Edexcel IGCSE Biology · Spec 2.59-2.61
The Blood
The four components of blood and their jobs, and how red blood cells are adapted to carry oxygen.
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The Blood
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Explained
Four components of blood, and what each one is for
Blood is a tissue, made of a liquid with three kinds of solid suspended in it. Learn the four parts by their jobs, because questions almost always ask what a component does rather than what it looks like.
Red blood cells
These carry oxygen, and every feature of them serves that purpose.
They contain haemoglobin, which binds oxygen in the lungs to form oxyhaemoglobin and releases it in respiring tissues where oxygen is scarce. They have no nucleus, which leaves more room for haemoglobin. They are a biconcave disc, which gives a larger surface area for oxygen to diffuse across and a thin centre so no part of the cell is far from the surface. And they are small and flexible, so they can squeeze through capillaries in single file.
Haemoglobin contains iron, which is why a shortage of iron in the diet causes anaemia.
White blood cells
These defend the body, and there are two types with two different methods.
Phagocytes engulf and digest pathogens. Lymphocytes produce antibodies, which are proteins that bind to a specific pathogen and mark it for destruction.
Unlike red blood cells, white blood cells do have a nucleus, and they can change shape to squeeze out of capillaries into infected tissue.
Platelets and plasma
Platelets are fragments of cells rather than whole cells. They clump at a wound and start the clotting process, which seals the cut, stops blood loss and keeps pathogens out.
Plasma is the pale yellow liquid, mostly water, that everything travels in. It carries the blood cells themselves, dissolved carbon dioxide, urea from the liver to the kidneys, digested food such as glucose and amino acids, hormones, and heat around the body.
What the examiners and mark scheme say
A principal examiner report for Edexcel International GCSE Biology describes a question where candidates interpreted a table of blood counts. Full marks came from linking each number to a consequence: more phagocytes to engulf pathogens, so infection is fought better; more lymphocytes to produce antibodies; no difference in red blood cell numbers, so no difference in oxygen transport.
Notice that no difference was itself worth a mark. Where data shows no change, saying so and explaining what it means is credited rather than ignored.
The report also flags a precision error in an otherwise full mark answer, where the candidate confused antigens with antibodies. An antigen is the marker on the pathogen. An antibody is the protein the lymphocyte makes to bind to it. The words look alike and mean opposite ends of the same interaction.
On the calculation part, answers had to be given in standard form, and the report notes that responses gaining full marks were the ones that did so.
Answering an adaptation question
Give the feature, then what it allows, then why that helps. No nucleus, so there is more space for haemoglobin, so more oxygen can be carried. Biconcave, so the surface area is larger, so oxygen diffuses in and out faster.
A feature listed without its consequence is usually half a mark's worth of an answer, and the consequence is the part that shows you understand why the cell looks like that.
Spec 2.59-2.61
What you need to know
- Name the four components of blood
- Explain how red blood cells are adapted to their job
- State the jobs of white blood cells, platelets and plasma
Active recall
Quick check
Answer each question before opening the answer.
Name the four components of blood and one role of each.
Red blood cells — carry oxygen; white blood cells — defence against pathogens; platelets — clotting; plasma — transports dissolved substances.
Give two adaptations of a red blood cell.
Biconcave shape (large surface area), no nucleus (more room), full of haemoglobin to carry oxygen.
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