Four components — RBCs, WBCs, platelets, plasma
Red blood cells transport oxygen using haemoglobin. White blood cells defend against pathogens — phagocytes engulf them, lymphocytes make antibodies. Platelets are cell fragments (not whole cells) that trigger clotting. Plasma is the pale yellow liquid, about 55% of blood volume, carrying glucose, amino acids, urea, hormones, , antibodies and heat. For the red blood cell function mark you must name haemoglobin and oxygen.
Haemoglobin — a reversible oxygen carrier
Haemoglobin is the iron-containing protein filling each red blood cell. In the lungs, where concentration is high, it combines with oxygen to form oxyhaemoglobin; at respiring tissues, where is low, oxyhaemoglobin releases it — the reaction is reversible. The cell is adapted to this: a biconcave disc (large surface area), no nucleus (more room for haemoglobin), and a small size with a thin membrane (short diffusion distance).
Clotting — fibrinogen to fibrin to scab
Blood clotting (Extended) runs in a fixed order: platelets and damaged tissue release clotting factors; those factors convert the soluble plasma protein fibrinogen into insoluble fibrin; the fibrin fibres form a mesh across the wound that traps red blood cells and platelets, giving a clot that dries into a scab. The clot does two jobs — it prevents further blood loss, and it stops pathogens entering through the wound.
Drawn from real examiner reports.
Phagocytes ≠ lymphocytes
Phagocytes engulf and digest pathogens by phagocytosis — non-specific, any pathogen. Lymphocytes produce antibodies against one specific antigen, plus antitoxins against bacterial toxins. Swapping the two is the commonest white-blood-cell error. On a diagram, the phagocyte has a multi-lobed nucleus; the lymphocyte has a large round one that nearly fills the cell.
Flagged w22 P22 Q18 · s22 P23 Q18
Fibrinogen → fibrin, never reversed
Fibrinogen is the soluble plasma protein you start with; fibrin is the insoluble product that builds the clot mesh. "Fibrin is converted to fibrinogen" scores zero. The adjectives soluble and insoluble are usually mark-worthy, and so is the word mesh (or network) — writing that "fibrin forms" on its own rarely earns the structural mark.
Flagged s22 P22 Q18
Plasma does not carry oxygen
Oxygen is carried by haemoglobin inside red blood cells, as oxyhaemoglobin — not dissolved in plasma. Listing "oxygen" as a substance transported in plasma is a standard trap. Plasma does carry dissolved carbon dioxide, so the two gases are not handled the same way; do not assume symmetry between them.
"Food and waste" is too vague
Asked to name substances transported in plasma, "food and waste products" earns nothing — examiners want the named dissolved substance. Use glucose, amino acids, urea, hormones (e.g. insulin), , antibodies, mineral ions or vitamins. Plasma also distributes heat from metabolically active organs such as the liver and muscles.
Oxygen transport without haemoglobin
"Red blood cells carry oxygen" misses the mechanism mark. The full answer names haemoglobin, states that it combines with oxygen in the lungs to form oxyhaemoglobin, and that oxyhaemoglobin releases oxygen at respiring tissues. "Carries blood" or "is red" scores nothing at all.
Clotting has two functions, not one
"State the role of blood clotting" normally carries 2 marks: (1) it prevents further blood loss from the wound; (2) it prevents pathogens entering the body through the wound. Answering only "it stops bleeding" throws away the second mark — the barrier-to-pathogens role is half of what is being asked.
Chain structure → consequence → function
For "explain how red blood cells are adapted", never stop at the feature. Structure: no nucleus. Consequence: more space inside the cell. Function: more haemoglobin, so more oxygen carried. One mark per completed step — link them with "so that".
Describe clotting as ordered steps
For a 3–4 mark "describe the process" question: clotting factors released by platelets and damaged tissue → soluble fibrinogen converted to insoluble fibrin → fibrin mesh traps red blood cells → clot dries to a scab. "Platelets clump together" alone misses every fibrin mark.
Identify the cell before naming its job
When a diagram labels a white blood cell, decide which type it is from the nucleus shape first, then give the matching function. Assigning antibody production to a cell that is clearly a phagocyte is the exact error examiners report on labelled-diagram questions.
Blood is a tissue consisting of four components suspended in or associated with a liquid medium.
| Component | Key features | Main function(s) |
|---|---|---|
| Red blood cells (RBCs) | Biconcave disc; no nucleus; packed with haemoglobin | Transport as oxyhaemoglobin; carry some |
| White blood cells (WBCs) | Have a nucleus; two types (phagocytes, lymphocytes) | Phagocytes: destroy pathogens by phagocytosis; Lymphocytes: produce antibodies |
| Platelets | Cell fragments; no nucleus | Initiate blood clotting at wound sites |
| Plasma | Pale yellow liquid (~55% of blood volume) | Transports dissolved glucose, amino acids, urea, hormones, , antibodies, heat |
What is haemoglobin and what is its role in red blood cells?
(a) State the four components of blood. [4]
(b) State the function of plasma in the transport of substances. Give TWO named examples of substances transported in plasma. [3]
