A-Level Biology May/June 2024 Q4(c)(i): Outline and explain the sequence of events that occurs, which allows amino acids to be…

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  1. Proton pumps in the cell surface membrane of the companion cell use energy derived from the hydrolysis of ATP to actively transport protons (H$^+$) out of the cell's cytoplasm.

  2. These protons are pumped into the apoplast (cell wall), causing them to accumulate there. This establishes a high concentration of protons in the apoplast, creating a steep proton gradient across the companion cell membrane.

  3. The protons then move back into the companion cell cytoplasm, down their electrochemical gradient. This occurs via facilitated diffusion through a specific membrane protein called a co-transporter.

  4. As a proton moves through this co-transporter protein, it brings an amino acid molecule with it. This co-transport mechanism allows the amino acid to be moved from the apoplast into the companion cell against its concentration gradient.

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  How the marks are awarded

  • B1 — The answer states that protons are moved by active transport out of the companion cell, specifying that this process uses energy from ATP.
  • B1 — The answer correctly identifies that the accumulation of protons in the apoplast establishes a proton gradient.
  • B1 — The answer describes protons moving back into the cell down their gradient via facilitated diffusion.
  • B1 — The answer links the movement of protons to the transport of amino acids via a co-transporter protein, and correctly states the amino acid moves against its concentration gradient.

  Common mistakes

  • Confusing the roles of active transport and facilitated diffusion, for example, stating that protons diffuse out of the cell or that amino acids are actively pumped by the co-transporter.
  • Forgetting to mention the creation of a proton gradient in the apoplast, which is the crucial intermediate step that powers the co-transport.
  • Incorrectly stating that protons are pumped into the phloem sieve tube element instead of the apoplast (cell wall).
  • Using imprecise language, such as 'diffusion' instead of 'facilitated diffusion' for the movement of protons through the co-transporter, or failing to mention the co-transporter protein.

  Examiner tip: For 'outline' or 'describe sequence' questions, learn the process as a numbered list of cause-and-effect steps to ensure your answer is logical and complete.

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