Describe the role of platelets and fibrin in blood clotting.

Cambridge International IGCSE Biology (0610) — 9.4 Blood · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

Platelets adhere to the damaged vessel wall and aggregate to form a platelet plug, which blocks the cut.
Fibrinogen is converted to fibrin by the coagulation cascade.
Fibrin strands weave through the platelet plug, forming a stable mesh.
The clot stops excessive bleeding and prevents pathogens from entering the body.

Examiner tips

Use the exact terms ‘platelets’, ‘fibrinogen’, ‘fibrin’, ‘platelet plug’, ‘mesh’ – these are the words the mark scheme recognises.
Show the sequence: platelet adhesion → fibrinogen → fibrin → clot formation.
Mention the protective function (prevent blood loss and infection).

Common mistakes

Confusing fibrinogen with fibrin or omitting the conversion step.
Forgetting to state the protective role of the clot (blood loss or infection).

Mark scheme (4 marks)

Platelets stick to the broken vessel wall and clump together, blocking the cut.
Fibrinogen is converted to fibrin.
Fibrin forms a mesh across the wound.
Blood clotting prevents excessive blood loss and/or prevents entry of pathogens / microorganisms into the body.

Key terms in this question

platelets · fibrin · blood clotting