Four components, four functions
Red blood cells transport oxygen using haemoglobin, a red pigment (protein) inside the cell that combines with oxygen. White blood cells defend the body by phagocytosis and antibody production. Platelets are cell fragments that cause clotting (mechanism not required). Plasma is the pale-yellow liquid carrying blood cells, ions, nutrients (glucose, amino acids), urea, hormones and carbon dioxide. Exactly four — name each with its own function.
Identifying cells in a photomicrograph
Red blood cells have no nucleus and are very numerous — they crowd most of the field of view. White blood cells have a nucleus, are larger, and are much less numerous: typically only one or two are visible per field of view among hundreds of red blood cells. Platelets are tiny cell fragments, much smaller than either cell type and with no nucleus. Settle the identification before attributing any function to the cell shown.
(Extended) Lymphocytes, phagocytes, clotting
At Extended level "white blood cells" splits into two types with two functions: lymphocytes carry out antibody production; phagocytes carry out engulfing pathogens by phagocytosis. A lymphocyte does not engulf pathogens; a phagocyte does not produce antibodies. Blood clotting, carried out by platelets, has two stated roles: it prevents further loss of blood from a wound, and it prevents the entry of pathogens through the break in the skin.
Drawn from real examiner reports.
Haemoglobin is not a component
The components of blood are red blood cells, white blood cells, platelets and plasma. Haemoglobin is not one of them — it is a protein found inside a red blood cell, so adding it as a fifth item to a "list the components of blood" answer earns nothing. Nor should red blood cells be listed a second time as a separate component of themselves.
Flagged Nov 2022 P42 Q1b
Plasma, not red cells, carries urea
When a question asks what transports hormones, urea or carbon dioxide around the body, the answer is plasma. Candidates routinely write "red blood cells", over-generalising the oxygen-carrying role that belongs specifically to red cells and haemoglobin. Plasma is the liquid that carries everything else dissolved or suspended in it — including the blood cells themselves.
Flagged Nov 2022 P42 Q1b · Nov 2023 P31 Q4c(ii)
Clotting is platelets, not white cells
White blood cells carry out phagocytosis and antibody production. Platelets carry out clotting. Writing "white blood cells clot the blood" — or the reverse, "platelets fight infection" — answers about the wrong component and is not credited, even though clotting and infection defence are both genuine jobs done by blood.
Flagged Nov 2022 P42 Q1b(i)
(Extended) Lymphocyte ≠ phagocyte
A question naming lymphocytes or phagocytes wants that cell type's own function. "Lymphocytes fight disease" is true but omits the required term antibody production; "phagocytes fight disease" omits engulfing pathogens by phagocytosis. A scientifically true but imprecise statement does not score — give the syllabus's own term for the named cell.
Swapping which cell has a nucleus
On a stained blood smear the nucleated cell is the white blood cell; the red blood cell has no nucleus. Labelling a red blood cell as nucleated — or reading cell density as if it were cell type — then sends every later mark the wrong way, because the function you attribute belongs to the other cell entirely.
Plasma is not the same as blood
Plasma is only the liquid part of blood — the component that carries everything else. "Blood" means plasma plus the red blood cells, white blood cells and platelets suspended within it. Answering "blood" where the question asks which component transports a substance loses the precision the mark scheme is testing.
Build a one-to-one function table
Map each component to one function before writing: red cells → oxygen (via haemoglobin); white cells → phagocytosis and antibody production; platelets → clotting; plasma → cells, ions, nutrients, urea, hormones, carbon dioxide. Never let a function migrate onto another row.
Identify the cell, then state the function
Name the cell from the stated visual evidence — nucleus present or absent, relative size, relative number — before giving any function. (Extended) A lymphocyte's nucleus is large and round and fills most of the cell; a phagocyte's nucleus is lobed.
Convert mm to µm before dividing
Magnification = image size ÷ actual size, and both must be in the same unit: 5.6 mm is 5600 µm. Skipping the conversion makes the answer about 1000 times too small, and inverting the formula (actual ÷ image) is the other repeated error.
(Extended) Give both roles of clotting
A clot prevents further loss of blood AND prevents the entry of pathogens through the wound. A question worth 2 or 3 marks expects both; one role, however well explained, caps your score. Match the number of points you make to the mark tariff.
List the four components of blood.
List the four components of blood and state one function of each. (4 marks)
