Cells need energy to stay alive. They release it from food in respiration. The food is mostly glucose. Usually this uses oxygen. This is called aerobic resp...
Question 1 Report
Cells need energy to stay alive. They release it from food in respiration. The food is mostly glucose. Usually this uses oxygen. This is called aerobic respiration. Sometimes there is not enough oxygen. Then cells release energy without it. This is anaerobic respiration. During hard exercise the muscles cannot get oxygen fast enough. So they respire without oxygen for a time. This releases energy. But it also makes lactic acid. The acid builds up in the muscles. It makes them ache and tire. Yeast can respire this way too. A student had just finished a sprint. They were asked why their muscles ached.
(a) Write the word equation for anaerobic respiration in human muscle. [2]
(b) Explain why the muscles respire anaerobically during hard exercise. [2]
(c) Explain why anaerobic respiration releases less energy from each glucose molecule than aerobic respiration. [2]
(d) Explain why the athlete continues to breathe deeply and quickly for some time after the exercise has stopped. [3]
(e) State the products of anaerobic respiration in yeast. [1]
Answer Details
This question is about anaerobic respiration in exercising muscle: why it happens, its lower energy yield, and the oxygen debt repaid afterwards.
(a) The word equation for anaerobic respiration in human muscle:
glucose \(\rightarrow\) lactic acid [1] (+ energy released) [1].
Note that no oxygen is used and no carbon dioxide or water is produced, unlike aerobic respiration.
(b) During hard exercise the muscles respire anaerobically because oxygen cannot be delivered to the muscles fast enough to meet demand [1], so the muscles release energy without oxygen (anaerobically) to keep contracting [1].
(c) Anaerobic respiration releases less energy per glucose molecule because the glucose is not completely broken down [1] (it is only partly broken down to lactic acid), so less energy is released than when glucose is fully broken down using oxygen to carbon dioxide and water [1].
(d) After exercise the athlete keeps breathing deeply and quickly because lactic acid has built up in the muscles, creating an oxygen debt [1]; extra oxygen is needed to break down (oxidise) this lactic acid [1]; so the deep, fast breathing continues to supply that extra oxygen until the debt is repaid [1].
(e) The products of anaerobic respiration in yeast are ethanol (alcohol) and carbon dioxide [1].
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