Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
278
Figure 12.17 shows what happens to the oxygen uptake
of a person before, during and after taking strenuous
exercise. Standing still, the person absorbs oxygen at
the resting rate of 0.2 dm 3 min −1 . (This is a measure
of the person’s metabolic rate.) When exercise begins,
more oxygen is needed to support aerobic respiration in
the person’s muscles, increasing the overall demand to
2.5 dm 3 min −1 . However, it takes four minutes for the heart
and lungs to meet this demand, and during this time lactic
fermentation occurs in the muscles. Thus the person builds
up an oxygen deficit. For the next three minutes, enough
oxygen is supplied. When exercise stops, the person
continues to breathe deeply and absorb oxygen at a higher
rate than when at rest. This post-exercise uptake of extra
oxygen, which is ‘paying back’ the oxygen deficit, is called
the oxygen debt. The oxygen is needed for:
■■
■■
■■
conversion of lactate to glycogen in the liver
reoxygenation of haemoglobin in the blood
a high metabolic rate, as many organs are operating at
above resting levels.
Respiratory substrates
Although glucose is the essential respiratory substrate
for some cells such as neurones in the brain, red blood
cells and lymphocytes, other cells can oxidise lipids and
amino acids. When lipids are respired, carbon atoms are
removed in pairs, as acetyl coenzyme A, from the fatty
acid chains and fed into the Krebs cycle. The carbon–
hydrogen skeletons of amino acids are converted into
pyruvate or into acetyl coenzyme A.
Energy values of respiratory substrates
Most of the energy liberated in aerobic respiration comes
from the oxidation of hydrogen to water when reduced
NAD and reduced FAD are passed to the electron
transport chain. Hence, the greater the number of
hydrogens in the structure of the substrate molecule, the
greater the energy value. Fatty acids have more hydrogens
per molecule than carbohydrates do, and so lipids have a
greater energy value per unit mass, or energy density, than
carbohydrates or proteins. The energy value of a substrate
is determined by burning a known mass of the substance
in oxygen in a calorimeter (Figure 12.18).
crucible
substrate
thermometer
water
oxygen
Figure 12.18 A simple calorimeter in which the energy value
of a respiratory substrate can be measured.
rest
exercise
recovery
2.5
oxygen deficit
Oxygen uptake / dm 3 min –1
2.0
1.5
1.0
0.5
post-exercise
oxygen uptake
(oxygen debt)
0
0
1
2
3
4
5
6 7 8
Time / min
9
10
11 12 13
30
Figure 12.17 Oxygen uptake before, during and after strenuous exercise.