▶️ Answer/Explanation
(a)
After exercise, it takes 20 minutes for the athlete’s breathing rate to return to its resting rate.
The heart rate decreases by 90 beats per minute between 20 and 40 minutes.
The heart rate and breathing rate do not immediately return to their resting rates because anaerobic respiration has occurred.
Lactic acid has built up in muscles / blood causing an oxygen debt.
The heart rate remains high to transport lactic acid to the liver.
The breathing rate remains high to supply oxygen for the aerobic respiration of lactic acid.
Explanation for (a):
- Graph Analysis: The graphs show the “period of exercise” ending at minute $20$. Looking at the top graph, the breathing rate returns to the baseline level ($12$ breaths/min) at minute $40$. The time taken is $40 – 20 = 20$ minutes. For the heart rate, the value at minute $20$ is $150$ bpm, and at minute $40$, it is $60$ bpm. The decrease is $150 – 60 = 90$ bpm.
- Biological Context (Oxygen Debt): During vigorous exercise, the body respires anaerobically, producing lactic acid which accumulates in the muscles and blood. This creates an “oxygen debt.” To remove this toxic lactic acid, it must be transported to the liver. There, it is broken down, a process that requires oxygen (specifically, the aerobic respiration of lactic acid). This is why heart rate (for transport) and breathing rate (for oxygen supply) remain elevated after exercise stops.
(b)(i) liver
Explanation: Excess amino acids are broken down in the liver via a process called deamination to produce urea.
(b)(ii) kidney
Explanation: The kidneys filter the blood to remove urea, which is then excreted from the body in urine.
(c) $C_{6}H_{12}O_{6} \rightarrow 2C_{2}H_{5}OH + 2CO_{2}$
Explanation: This is the balanced chemical equation for anaerobic respiration in yeast (fermentation). Glucose ($C_{6}H_{12}O_{6}$) is broken down into ethanol ($C_{2}H_{5}OH$) and carbon dioxide ($CO_{2}$) without the use of oxygen.
(d) bread (or alcohol / wine / beer)
Explanation: In bread making, the carbon dioxide produced by yeast during anaerobic respiration gets trapped in the dough, causing it to rise.