Explain why a cell membrane is found in both plant cells and bacterial cells, and explain how the nucleus of a plant cell differs in structure from the genetic material in a bacterial cell.

AQA A-Level Biology (7402) — 3.2.1 Cell Structure · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

A cell membrane is required in both plant cells and bacterial cells because it controls the passage of substances into and out of the cell, allowing the cell to maintain its internal environment.

In a plant cell the genetic material is enclosed within a nucleus.
In a bacterial cell there is no nucleus; the DNA is a single circular loop that lies freely in the cytoplasm.

Examiner tips

  • Use the word ‘controls’ to describe the membrane’s function. Mention that both cell types need selective permeability. State that plant DNA is in a nucleus, bacterial DNA is not. Include that bacterial DNA is a single circular loop in the cytoplasm.

Common mistakes

  • Confusing the membrane with the cell wall. Saying the bacterial DNA is in a nucleus. Omitting that bacterial DNA is a single circular loop.

Mark scheme (5 marks)

  1. The cell membrane controls what substances pass into and out of the cell
  2. Both plant cells and bacterial cells need to control which substances enter and leave, so both require a cell membrane
  3. In a plant cell the genetic material / DNA is contained within a nucleus
  4. In a bacterial cell there is no nucleus / the genetic material is not enclosed in a nucleus
  5. The bacterial genetic material / DNA is found in the cytoplasm (as a single circular loop of DNA)

Key terms in this question

cell membrane · nucleus · genetic material · bacterial cell

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