A-Level Biology October/November 2024 Q2(d)(ii): One consequence of the pneumonia that results from P. jirovecii infection is a decrease…

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  A severe Pneumocystis jirovecii infection leads to an inflammatory response in the lungs. The accumulation of the fungal cells, fluid, and immune cells (like macrophages) within the alveoli causes the alveolar walls to become thicker. [1]

  This thickening of the respiratory surface increases the diffusion distance for gases. According to Fick's law, this increased distance significantly reduces the rate of diffusion of oxygen from the air in the alveoli into the blood in the surrounding capillaries. [2]

  As a result of the reduced rate of diffusion, less oxygen enters the bloodstream per unit of time. This means that less oxygen is available to bind with haemoglobin in red blood cells, leading to the formation of a smaller quantity of oxyhaemoglobin. Consequently, the oxygen-carrying capacity of the blood is reduced, and less oxygen is delivered to respiring body tissues. [3]

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  How the marks are awarded

  • M1 — The first mark is awarded for explaining that the infection causes the alveolar walls to become thicker, which is a direct consequence of the pathogen's presence and the resulting inflammation.
  • M1 — The second mark is for linking the thicker alveolar wall to an increased diffusion distance and a subsequent decrease in the rate of oxygen diffusion between the alveolus and the capillary blood.
  • M1 — The third mark is for explaining the consequence of reduced diffusion on blood chemistry: less oxygen binds to haemoglobin, resulting in less oxyhaemoglobin being formed and transported to the tissues.

  Common mistakes

  • Providing a vague answer, such as 'the infection damages the lungs', without specifying the effect on diffusion distance, surface area, or the diffusion gradient.
  • Stating that the pathogen 'uses up all the oxygen' as the sole explanation, which is an oversimplification and ignores the primary impact on the gas exchange surface.
  • Confusing ventilation with gas exchange, for example, by only stating that it is 'harder to breathe' without explaining why this leads to less oxygen in the blood.
  • Incorrectly describing the effect on the diffusion gradient, for instance, by not linking impaired ventilation (due to loss of elasticity) to a reduced oxygen concentration gradient.

  Examiner tip: For 'explain' questions involving a physiological process, always trace the sequence of events logically, linking the change in structure to its effect on function at each step.

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