Blood: Students should know the functions of each of these blood components.

The idea

Blood is a tissue that transports substances, defends the body and distributes heat. Red blood cells carry oxygen: they contain haemoglobin, which binds oxygen in the lungs to form oxyhaemoglobin and releases it in respiring tissues. White blood cells defend against pathogens; some engulf and digest microbes by phagocytosis, while lymphocytes produce antibodies and antitoxins. Platelets are cell fragments that help blood clot at a wound, reducing blood loss and preventing pathogens entering. Plasma is the pale yellow liquid that transports dissolved substances, including carbon dioxide, digested food such as glucose and amino acids, urea, hormones and heat.

For example: After a meal glucose dissolves in plasma and is delivered to cells, while most oxygen is carried inside red cells.

What earns the marks

• Red blood cells transport oxygen using haemoglobin, which binds oxygen to form oxyhaemoglobin in the lungs and releases it in tissues.
• White blood cells defend the body: phagocytes engulf and digest pathogens, and lymphocytes produce antibodies and antitoxins.
• Platelets are cell fragments that cause blood to clot at a wound, reducing blood loss and helping prevent infection.
• Plasma is the liquid that transports dissolved carbon dioxide, digested food, urea, hormones and heat around the body.
• A strong answer links each component to its function rather than merely naming the components.

Common mistakes and how to put them right

• Saying red blood cells carry carbon dioxide.

  Put it right by stating that most carbon dioxide is transported dissolved in plasma.

• Describing platelets as whole cells.

  Put it right by calling them cell fragments that help clotting.

• Confusing the roles of phagocytes and lymphocytes.

  Put it right by stating that phagocytes engulf pathogens while lymphocytes produce antibodies and antitoxins.

Exam tips

• Use the component names red blood cells, white blood cells, platelets and plasma, and give one clear function for each.
• When asked to explain, link structure to function, for example haemoglobin binding oxygen in red blood cells.
• Check that each function names the substance transported or the defence provided, not just 'helps the body'.