Answers · Blood

Blood — final check key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. BK1 · Final check

Model answer. (a) Red blood cells, white blood cells, platelets and plasma. (b) F is a red blood cell: a pink disc with no nucleus and a paler centre, like most of the cells in the field. E and W are white blood cells: each has a nucleus, stained purple; either letter is right. N is purple too, but it is a tiny fragment, far smaller than a red cell: a platelet, not a white cell.

A full answer includes

(a) I stated red blood cells, white blood cells, platelets and plasma.
(a) I did not give haemoglobin: it is inside the red cells.
(b) I gave F as the red blood cell.
(b) I gave its feature: no nucleus (or: a paler centre).
(b) I gave E or W as the white blood cell.
(b) I gave its feature: a nucleus, stained purple.
(b) I did not give N as a white cell: it is a tiny fragment, a platelet.

Also accept

(a) red blood cells, white blood cells, platelets, plasma; (b) red: F, no nucleus; white: E or W, a purple nucleus

Watch for

“(a) Red cells, white cells, platelets and haemoglobin.” — Haemoglobin is inside the red cells. The fourth component is plasma.
“(b) N is a white blood cell, because it is stained purple.” — N is far smaller than a red cell: a fragment of a cell, a platelet. A white cell is a whole cell with a purple nucleus inside it.
“(b) W is a white cell because it is larger than the red cells.” — W is only a little larger than the pink discs, so size is a weak clue. Its purple nucleus is what shows it is a white cell.

2. BK2 · Final check

Model answer. W is the lymphocyte: one large round purple nucleus fills most of the cell, with only a thin rim of cytoplasm. E is the phagocyte: its nucleus is in four joined lobes and its cytoplasm is grainy. E is also the larger cell, but the shape of the nucleus is the safer clue, because some lymphocytes are as large as a phagocyte.

A full answer includes

I gave W as the lymphocyte.
I described its nucleus: one large round nucleus that fills most of the cell.
I gave E as the phagocyte.
I described its nucleus in lobes (or: its grainy cytoplasm).
I did not decide from size alone.

Also accept

W lymphocyte (large round nucleus); E phagocyte (nucleus in lobes, grainy cytoplasm)

Watch for

“W is the phagocyte, because its nucleus is the biggest.” — A big round nucleus filling the cell marks a lymphocyte. A phagocyte's nucleus is in lobes.
“F is the lymphocyte, because it is small and round.” — F has no nucleus: it is a red blood cell. A lymphocyte has a large purple nucleus.

3. FK1 · Final check

Model answer. Haemoglobin in the red blood cells combines with oxygen in the lungs and gives it up in the tissues. With fewer red cells there is less haemoglobin, so less oxygen is carried to the tissues, such as the muscles. The cells have less oxygen for respiration, so less energy is released, and the person feels tired and weak.

A full answer includes

I wrote that haemoglobin combines with oxygen in the lungs.
I wrote that with less haemoglobin, less oxygen is carried to the tissues.
I wrote that the cells then have less oxygen for respiration.
I wrote that less energy is released.
I wrote that this is why the person feels tired and weak.
I did not write that the blood makes less oxygen.

Also accept

less haemoglobin; less oxygen carried; less respiration in the tissues

Watch for

“Their blood makes less oxygen.” — Blood does not make oxygen: it carries oxygen that was breathed in. Less haemoglobin means less of it can be carried.

4. FK2 · Final check

Model answer. Lymphocytes produce antibodies. Phagocytes engulf pathogens by phagocytosis: they take them in and digest them.

A full answer includes

(a) I stated that lymphocytes produce antibodies.
(b) I stated that phagocytes engulf pathogens by phagocytosis.
I did not write that lymphocytes engulf pathogens.

Also accept

lymphocytes antibodies; phagocytes phagocytosis

Watch for

“Both engulf pathogens.” — Only phagocytes engulf. Lymphocytes produce antibodies.

5. FK3 · Final check

Model answer. C. White blood cells defend the body by phagocytosis and antibody production. Platelets help blood to clot. Plasma carries dissolved substances such as urea and hormones, as well as the blood cells.

A full answer includes

I chose row C.
I did not choose row D: haemoglobin and the oxygen it carries stay inside the red cells.

Also accept

Watch for

“Row A: platelets transport oxygen.” — Platelets help blood to clot. Oxygen is carried by haemoglobin in the red blood cells.
“Row B: white cells clot; platelets do phagocytosis and antibody production.” — These two are swapped. White blood cells defend the body; platelets help blood to clot.
“Row D: plasma transports haemoglobin and oxygen.” — Haemoglobin stays inside the red blood cells, and it carries the oxygen there. Plasma carries dissolved substances such as urea, hormones and carbon dioxide.

6. CK1 · Final check

Model answer. A blood clot seals a cut, so (a) it prevents blood loss and (b) it prevents pathogens from entering the body. The hardened resin seals the wound in the bark in the same way: (a) the tree loses less sap through the wound, and (b) pathogens such as fungi and bacteria cannot enter the tree through it.

A full answer includes

(a) I stated that a clot prevents blood loss.
(a) I suggested that the resin stops the tree losing sap through the wound.
(b) I stated that a clot prevents pathogens entering the body.
(b) I suggested that the resin stops pathogens, such as fungi or bacteria, entering the tree through the wound.
I did not write that a clot kills pathogens.

Also accept

(a) prevents blood loss; the resin stops sap being lost; (b) prevents pathogens entering; the resin keeps fungi and bacteria out

Watch for

“A clot kills the pathogens in a cut.” — A clot keeps pathogens out by sealing the opening; it does not kill them. White blood cells destroy any that get in.

7. CK2 · Final check

Model answer. The pupil has swapped the two proteins. The soluble protein dissolved in the plasma is fibrinogen. At a cut, fibrinogen is converted into fibrin, which is insoluble, and the fibrin forms the mesh of threads that traps blood cells.

A full answer includes

I wrote that the soluble protein in the plasma is fibrinogen, not fibrin.
I wrote that fibrinogen is converted into fibrin, not the other way round.
I wrote that fibrin is insoluble.
I wrote that the mesh of threads is made of fibrin.
I did not write that platelets turn into fibrin.

Also accept

the names are swapped: soluble fibrinogen is converted into insoluble fibrin, which forms the mesh

Watch for

“Only the word "soluble" is wrong.” — The names are swapped too: the protein in the plasma is fibrinogen, and the mesh is made of fibrin.
“Platelets are converted into fibrin.” — Fibrin is made from fibrinogen, a protein dissolved in the plasma. Platelets help to start the process.