Cambridge International A Level Biology Revision Guide

Chapter 12: Energy and respiration
QUESTIONS
glucose
12.4 Calculate the number of reduced NAD and reduced
FAD molecules produced for each molecule of
glucose entering the respiration pathway when
oxygen is available.
12.5 Using your answer to Question 12.4, calculate
the number of ATP molecules produced for each
molecule of glucose in oxidative phosphorylation.
12.6 Explain why the important contribution of the
Krebs cycle to cellular energetics is the release of
hydrogens and not the direct production of ATP.
12.7 Explain how the structure of a mitochondrion is
adapted for its functions in aerobic respiration.
ADP
ATP
pyruvate
2H
reduced
NAD
ethanal
2H
NAD
ethanol
CO 2
Respiration without oxygen
When free oxygen is not present, hydrogen cannot be
disposed of by combination with oxygen. The electron
transfer chain therefore stops working and no further ATP
is formed by oxidative phosphorylation. If a cell is to gain
even the two ATP molecules for each glucose yielded by
glycolysis, it is essential to pass on the hydrogens from the
molecules of reduced NAD that are made in glycolysis.
There are two different anaerobic pathways that solve the
problem of ‘dumping’ this hydrogen. Both pathways take
place in the cytoplasm of the cell.
In various microorganisms such as yeast, and in
some plant tissues, the hydrogen from reduced NAD is
passed to ethanal (CH 3
CHO). This releases the NAD and
allows glycolysis to continue. The pathway is shown in
Figure 12.15. First, pyruvate is decarboxylated to ethanal;
then the ethanal is reduced to ethanol (C 2
H 5
OH) by the
enzyme alcohol dehydrogenase. The conversion of glucose
to ethanol is referred to as alcoholic fermentation.
In other microorganisms, and in mammalian muscles
when deprived of oxygen, pyruvate acts as the hydrogen
acceptor and is converted to lactate by the enzyme lactate
dehydrogenase (named after the reverse reaction, which
it also catalyses). Again, the NAD is released and allows
glycolysis to continue in anaerobic conditions. This
pathway, known as lactic fermentation, is shown in
Figure 12.16.
Figure 12.15 Alcoholic fermentation.
ADP
ATP
glucose
pyruvate
2H
Figure 12.16 Lactic fermentation.
reduced
NAD
NAD
lactate
These reactions ‘buy time’. They allow the continued
production of at least some ATP even though oxygen is
not available as the hydrogen acceptor. However, as the
products of anaerobic reaction, ethanol or lactate, are
toxic, the reactions cannot continue indefinitely. The
pathway leading to ethanol cannot be reversed, and the
remaining chemical potential energy of ethanol is wasted.
The lactate pathway can be reversed in mammals. Lactate
is carried by the blood plasma to the liver and converted
back to pyruvate. The liver oxidises some (20%) of the
incoming lactate to carbon dioxide and water via aerobic
respiration when oxygen is available again. The remainder
of the lactate is converted by the liver to glycogen.
2H
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