Explain why a hormone such as adrenaline can only affect certain cells in the body, even though it travels in the blood and reaches virtually every tissue.

OCR A-Level Biology A (H420) — 5.4 Hormonal communication · Explain · 4 marks · View as Markdown

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

Adrenaline is released from the adrenal medulla directly into the bloodstream during the 'fight or flight' response. Despite circulating throughout the body, its effects are highly specific — for example, it causes increased heart rate and dilation of the pupils, but does not affect all cell types equally.

Model answer (4 marks)

Adrenaline can only affect cells that possess the specific complementary receptor for adrenaline.

The receptor is a protein with a particular tertiary structure that matches the shape of adrenaline.

Cells that lack this receptor cannot bind adrenaline, so no intracellular response is triggered.

Because adrenaline is a non‑steroid, hydrophilic hormone, it cannot cross the cell membrane; it binds to a cell‑surface receptor, which activates a second‑messenger cascade inside the target cell only.

Examiner tips

  • Mention the complementary receptor and its shape; link to the second‑messenger system; keep the answer concise and use the exact terminology.
  • Use the word ‘specific’ to show why only certain cells respond.

Mark scheme (4 marks)

  1. Adrenaline only affects cells that possess the specific complementary receptor for adrenaline
  2. The receptor is specific to adrenaline due to its complementary shape / tertiary structure
  3. Cells lacking this receptor cannot bind adrenaline, so no intracellular response is triggered
  4. Adrenaline is a non-steroid / hydrophilic hormone so it cannot enter cells; it binds to a cell-surface receptor, triggering a second messenger cascade inside target cells only

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