Blood: Students should be able to recognise different types of blood cells in a photograph or diagram, and explain how they are adapted to their functions.

The idea

Blood is a tissue containing plasma and three cell types, each shaped for its job. Red blood cells are biconcave discs packed with haemoglobin, giving a large surface area for oxygen uptake and no nucleus for more haemoglobin. White blood cells are larger, have a nucleus and can change shape to engulf pathogens or produce antibodies. Platelets are small cell fragments that help blood clot at wounds. In a photograph or diagram, identify cells by shape, size and features: red cells stain pink and lack a nucleus; white cells have a dark nucleus; platelets are tiny fragments. Then link each feature to its function, for example the biconcave shape increases surface area for diffusion of oxygen.

What earns the marks

• Red blood cells are biconcave discs, which increases surface area to volume ratio for faster oxygen diffusion.
• Red blood cells contain haemoglobin, which binds oxygen in the lungs and releases it in respiring tissues.
• Red blood cells have no nucleus, creating more space for haemoglobin and therefore more oxygen transport.
• White blood cells have a nucleus and can change shape to engulf pathogens by phagocytosis.
• White blood cells produce antibodies or antitoxins to destroy pathogens or neutralise toxins.
• Platelets are small cell fragments that release substances causing blood to clot, sealing wounds and reducing blood loss.
• Plasma is the liquid that transports dissolved substances such as carbon dioxide, glucose and urea.

Common mistakes and how to put them right

• Calling platelets cells; they are cell fragments, so describe them as fragments that help clotting.

• Stating that red blood cells have a nucleus.

  Put it right: They lose it as they mature, so say they have no nucleus to carry more haemoglobin.

• Confusing white blood cells with red blood cells in a diagram.

  Put it right: Use the dark nucleus and larger, irregular shape to identify white cells.

Exam tips

• When identifying cells, quote two visible features, such as biconcave shape and absence of a nucleus, before naming the cell.
• Link every adaptation to a function using 'because' or 'so that', for example 'no nucleus so more haemoglobin can be carried'.
• For explain questions, give the feature, the function and the benefit to the organism in one clear chain.