is available, some of this energy can be released via the
Krebs cycle and oxidative phosphorylation. However, the
pyruvate first enters the link reaction, which takes place in
the mitochondria (page 275).
The link reaction
Pyruvate passes by active transport from the
cytoplasm, through the outer and inner membranes of
a mitochondrion and into the mitochondrial matrix.
Here it is decarboxylated (this means that carbon dioxide
is removed), dehydrogenated (hydrogen is removed)
and combined with coenzyme A (CoA) to give acetyl
coenzyme A. This is known as the link reaction
(Figure 12.9). Coenzyme A is a complex molecule
composed of a nucleoside (adenine plus ribose) with a
vitamin (pantothenic acid), and acts as a carrier of acetyl
groups to the Krebs cycle. The hydrogen removed from
pyruvate is transferred to NAD.
pyruvate + CoA + NAD
acetyl CoA + CO 2
+ reduced NAD
Fatty acids from fat metabolism may also be used to
produce acetyl coenzyme A. Fatty acids are broken down
in the mitochondrion in a cycle of reactions in which
each turn of the cycle shortens the fatty acid chain by
a two-carbon acetyl unit. Each of these can react with
coenzyme A to produce acetyl coenzyme A, which, like
that produced from pyruvate, now enters the Krebs cycle.
The Krebs cycle
The Krebs cycle (also known as the citric acid cycle or
tricarboxylic acid cycle) was discovered in 1937 by Hans
Krebs. It is shown in Figure 12.9.
The Krebs cycle is a closed pathway of enzymecontrolled
reactions.
■■
■■
■■
Acetyl coenzyme A combines with a four-carbon
compound (oxaloacetate) to form a six-carbon
compound (citrate).
The citrate is decarboxylated and dehydrogenated in a
series of steps, to yield carbon dioxide, which is given
off as a waste gas, and hydrogens which are accepted by
the carriers NAD and FAD (page 275).
Oxaloacetate is regenerated to combine with another
acetyl coenzyme A.
For each turn of the cycle, two carbon dioxide molecules
are produced, one FAD and three NAD molecules are
reduced, and one ATP molecule is generated via an
intermediate compound.
Although part of aerobic respiration, the reactions
link
reaction
oxaloacetate (4C)
(4C)
(4C)
reduced
FAD
ATP
ADP
reduced
NAD
NAD
(4C)
pyruvate (3C)
NAD
CO 2
reduced
NAD
acetyl (2C) CoA
FAD
Krebs cycle
(4C)
reduced
NAD
NAD
reduced
NAD
NAD
Figure 12.9 The link reaction and the Krebs cycle.
CoA
of the Krebs cycle make no use of molecular oxygen.
However, oxygen is necessary for the final stage of aerobic
respiration, which is called oxidative phosphorylation.
The most important contribution of the Krebs cycle to
the cell’s energetics is the release of hydrogens, which can
be used in oxidative phosphorylation to provide energy to
make ATP.
QUESTION
citrate (6C)
(6C)
(5C)
12.2 Explain how the events of the Krebs cycle can be
cyclical.
CO 2
CO 2
Oxidative phosphorylation and the
electron transport chain
In the final stage of aerobic respiration, oxidative
phosphorylation, the energy for the phosphorylation
of ADP to ATP comes from the activity of the electron
transport chain. Oxidative phosphorylation takes place in
the inner mitochondrial membrane (Figure 12.10).
273