Cells and organisation · GCSE Biology
Blood components
GCSE Biology revision on blood: plasma, red blood cells, white blood cells and platelets — structure, function, and how each component is adapted for transport, defence or clotting.
Plasma carries everything dissolved. Red = haemoglobin, no nucleus, biconcave, O₂. White = defence (phagocytes, lymphocytes). Platelets = fragments, clotting. Do not swap red and white jobs.
The important bits
What you need to know
- 1
Blood is a tissue composed of plasma (liquid) and cells: red blood cells, white blood cells, and platelets. It transports substances and helps defence and clotting.
- 2
Plasma is the yellowish liquid that carries dissolved glucose, amino acids, carbon dioxide, urea, hormones, antibodies, and heat. It is mostly water, which is an excellent solvent and transport medium.
- 3
Red blood cells (erythrocytes) are biconcave discs with no nucleus at maturity. They are packed with haemoglobin, which binds oxygen in the lungs to form oxyhaemoglobin and releases oxygen in respiring tissues.
- 4
The biconcave shape increases surface area for faster oxygen diffusion. No nucleus leaves more space for haemoglobin. Red cells are made in bone marrow and live about 120 days before being recycled.
- 5
White blood cells (leukocytes) defend the body. Phagocytes engulf pathogens by phagocytosis. Lymphocytes produce antibodies specific to antigens on pathogens, and some produce antitoxins.
- 6
White blood cells have a nucleus and can change shape to move through vessel walls to sites of infection. They are larger than red blood cells and far fewer in number.
- 7
Platelets are small cell fragments (no nucleus) involved in blood clotting. At a wound, they trigger a cascade that forms fibrin threads, trapping blood cells to make a scab and prevent entry of pathogens.
- 8
Anaemia can result from too few red cells or too little haemoglobin, reducing oxygen delivery. Leukaemia involves abnormal white cell production — context only; GCSE focus is normal structure and function.
Quotations worth analysing
Short evidence. Real method.
“Red blood cells contain haemoglobin and have no nucleus, giving more space for oxygen transport.”
Biconcave shape and no nucleus are two adaptations — name both in longer answers.
“White blood cells defend the body by phagocytosis, antibody production and antitoxin production.”
Three functions for white cells. Red cells do not produce antibodies.
“Platelets are involved in blood clotting at wounds.”
Platelets are fragments, not phagocytes. Clotting prevents blood loss and pathogen entry.
Go deeper
Table question: match component to structure and function
Plasma: liquid, no cells, carries dissolved solutes and heat. Red cell: biconcave, no nucleus, haemoglobin, oxygen transport. White cell: nucleus, phagocytosis or antibodies. Platelet: fragment, clotting. For each row add one adaptation: red cell shape → surface area; white cell shape change → reach infection; platelets → scab formation. “Red cells fight disease” is the classic swap — that is white cells. “White cells carry oxygen” is the reverse error.
Go deeper
Haemoglobin — loading and unloading in one paragraph
In the lungs, high oxygen concentration favours binding: haemoglobin + O₂ → oxyhaemoglobin. In respiring tissues, low oxygen concentration favours release. Carbon dioxide is mostly transported dissolved in plasma or as hydrogencarbonate ions, not primarily on haemoglobin at GCSE depth. The biconcave disc maximises exchange surface per cell. Students who say “oxygen sticks permanently” miss unloading in muscles. Link to organisation: red cells are specialised cells in the circulatory system tissue.
Go deeper
Clotting is not the same as defence
Platelets trigger clotting: platelets → fibrin mesh → scab. White cells kill or mark pathogens. Both happen at a cut, but different components. Clotting is physical barrier; phagocytosis is biological attack. In exams, if asked what platelets do, do not write “engulf bacteria” unless you mean they are not phagocytes — platelets are not white cells. Scab prevents blood loss and blocks pathogens; white cells deal with any that entered.
See the idea in action
Explain how red blood cells are adapted to carry oxygen (4 marks). They contain haemoglobin, which binds oxygen in the lungs and releases it in tissues. They have no nucleus, so more space inside for haemoglobin. They are biconcave discs, increasing surface area for oxygen to diffuse in and out quickly. They are small enough to pass through capillaries in single file, maintaining exchange close to cells.
Exam technique
Turn knowledge into marks
Learn a four-row table: plasma, red, white, platelets — structure and function. Red = oxygen only. White = three defences. Never give red cells a nucleus or antibodies.
Common mistakes
Do not give these marks away
- 01
Saying red blood cells defend against disease or produce antibodies.
- 02
Claiming white blood cells carry oxygen or lack a nucleus.
- 03
Calling platelets complete cells with nuclei, or confusing them with phagocytes.
Which blood component contains haemoglobin?
APlasma
BRed blood cells
CPlatelets
DLymph only
Show the answer
Red blood cells. Haemoglobin in red blood cells binds oxygen. Plasma carries dissolved substances; platelets clot; lymph is a separate fluid.
Quick questions
If this is the bit you searched
What is in blood plasma GCSE?
Water with dissolved glucose, amino acids, carbon dioxide, urea, hormones, antibodies and other solutes. It carries heat.
Why do red blood cells have no nucleus?
To leave more room for haemoglobin, increasing oxygen-carrying capacity.
What do platelets do?
They are cell fragments that trigger blood clotting at wounds, forming a scab to stop bleeding and block pathogens.
How do white blood cells defend the body?
Phagocytes engulf pathogens; lymphocytes produce antibodies and antitoxins against specific threats.