A-Level Biology May/June 2025 Q2(d): Cells that have been affected by melittin break down into cell fragments. These cell fr…

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  The process begins when the cell fragment attaches to specific receptors on the cell surface membrane of the phagocyte. The phagocyte's membrane then extends and flows around the fragment, with pseudopodia enveloping it. This process is a form of endocytosis called phagocytosis.

  The membrane fuses to enclose the fragment within a vesicle, forming a phagocytic vacuole (also known as a phagosome) inside the cell's cytoplasm. Lysosomes, which are organelles containing hydrolytic enzymes, then migrate towards and fuse with the membrane of the phagocytic vacuole. This fusion releases the digestive enzymes into the vacuole, forming a phagolysosome. The hydrolytic enzymes, such as proteases and nucleases, then break down the cell fragment into smaller, soluble molecules which can be absorbed by the phagocyte.

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  How the marks are awarded

  • B1 — Describing the initial binding of the fragment to the phagocyte's membrane: "attaches to specific receptors on the cell surface membrane of the phagocyte".
  • B1 — Stating that a vesicle is formed to enclose the fragment: "enclosing the fragment within a vesicle, forming a phagocytic vacuole".
  • B1 — Describing the fusion of the lysosome with the vacuole: "Lysosomes... migrate towards and fuse with the membrane of the phagocytic vacuole".
  • B1 — Explaining that the fragment is broken down by hydrolytic enzymes: "hydrolytic enzymes... then break down the cell fragment".

  Common mistakes

  • Using vague language like 'the cell eats it' or 'it is absorbed' instead of precise terminology like 'engulfs', 'phagocytosis', and 'phagosome'.
  • Forgetting to mention the role of lysosomes, or incorrectly stating that another organelle (e.g., Golgi apparatus) fuses with the vacuole.
  • Describing the steps in the wrong sequence, for example, stating that enzymes act on the fragment before the lysosome has fused with the vacuole.
  • Confusing phagocytosis (cell eating of solids) with pinocytosis (cell drinking of liquids).

  Examiner tip: For questions asking to describe a biological process, memorise the key stages in a specific sequence and the precise scientific terminology for each stage.

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