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).

Model answer (4 marks)

In a multicellular organism many cells are isolated from the external environment by other cells and tissues, so a change in, for example, glucose or pH cannot reach all cells by simple diffusion alone. Cells have specialised functions and therefore different metabolic needs; a rise in blood glucose must be sensed by one tissue and a response produced by another. A dedicated communication system (neural or hormonal) therefore transmits a signal from a receptor cell to an effector cell, coordinating the corrective action and maintaining homeostasis.

In contrast, a single‑cell organism is in direct contact with its surroundings. Any change in the environment is immediately sensed by the whole cell and the cell can alter its metabolism or behaviour directly, without the need for inter‑cellular signalling.

Examiner tips

  • Use the word ‘direct contact’ to contrast with multicellular isolation
  • Mention both nervous and hormonal signals
  • Explain coordination of different tissues
  • Show that a single cell can respond as a whole

Common mistakes

  • Confusing diffusion with signalling in multicellular context
  • Forgetting that single cells need no inter‑cell communication
  • Using vague terms like ‘cellular response’ without linking to homeostasis

Mark scheme (4 marks)

  1. In multicellular organisms, cells are not all in direct contact with the external environment, so conditions at internal cells cannot be regulated by simple diffusion alone.
  2. Multicellular organisms have specialised cells and tissues with different metabolic demands, so conditions must be coordinated between different parts of the body.
  3. A communication system allows signals (nervous or hormonal) to be sent between receptor cells detecting a change and effector cells that bring about a corrective response.
  4. Single-celled organisms are in direct contact with their external environment, so any change in conditions is detected and responded to by the whole cell without the need for intercellular communication.

Key terms in this question

homeostasis · communication system · multicellular

Related

More Communication and homeostasis questions

▶ Try answering this question with AI marking (free) →