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 all tissues.

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 during a stress response and circulates throughout the body via the bloodstream. Despite this widespread distribution, only specific cells respond to adrenaline.

Model answer (4 marks)

Only cells that possess the specific complementary receptor for adrenaline can respond.

The receptor has a complementary shape to adrenaline, so only adrenaline can bind to it (shape specificity).

Binding of adrenaline to its receptor initiates a response or activates a second‑messenger cascade inside the cell.

Non‑target cells lack this receptor and so cannot respond, even when adrenaline is present.

Examiner tips

Use the phrase "specific complementary receptor" to show understanding of receptor–ligand specificity. Mention shape specificity and the initiation of a second‑messenger cascade. Explain that cells lacking the receptor cannot respond.
Keep the answer concise – four short points matching the mark scheme.
Use UK spelling (e.g. "cascades").

Common mistakes

Saying adrenaline can affect all cells because it is in the blood. Forgetting to mention the receptor’s shape specificity. Including irrelevant details such as adrenaline’s synthesis site.

Mark scheme (4 marks)

Only target cells possess the specific complementary receptor for adrenaline
The receptor has a complementary shape to adrenaline, so only adrenaline can bind to it (shape specificity)
Binding of adrenaline to its receptor initiates a response / activates a second messenger cascade inside the cell
Non-target cells lack this receptor and so cannot respond, even when adrenaline is present

Key terms in this question

adrenaline