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Respiration: Question 4

Syllabus 12.3

Structured Extended 8 marks

A commuter is running late for work. Seeing the doors of their train about to close, they sprint as fast as possible along the platform for the last 15 seconds before the doors shut, then stand catching their breath once safely on board.

(a) Explain, in terms of the chemical reactions taking place inside the commuter's leg muscle cells, why their muscles begin to ache slightly during this short, fast sprint, even though they are breathing throughout. [3]

(b) Once on the train, the commuter's breathing stays fast and deep, and their heart continues to beat quickly for several minutes, even though they are now standing still. Describe two ways in which this continued fast breathing and heart rate help to repay the oxygen debt built up during the sprint. [4]

(c) State the organ in which the lactic acid produced during the sprint is eventually broken down by aerobic respiration. [1]

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Worked solution

Part (a): Why the muscles ache during the sprint

Sprinting flat out demands energy much faster than usual. Even though the commuter is already breathing throughout the sprint, aerobic respiration alone cannot deliver enough oxygen to the leg muscle cells quickly enough to release all the energy needed at this rate.

To make up the shortfall, the muscle cells respire anaerobically as well as aerobically, breaking glucose down without using oxygen. This anaerobic reaction produces lactic acid, which starts to build up in the muscle tissue. This build-up of lactic acid is what causes the muscles to ache, and it creates an oxygen debt. An amount of oxygen that will be needed later to deal with the lactic acid that has accumulated.

Part (b): Repaying the oxygen debt

Once safely on the train, the commuter’s body works to repay the oxygen debt built up during the sprint, even though they are now standing still:

  1. Continued fast, deep breathing takes in extra oxygen, over and above what is needed for resting activities. This extra oxygen is needed to allow the lactic acid that has built up to be broken down by aerobic respiration.
  2. Continued fast heart rate keeps blood flowing around the body quickly. This transports the lactic acid, dissolved in the blood, away from the leg muscles to the liver, where it can be dealt with.

Both responses continue for as long as it takes to clear the lactic acid and repay the oxygen debt, which is why the commuter’s breathing and heart rate stay raised for several minutes after the sprint has ended, even at rest.

Part (c): Where the lactic acid is broken down

The lactic acid transported to the liver is broken down there by aerobic respiration, using the oxygen delivered by the continued fast, deep breathing.

Final answers

  • (a) The sprint needs energy faster than aerobic respiration alone can provide, so the muscle cells also respire anaerobically, producing lactic acid, which builds up and causes the ache while creating an oxygen debt.
  • (b) Continued fast, deep breathing supplies extra oxygen for breaking down the lactic acid; continued fast heart rate transports the lactic acid in the blood from the muscles to the liver.
  • (c) The liver.