A runner took part in a 400 m race. During the race her leg muscles could not be supplied with enough oxygen, so they respired anaerobically to keep releasi...
Question 1 Report
A runner took part in a 400 m race. During the race her leg muscles could not be supplied with enough oxygen, so they respired anaerobically to keep releasing energy. Her heart rate was measured every minute during the recovery period after the race, and the readings were plotted on the graph. The dashed horizontal line shows her normal resting heart rate, so it can be compared with the readings. Straight after the race her heart rate was well above the resting level, and over the following minutes it gradually came back down. The graph shows how her heart rate changed throughout recovery.
(a) Name the substance produced when muscle cells respire anaerobically. [1]
(b) State the word equation for anaerobic respiration in muscle. [2]
(c) State, from the graph, the runner's heart rate 2 minutes after exercise. [1]
(d) Describe how the heart rate changes during the recovery period. [2]
(e) Explain why the heart rate stays above the resting level for several minutes after the race. [3]
(f) State whether anaerobic respiration releases more or less energy per glucose molecule than aerobic respiration. [1]
Answer Details
(a) Substance produced in anaerobic respiration in muscle [1]
Lactic acid.
(b) Word equation for anaerobic respiration in muscle [2]
\(\text{glucose} \rightarrow \text{lactic acid}\)
1 mark for glucose as the reactant, 1 mark for lactic acid as the product.
(c) Heart rate 2 minutes after exercise (from the graph) [1]
120 beats per minute.
(d) How the heart rate changes during recovery [2]
The heart rate falls steadily (decreases) [1]; returning towards the resting rate by about 5 minutes [1].
(e) Why the heart rate stays above resting for several minutes [3]
During the race an oxygen debt built up [1]; extra oxygen must now be delivered to the muscles to break down the lactic acid produced by anaerobic respiration [1]; so the heart keeps beating fast to transport this oxygen (and to carry away carbon dioxide) until the debt is repaid [1].
(f) Energy released compared with aerobic respiration [1]
Less energy is released per glucose molecule than in aerobic respiration.
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