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Scientists investigated the effect of a decrease in pH on muscle contraction - AQA - A-Level Biology - Question 4 - 2019 - Paper 1
Question 4
Scientists investigated the effect of a decrease in pH on muscle contraction. The scientists did the investigation with four different preparations of isolated muscl... show full transcript
Worked Solution & Example Answer:Scientists investigated the effect of a decrease in pH on muscle contraction - AQA - A-Level Biology - Question 4 - 2019 - Paper 1
Step 1
A student looked at the results and concluded that a decrease in pH does cause a decrease in the force of muscle contraction. Use Figure 3 to evaluate this conclusion.
Answer
The graph in Figure 3 shows the force of muscle contraction for B (mouse muscle fibres at 0.5 pH units below typical) and D (rabbit muscle fibres at 0.5 pH units below typical) compared with their respective controls. At 32°C, B demonstrates a decrease in muscle contraction force compared to the control (A), indicating that the decrease in pH does indeed reduce the force of muscle contraction. However, D shows a different trend, as rabbit muscle fibres appear to maintain contraction force closer to the control suggesting that the effect of pH on muscle contraction may vary between species. Thus, while the conclusion regarding B is valid, it may not universally apply to all muscle fibres.
Step 2
Explain how a decrease in the concentration of calcium ions within muscle tissues could cause a decrease in the force of muscle contraction.
Answer
Calcium ions play a crucial role in muscle contraction by enabling the interaction between actin and myosin, the proteins responsible for muscle contraction. When calcium ions bind to troponin, it causes a conformational change that moves tropomyosin, thereby exposing binding sites on actin for myosin heads to attach. If the concentration of calcium ions decreases, fewer troponin complexes would be activated, leading to less tropomyosin being moved aside. As a result, fewer actin sites would be available for myosin binding, reducing the number of actin-myosin bridges formed and thereby decreasing the force of muscle contraction.
Step 3
Explain why converting pyruvate to lactate allows the continued production of ATP by anaerobic respiration.
Answer
During prolonged exercise, when oxygen levels are insufficient for aerobic respiration, cells convert pyruvate to lactate to regenerate NAD extsuperscript{+}. This regeneration is crucial because NAD extsuperscript{+} is necessary for glycolysis to continue. Glycolysis produces ATP anaerobically, and maintaining NAD extsuperscript{+} availability ensures that ATP can be generated even in an anaerobic environment. By converting pyruvate to lactate, cells can sustain energy production temporarily until oxygen becomes available again.