Answers · Blood

What is in blood? — answer key

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

1. BQ1 · Practice

Model answer. Blood has four components: red blood cells, white blood cells, platelets and plasma. The cells and platelets are carried along in the plasma.

A full answer includes

I chose: red blood cells, white blood cells, platelets and plasma.
I did not choose a list with haemoglobin in it: haemoglobin is inside the red cells.

Also accept

red blood cells, white blood cells, platelets and plasma

Watch for

“Blood is red cells, white cells, oxygen and water.” — Oxygen is carried by the red cells, and water is only part of the plasma. Neither is a component on its own.
“Haemoglobin is one of the four components.” — Haemoglobin is a protein inside the red blood cells. It is part of a component, not a component itself.
“Fibrin is one of the four components.” — Fibrin forms only at a cut, from a protein dissolved in the plasma. The component you left out is platelets.

2. BQ2 · Practice

Options as numbered on the pupil sheet: 1 — white blood cells and platelets; 2 — red blood cells; 3 — haemoglobin dissolved in water; 4 — plasma.

Model answer. Z, the pale yellow liquid at the top, a little more than half of the blood, is plasma. X, the very thin whitish band, holds the white blood cells and platelets. Y, the dark red layer at the bottom, is the red blood cells, the heaviest part, which sink. No layer is haemoglobin dissolved in water: if it were, the liquid at the top would be red.

A full answer includes

I matched Z to plasma.
I matched X to white blood cells and platelets.
I matched Y to red blood cells.
I did not use "haemoglobin dissolved in water": haemoglobin stays inside the red cells.

Also accept

Z plasma; X white cells and platelets; Y red cells

Watch for

“X is the plasma, because it is pale.” — The plasma is the large pale yellow layer at the top. The thin band is the white blood cells and platelets.
“Z is haemoglobin dissolved in water.” — Z is pale yellow. Haemoglobin is red and stays inside the red cells, which have sunk to the bottom.

3. BQ3 · Practice

Model answer. White blood cells. The stain colours a nucleus purple, and that purple nucleus is what makes a white cell stand out among the red cells. Without the stain the nucleus is still there, but the whole white cell is almost colourless. Red cells are full of haemoglobin, a red protein, so they keep a pale colour of their own and are still easy to see; they never had a nucleus. (Platelets, which are fragments of cells, not cells, are also hard to see without the stain.)

A full answer includes

I chose: white cells, because without the stain their nucleus is not purple.
I did not choose red cells: their haemoglobin gives them a colour of their own, stained or not.
I did not choose a reason in which a cell loses its nucleus: the stain only colours a nucleus.

Also accept

white cells, because without the stain their nucleus is not purple

Watch for

“Red cells, because without the stain they have no colour at all.” — Red cells are full of haemoglobin, a red protein, so they keep a pale colour of their own without any stain.
“White cells, because without the stain they lose their nucleus.” — The stain only colours the nucleus; it does not make it. The nucleus is still there, but it is now almost colourless, like the rest of the cell.
“Red cells, because without the stain their nucleus cannot be seen.” — Red blood cells have no nucleus at all, stained or not.

4. BQ4 · Practice

Model answer. A lymphocyte has one large round nucleus that fills most of the cell, with only a thin rim of cytoplasm. A phagocyte has a nucleus in several lobes and grainy cytoplasm.

A full answer includes

I chose: a large round nucleus filling the cell.
I did not choose grainy cytoplasm: that is a clue to a phagocyte.

Also accept

one large round nucleus filling most of the cell

Watch for

“A nucleus in several joined lobes.” — A nucleus in lobes is the mark of a phagocyte, not a lymphocyte.
“No nucleus and a paler centre.” — Every white blood cell has a nucleus. No nucleus and a pale centre mark a red blood cell.
“Grainy cytoplasm around a small nucleus.” — Grainy cytoplasm belongs to a phagocyte. A lymphocyte has only a thin rim of clear cytoplasm round a large nucleus.

5. BQ10 · Practice

Model answer. Count every cell inside the square, and the cells that cross the top or left side; leave out those that cross the bottom or right side. 13 + 3 + 1 = 17. The cells on the bottom and right sides are counted in the next squares instead, so over many squares every cell on a line is counted once.

Checked automatically

Red cells counted in the square: 17 (exact value)
- Common wrong answer 23 : You counted every cell on a line. Leave out the cells that cross the bottom or right side.
- Common wrong answer 13 : That is only the cells wholly inside. Add the cells that cross the top or left side.
- Common wrong answer 19 : You counted the bottom and right sides. The rule counts the top and left sides.

A full answer includes

I counted the 13 cells inside the square.
I added the 3 cells on the top side and the 1 on the left side.
I wrote 17.
I did not count the cells on the bottom or right side.

Also accept

Watch for

“All 23: every cell that touches the square counts.” — Each side is shared with the next square, so counting every cell on a line counts some cells twice. Count cells across the top or left side only.

6. BQ11 · Practice

Model answer. (a) J is a red blood cell: a pink disc with no nucleus and a paler centre, like most of the cells in the picture. (b) K and M are white blood cells: each has a nucleus, stained purple, and red cells have none. K is about the size of a red cell, so size alone would not find it: look for the nucleus. (c) L is a platelet: a tiny purple fragment of a cell, far smaller than a red cell.

A full answer includes

(a) I identified J.
(a) I gave a reason: it has no nucleus (or: a paler centre).
(b) I identified both K and M.
(b) I gave the reason: each has a nucleus, stained purple.
(b) I did not give "larger" as my only reason: K is about the size of a red cell.
(c) I identified L.
(c) I gave the reason: it is a tiny fragment, far smaller than a red cell.

Also accept

(a) J, no nucleus; (b) K and M, a purple nucleus; (c) L, a tiny fragment

Watch for

“L is a white blood cell, because it is purple.” — L is far smaller than a red cell: it is a platelet, a fragment of a cell. A white cell is a whole cell with a purple nucleus inside it.
“K is a red blood cell, because it is the same size as J.” — K has a large purple nucleus. Red blood cells have no nucleus, so K is a white blood cell.

7. BQ13 · Practice

Model answer. A. K's nucleus is round and fills most of the cell, with only a thin rim around it: K is a lymphocyte. M's nucleus is in several rounded lobes and its cytoplasm is grainy: M is a phagocyte. M is also the larger cell, but size is weaker evidence, because some lymphocytes are as large as a phagocyte (not C). Row B gives the right evidence but swaps the names, and K cannot be a red blood cell, because it has a nucleus (not D).

A full answer includes

I chose row A.
I used K's nucleus: it is round and fills most of the cell, so K is a lymphocyte.
I used M's nucleus: it is in lobes, so M is a phagocyte.
I did not choose row C: size is weaker evidence than the shape of the nucleus.

Also accept

Watch for

“Row B: K is a phagocyte and M is a lymphocyte.” — The names are swapped. A round nucleus that fills the cell marks a lymphocyte; a nucleus in lobes marks a phagocyte.
“Row C: the larger white cell, M, must be the phagocyte.” — The names are right, but size is weak evidence: some lymphocytes are as large as a phagocyte. The shape of the nucleus is the better evidence.
“Row D: K is a red blood cell, because it is the size of one.” — K has a purple nucleus, and red blood cells have none. In a smear of human blood, a cell with a nucleus is a white blood cell.

8. BQ5 · Practice

Model answer. (a) Agree: a white blood cell is a whole cell with a purple-stained nucleus, and no cell in the picture has one. The pink discs have only a paler centre, where they are thinnest, and the tiny purple specks are far smaller than a cell: they are platelets. (b) White cells are far rarer than red cells: there are hundreds of red cells for each white cell. The cells lie scattered at random, so one small view can hold no white cell just by chance. Looking at more fields gives a fairer sample of the blood; a person with no white cells at all would be very ill.

A full answer includes

(a) I agreed with the pupil.
(a) I explained that no cell in the picture has a purple-stained nucleus.
(a) I did not take the pale centre of a pink disc for a nucleus.
(b) I wrote that white cells are far rarer than red cells.
(b) I wrote that one small view can hold no white cell just by chance.
(b) I wrote that looking at more fields gives a fairer sample of the blood.
(b) I did not write that the white cells are hidden under the red cells.

Also accept

(a) agree, no cell has a purple nucleus; (b) white cells are rare, so one small view can miss them by chance

Watch for

“(a) Disagree: the purple specks are white cells, because they are purple.” — The specks are far smaller than a red cell: they are platelets, fragments of cells. A white cell is a whole cell with a purple nucleus inside it.
“(a) Disagree: the pale centre of each pink disc is its nucleus.” — The pale centre is where a red cell is thinnest. A nucleus would be stained purple, and red cells have none.
“(b) One view is enough, because the smear is spread evenly.” — A smear is never perfectly even, and white cells are rare. One small view can miss them by chance, so more fields are needed.
“(b) The white cells are there, but hidden under the red cells.” — In a thin smear the cells lie side by side in one layer. The view has no white cell simply because white cells are rare.

9. BQ6 · Practice

Model answer. (a) The field has 50 times the area of the square, so it holds about 34 × 50 = 1700 red cells. (b) 1700 ÷ 5 = 340 red cells for each white cell. Both answers are numbers of cells, with no unit. (In the experiment the field is 0.45 mm across and 100 times the area of its square, so there you would multiply by 100 instead.)

Checked automatically

(a) Red cells in the field: 1700 (exact value)
- Common wrong answer 3400 : You multiplied by 100. This square is one fiftieth of the field: multiply by 50.
- Common wrong answer 34 : That is the count in the square. The field is 50 times bigger.
(b) Red cells for each white cell: 340 (exact value)
- Common wrong answer 680 : You used 3400 red cells. The field holds 34 × 50 = 1700.
- Common wrong answer 6.8 : You divided the square's count by the field's white cells. Use the field's red cells, 1700.

A full answer includes

(a) I multiplied 34 by 50.
(a) I wrote 1700 red cells.
(a) I did not multiply by 100: this square is one fiftieth of the field.
(b) I divided 1700 by 5.
(b) I wrote 340.
(b) I did not divide the square's count, 34, by 5.

Also accept

(a) 1700; (b) 340

Watch for

“There are about 7 red cells for each white cell: 34 ÷ 5.” — The 34 red cells were counted in the square only, while the 5 white cells were in the whole field. Scale the red count up to the whole field first.
“Multiply by 100, as in the experiment.” — This square is one fiftieth of the field, so 50 squares fill it: multiply by 50.

10. BQ7 · Practice

Model answer. (a) 24 mm ÷ 58 mm × 100 = 41.4%, which is 41% to 2 significant figures. In healthy blood the red cells are usually about 40 to 45% of the blood. (b) Nearly all of the rest is plasma, the pale yellow liquid at the top; the white cells and platelets make up only the thin band.

Checked automatically

(a) Red cells: 41 % (2 significant figures)
- Common wrong answer 24 %: That is the depth in mm. Divide by the whole 58 mm and multiply by 100.
- Common wrong answer 59 %: That is the rest, mostly plasma. The red cells are 24 mm of 58 mm.
- Common wrong answer 2.4 %: You divided the whole by the red layer. Divide the red layer, 24 mm, by the whole, 58 mm.

A full answer includes

(a) I divided 24 mm by 58 mm and multiplied by 100.
(a) I wrote 41%, to 2 significant figures.
(b) I stated plasma.
(b) I did not state white blood cells: with the platelets they make only the thin band.

Also accept

(a) 41%; (b) plasma

Watch for

“The rest is white blood cells.” — The white cells and platelets form only a band less than 1 mm deep. The rest, about 59%, is plasma.

11. BQ8 · Practice

Model answer. G is a lymphocyte: one large round purple nucleus fills most of the cell, with only a thin rim around it. H is a phagocyte: its nucleus is in three joined lobes and its cytoplasm is grainy. Here the lymphocyte is the larger cell. Phagocytes are usually larger, but some lymphocytes are large, so the shape of the nucleus decides.

A full answer includes

(a) I identified G as a lymphocyte.
(a) I gave its feature: one large round nucleus that fills most of the cell.
(b) I identified H as a phagocyte.
(b) I gave its feature: a nucleus in lobes (or: grainy cytoplasm).
I did not call G the phagocyte because it is the larger cell.

Also accept

G lymphocyte (large round nucleus); H phagocyte (nucleus in lobes, grainy cytoplasm)

Watch for

“G is the phagocyte, because it is the larger white cell.” — Size can mislead: some lymphocytes are large. G has one large round nucleus filling the cell, the mark of a lymphocyte.
“H is a lymphocyte, because its nucleus is small.” — H's nucleus is in three joined lobes, and its cytoplasm is grainy: the marks of a phagocyte.

12. BQ12 · Practice

Model answer. (a) The other four counts are all between 24 and 28, and 44 is far above them: 44 is anomalous. A count this high suggests a mistake, such as counting some cells twice, or a square in a thick part of the smear where the cells pile up. (b) Leave it out: (27 + 25 + 28 + 24) ÷ 4 = 104 ÷ 4 = 26 red cells in a square.

Checked automatically

(a) The anomalous count: 44 (exact value)
- Common wrong answer 24 : That is the smallest count, but it is close to the others. Look for the one far from the rest.
(b) Mean red cells in a square: 26 (2 significant figures)
- Common wrong answer 29.6 : You included the anomalous count, 44. Leave it out and divide by 4.
- Common wrong answer 20.8 : You left out 44 but still divided by 5. There are 4 counts left: divide 104 by 4.

A full answer includes

(a) I identified 44 as the anomalous count.
(b) I added the other four counts: 27 + 25 + 28 + 24 = 104.
(b) I divided 104 by 4.
(b) I wrote 26 red cells.
(b) I did not include 44 in the mean.

Also accept

(a) 44; (b) 26

Watch for

“Include every count in the mean, because they are all real counts.” — A count far from all the others is probably a mistake. Including it makes the mean too high: (27 + 25 + 28 + 44 + 24) ÷ 5 = 29.6.
“The anomalous count is 24, because it is the smallest.” — 24 is close to the other counts, 25, 27 and 28. Only 44 is far from the rest.

13. BQ9 · Practice

Model answer. A red blood cell has no nucleus, and it is thinner in the middle, so its centre looks paler. A nucleus stained purple, or a cell larger than most other cells, marks a white cell; a speck stained purple all over is a platelet.

A full answer includes

I chose: no nucleus inside the cell.
I chose: a centre paler than its edge.
I did not choose a nucleus stained purple: red cells have no nucleus.
I did not choose "stained purple all over": the stain colours nuclei and platelets, not red cells.

Also accept

no nucleus inside the cell; a centre paler than its edge

Watch for

“A nucleus stained purple.” — The stain colours nuclei purple, and red cells have no nucleus. A purple nucleus means a white cell.
“Larger than most other cells.” — Red cells are by far the commonest cells, so they set the usual size. A cell larger than most others is likely to be a white cell.
“Stained purple all over.” — The stain colours nuclei and platelets purple. A red cell has no nucleus, so it stays pink, with a paler centre.

14. BT1 · Apply

Model answer. The figure is red cells ÷ white cells: it shows how many there are of each kind compared with each other. It falls from several hundred to about 150 if there are more white cells than usual (for example, while the body fights an infection), or if there are fewer red cells than usual (for example, when the body makes too few of them). The answers can come in either order. The smear alone cannot tell which change happened, because it does not give the number of cells in a drop.

A full answer includes

I suggested more white cells than usual as one change.
I suggested fewer red cells than usual as the other change.
I did not suggest fewer white cells: that would make the figure larger.
I did not suggest more red cells: that would make the figure larger too.

Also accept

more white cells than usual; fewer red cells than usual (either order)

Watch for

“Fewer white cells than usual.” — The figure is red cells divided by white cells. Dividing by a smaller number of white cells gives a larger figure, not a smaller one.
“More red cells than usual.” — More red cells divided by the same number of white cells gives a larger figure, not a smaller one.
“The smear was spread too thinly.” — A thinner smear has fewer cells of both kinds in each field. The figure compares the two kinds with each other, so it stays the same.

15. BT2 · Apply

Model answer. (a) Field A alone gives no answer, because you cannot divide by zero, so add the counts of both fields before you divide. Red cells: (20 + 19) × 100 = 3900. White cells: 0 + 5 = 5. 3900 ÷ 5 = 780 red cells for each white cell. (b) Field A is a real result: its whole field was searched for white cells, and it held none. White cells are rare, so a field with none happens by chance. Leaving field A out drops its 2000 red cells but no white cells, so white cells look commoner than they are: field B alone gives 1900 ÷ 5 = 380, about half the figure from both fields.

Checked automatically

(a) Red cells for each white cell: 780 (exact value)
- Common wrong answer 380 : That is field B alone (1900 ÷ 5). Use both fields: add their counts before you divide.
- Common wrong answer 7.8 : Scale the red counts up to the whole fields first: each square is one hundredth of its field.

A full answer includes

(a) I added the red counts in the two squares: 20 + 19 = 39.
(a) I multiplied by 100 to get the red cells in both fields: 3900.
(a) I added the white cells: 0 + 5 = 5.
(a) I divided 3900 by 5.
(a) I wrote 780.
(b) I wrote that field A is a real count: white cells were searched for there, and there were none.
(b) I wrote that leaving it out makes white cells look commoner than they are.
(b) I did not call field A anomalous: white cells are rare, so a field can hold none by chance.

Also accept

(a) 780; (b) field A is a real count, and leaving it out makes white cells look commoner (380 instead of 780)

Watch for

“Leave field A out, because a field with no white cells must be a mistake.” — White cells are rare, so a field can hold none just by chance. Field A was searched and had none: that is a real result.
“Work out each field on its own and take the mean of the two answers.” — Field A on its own gives no answer, because you cannot divide by zero. Add the counts of both fields first, then divide.

16. BT3 · Apply

Model answer. A nucleus that is large and round and fills most of the cell marks a lymphocyte, so 75 of the 250 white cells are lymphocytes: 75 ÷ 250 × 100 = 30%. The 175 cells with a nucleus in lobes are phagocytes, the other 70%.

Checked automatically

Lymphocytes: 30 % (2 significant figures)
- Common wrong answer 70 %: Those are the cells with a nucleus in lobes: phagocytes. Use the cells with one large round nucleus.
- Common wrong answer 43 %: You divided by the 175 phagocytes. Divide by all 250 white cells.
- Common wrong answer 0.3 %: You divided correctly but did not multiply by 100.

A full answer includes

I took the 75 cells with one large round nucleus as the lymphocytes.
I divided 75 by 250 and multiplied by 100.
I wrote 30%.
I did not use the 175 cells with lobes: those are phagocytes.
I did not divide by 175: the whole is all 250 white cells.

Also accept

Watch for

“70%: the cells with a nucleus in lobes are the lymphocytes.” — A nucleus in lobes marks a phagocyte. A lymphocyte has one large round nucleus, so the lymphocytes are the 75.
“43%: 75 ÷ 175 × 100.” — That compares the lymphocytes with the phagocytes. A percentage of the white cells divides by all of them, 250.

17. BT4 · Apply

Model answer. When blood is spun, the cells sink and the plasma, the liquid part, is left on top: it is pale yellow, not red. All the red colour is in the packed layer of red blood cells at the bottom, because the red protein, haemoglobin, is inside the cells. So the red is not dissolved in the liquid. Plasma is mostly water, with other substances dissolved in it.

A full answer includes

I wrote that after spinning, the plasma at the top is pale yellow, not red.
I wrote that all the red is in the bottom layer, the red blood cells.
I wrote that the red protein, haemoglobin, is inside the cells.
I did not write that the red colour sinks on its own.

Also accept

plasma is pale yellow; the red is in the red blood cells

Watch for

“The red colour sinks to the bottom because it is heavy.” — Colour cannot sink on its own. What sinks are the red blood cells, which carry the red protein inside them.