Blood — AQA GCSE Biology
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Blood explained
Blood is a tissue because it contains cells suspended in a liquid matrix.
Read the full explanation
The liquid part is plasma, which transports dissolved substances such as glucose, amino acids, carbon dioxide, urea, hormones and heat. Suspended in plasma are red blood cells, white blood cells and platelets. Red blood cells contain haemoglobin, which binds oxygen in the lungs and releases it in tissues; they have no nucleus to maximise space for haemoglobin. White blood cells defend against pathogens through phagocytosis and antibody production. Platelets are cell fragments that help blood clot at wounds. For example, a cut triggers platelets to form a clot, while white blood cells fight infection. Recognising each component's role is essential.
Your focus
- Describe the composition of blood as plasma with suspended red blood cells, white blood cells and platelets.
- State the function of plasma, red blood cells, white blood cells and platelets.
- Explain how the structure of red blood cells relates to their function in oxygen transport.
Blood exam tips
Marking Points
- Blood is a tissue because it consists of cells suspended in a liquid matrix called plasma.
- Plasma transports dissolved substances including glucose, amino acids, carbon dioxide, urea and hormones.
- Red blood cells contain haemoglobin, which binds oxygen to form oxyhaemoglobin in the lungs and releases oxygen in tissues.
- Red blood cells have no nucleus, increasing the space available for haemoglobin.
- White blood cells defend the body against pathogens by phagocytosis and by producing antibodies.
- Platelets are small cell fragments that help blood to clot at the site of a wound.
- The components of blood can be separated by centrifugation, forming layers according to density.
Examiner Tips
- 💡When describing blood composition, name plasma first and then list the three suspended components.
- 💡Link each component to its function, for example red blood cells transport oxygen using haemoglobin.
- 💡Use the term tissue correctly by referring to cells suspended in a liquid matrix.
Common Mistakes
- Stating that plasma is a cell; the correction is that plasma is the liquid part of blood, while red blood cells, white blood cells and platelets are the cellular components.
- Saying red blood cells have a nucleus; the correction is that mature red blood cells lack a nucleus, which increases space for haemoglobin.
- Confusing the roles of white blood cells and platelets; the correction is that white blood cells defend against pathogens, while platelets help clotting.