Explain why multicellular organisms require a dedicated communication system to maintain homeostasis, whereas single-celled organisms do not.

OCR A-Level Biology A (H420) — 5.1 Communication and homeostasis · Explain · 4 marks · View as Markdown

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

Homeostasis is the maintenance of a stable internal environment despite changes in the external environment. In mammals, specialised systems coordinate responses across the body to keep conditions within narrow limits.

Model answer (4 marks)

Multicellular organisms have cells that are not in direct contact with the external environment, so they cannot respond independently to changes.

A dedicated communication system is therefore required to coordinate responses between specialised cells, tissues and organs across the body.

In contrast, single‑cell organisms have a large surface‑area‑to‑volume ratio, meaning the entire cell surface is exposed to the environment and can detect and respond directly.

Thus, in multicellular organisms receptors detect changes and transmit information via chemical or electrical signals so that effectors elsewhere can restore optimum conditions.

Examiner tips

  • Use the key terms ‘receptors’, ‘effectors’, ‘chemical/electrical signals’ and ‘surface‑area‑to‑volume ratio’.
  • Show the causal link: lack of direct contact → need for communication → homeostasis maintained.
  • Keep the answer concise and structured into four clear points to match the 4 marks.

Common mistakes

  • Confusing the role of receptors with effectors, or vice versa.
  • Failing to mention the surface‑area‑to‑volume ratio for single‑cell organisms.
  • Writing a long paragraph instead of separate points, losing marks for structure.

Mark scheme (4 marks)

  1. Multicellular organisms have cells that are not in direct contact with the external environment, so cannot respond independently to environmental changes
  2. A communication system is needed to coordinate responses between specialised cells, tissues and organs across the body
  3. Single-celled organisms have a large surface area to volume ratio, so the entire cell surface is exposed to the environment and can detect and respond to changes directly
  4. In multicellular organisms, receptors detect changes and pass information via chemical or electrical signals so that effectors elsewhere in the body can produce an appropriate response to restore optimum conditions

Key terms in this question

homeostasis · multicellular · communication system

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