Energy and Respiration: Question 9
Syllabus 12.2
During vigorous exercise, oxygen cannot always be delivered to muscle cells quickly enough to sustain fully aerobic respiration, so some ATP is generated anaerobically. After exercise has stopped, a person continues to breathe more deeply and rapidly than at rest for some time.
(a) Explain, in terms of the availability of NAD, why muscle cells respire anaerobically when oxygen delivery cannot keep pace with demand, and state what happens to the pyruvate produced by glycolysis under these conditions. [3] (b) Explain, in terms of the fate of the substance formed in (a), why extra oxygen (an "oxygen debt") is needed for some time after exercise has finished, even though the exercise itself has ended. [3]
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Worked solution
Part (a): Why muscle cells respire anaerobically, and the fate of pyruvate
Glycolysis, in the cytoplasm, produces a small amount of ATP and reduces NAD to reduced NAD as glucose is converted to pyruvate. Normally, this reduced NAD is reoxidised back to NAD by the electron transport chain, in the presence of oxygen, so that free NAD is continuously available for glycolysis to use again.
During vigorous exercise, if oxygen cannot be delivered to muscle cells as fast as it is being used, the electron transport chain (and, in turn, the link reaction and Krebs cycle) cannot keep up, so reduced NAD is no longer reoxidised quickly enough. Because only a small, limited pool of free NAD is present in the cytoplasm, glycolysis would stop as soon as this ran out. Unless NAD is regenerated by another route.
To solve this, pyruvate produced by glycolysis is reduced directly to lactate, using hydrogen transferred from reduced NAD. This reaction regenerates NAD, so that it is available again for glycolysis, allowing glycolysis (and its small net production of ATP) to continue even while oxygen delivery is insufficient for aerobic respiration.
Part (b): Why an oxygen debt is needed after exercise
Lactate produced during anaerobic respiration accumulates in the muscle cells and diffuses into the blood, but it is not simply a waste product to be discarded. It still represents a partially oxidised, energy-containing molecule that needs to be dealt with.
Once exercise stops and the demand for ATP falls, oxygen delivery can once again keep pace, and the lactate that built up during exercise is transported (largely to the liver) and oxidised, ultimately being broken down through aerobic respiration (with some being converted back to glucose or glycogen). Both of these processes require additional oxygen, beyond what is needed for the body’s resting metabolism.
This extra oxygen, consumed after exercise has finished in order to oxidise the accumulated lactate, is known as the oxygen debt. It explains why a person continues to breathe more deeply and rapidly than at rest for some time after stopping exercise: the elevated breathing rate persists until enough oxygen has been taken in to “repay” this debt and clear the lactate.
Final answers
- (a) Oxygen delivery cannot keep pace with demand, so reduced NAD cannot be reoxidised by the electron transport chain quickly enough; pyruvate is reduced to lactate, regenerating NAD so glycolysis can continue.
- (b) Accumulated lactate must later be oxidised (mainly in the liver), which requires extra oxygen after exercise has stopped; this extra oxygen requirement is the oxygen debt, and explains the raised breathing rate seen after exercise.