Plasma is the liquid component of blood. Explain how plasma is adapted to transport substances around the human body, and describe the role of platelets in protecting the body when a blood vessel is damaged.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Blood is made up of four main components: plasma, red blood cells, white blood cells and platelets. Each component has a specific role in maintaining health.
Model answer (5 marks)
Plasma is a pale‑yellow liquid that carries substances in solution.
It transports nutrients, hormones, urea and antibodies throughout the body.
Platelets stick to the broken or damaged vessel wall when a blood vessel is damaged.
They block the cut, forming a clot or scab, which prevents further blood loss.
This clot also stops pathogens from entering the blood vessel and allows the vessel to heal.
It transports nutrients, hormones, urea and antibodies throughout the body.
Platelets stick to the broken or damaged vessel wall when a blood vessel is damaged.
They block the cut, forming a clot or scab, which prevents further blood loss.
This clot also stops pathogens from entering the blood vessel and allows the vessel to heal.
Examiner tips
- Use the exact terms ‘plasma’, ‘platelets’, ‘clot’, ‘scab’, ‘nutrients’, ‘hormones’, ‘urea’, ‘antibodies’.
- Show the sequence: plasma transports → platelets stick → clot forms → prevents loss and infection.
- Answer all five points to gain full marks.
Common mistakes
- Confusing plasma with whole blood or describing it as a solid.
- Omitting the role of platelets in clot formation or pathogen defence.
- Using vague terms like ‘blood’ instead of ‘plasma’ or ‘platelets’.
Mark scheme (5 marks)
- Plasma is a liquid / pale yellow liquid that carries substances in solution
- Plasma transports named substances such as nutrients, hormones, urea OR antibodies
- Platelets stick to the broken / damaged vessel wall (when a blood vessel is damaged)
- Platelets block the cut / form a clot / scab, preventing further blood loss
- This prevents pathogens from entering the blood vessel / allows the vessel to heal
Key terms in this question
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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