A-Level Biology May/June 2024 Q4(e): Discuss how comparing each of the results with the control provides information about:…

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  Comparing the results with the control indicates that the primary mechanism for nitrate ion uptake is active transport. This is an active process that requires energy.

  1. Comparison with 'no oxygen' condition: When oxygen is removed, nitrate uptake is significantly reduced. This is because oxygen is essential for aerobic respiration. Without oxygen, aerobic respiration ceases, which drastically reduces the production of ATP. As active transport requires energy from ATP to move nitrate ions against their concentration gradient, a lack of ATP inhibits this process.

  2. Comparison with 'low temperature' condition: At a lower temperature, nitrate uptake is also significantly reduced. This is because the enzymes involved in respiration have lower kinetic energy and thus a lower rate of activity. This reduced rate of respiration leads to a lower rate of ATP synthesis, which in turn limits the rate of active transport.

  The small amount of uptake that may still occur in the absence of oxygen or at very low temperatures could be attributed to some facilitated diffusion occurring down a concentration gradient.

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  How the marks are awarded

  • B1 — The model answer correctly identifies that the main uptake mechanism is active transport, based on the evidence from the experiments.
  • B1 — The answer explains that the reduction in uptake without oxygen is due to oxygen's role in aerobic respiration.
  • B1 — The answer correctly links lower temperatures to a reduced activity of enzymes that are involved in the process of respiration.
  • B1 — The answer makes the crucial link between respiration (affected by oxygen and temperature) and the production of ATP, which is explicitly stated as being necessary for active transport.

  Common mistakes

  • Stating that low temperature or lack of oxygen 'stops' uptake, which is too absolute. The data usually shows a significant reduction, not a complete halt.
  • Vaguely mentioning that 'energy is needed' without specifying that this energy is in the form of ATP produced by respiration.
  • Confusing the effect of temperature on enzymes with its effect on membrane fluidity. While fluidity is affected, the primary reason for the drastic drop in uptake is the impact on respiratory enzyme kinetics.
  • Failing to use the comparative language required by the question (e.g., 'reduced compared to the control') and instead just stating facts about active transport.

  Examiner tip: When asked to interpret data, always explicitly connect the experimental condition (e.g., low temperature) to the specific metabolic process it affects (e.g., enzyme activity in respiration) and then explain the consequence for the overall process being measured (e.g., active transport).

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