Answers · Blood

What each part does — answer key

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

1. FQ1 · Practice

Model answer. A. Potassium ions, like other ions, are absorbed from the gut and carried dissolved in the plasma (not B: haemoglobin combines with oxygen, not with ions). Most of the oxygen is carried combined with haemoglobin inside the red cells, as oxyhaemoglobin; only a little dissolves in the plasma (not D). The red blood cells are whole cells: the flowing plasma carries them along, but they do not dissolve in it (not C).

A full answer includes

I chose row A.
I used the ions: they are dissolved in the plasma.
I used the oxygen: most of it is combined with haemoglobin.
I used the red cells: they are whole cells, carried along by the plasma.
I did not choose row C: a cell does not dissolve.

Also accept

Watch for

“Row B: the ions are combined with haemoglobin and the oxygen is dissolved in the plasma.” — Haemoglobin combines with oxygen, not with ions. Ions dissolve in the water of the plasma.
“Row C: the red blood cells are dissolved in the plasma.” — A red blood cell is a whole cell. The plasma carries it along as it flows, but the cell does not dissolve.
“Row D: most of the oxygen is dissolved in the plasma.” — Only a little oxygen dissolves in the plasma. Most of it is combined with haemoglobin in the red cells.

2. FQ2 · Practice

Options as numbered on the pupil sheet: 1 — help the blood to clot at a cut; 2 — carry the cells and dissolved substances; 3 — carry oxygen, using haemoglobin; 4 — carry oxygen dissolved in its water; 5 — engulf pathogens and make antibodies.

Model answer. Red blood cells transport oxygen, using haemoglobin. White blood cells defend the body, by phagocytosis and by producing antibodies. Platelets help blood to clot. Plasma transports the blood cells and dissolved substances such as ions, nutrients, urea, hormones and carbon dioxide. Plasma does not carry the oxygen: the red cells do.

A full answer includes

I matched red blood cells to carrying oxygen, using haemoglobin.
I matched white blood cells to engulfing pathogens and making antibodies.
I matched platelets to helping the blood to clot.
I matched plasma to carrying the cells and dissolved substances.
I did not match plasma to carrying oxygen dissolved in its water.

Also accept

red: oxygen; white: phagocytosis and antibodies; platelets: clotting; plasma: transport

Watch for

“Plasma carries the oxygen, dissolved in its water.” — Oxygen is carried by red blood cells, combined with haemoglobin. Plasma carries the cells and dissolved substances such as carbon dioxide.
“Platelets engulf pathogens and white cells help the blood to clot.” — These two are swapped. Platelets are fragments that help blood to clot; white cells defend the body.

3. FQ3 · Practice

Model answer. The red blood cell is packed with haemoglobin, a red protein. As the cell passes through the lungs, the haemoglobin combines with oxygen to form oxyhaemoglobin. In the working leg muscle, where oxygen is low, the oxyhaemoglobin gives up its oxygen, and the muscle cells use it in respiration to release energy.

A full answer includes

(1) I wrote haemoglobin.
(2) I wrote lungs.
(3) I wrote oxyhaemoglobin.
(4) I wrote respiration.
I did not write the leg muscle in (2): the cell gives oxygen up there, it does not pick it up.

Also accept

(1) haemoglobin; (2) lungs; (3) oxyhaemoglobin; (4) respiration

Watch for

“The cell picks up its oxygen in the leg muscle.” — It is the other way round: the blood picks up oxygen in the lungs, where there is a lot of it, and gives it up in the muscle, where it is used.
“The muscle cells use the oxygen to make haemoglobin.” — Muscle cells use the oxygen in respiration, to release energy from glucose.

4. FQ4 · Practice

Options as numbered on the pupil sheet: 1 — forms a mesh of fibrin threads at a cut; 2 — takes pathogens inside itself and digests them; 3 — releases proteins that stick to pathogens.

Model answer. Phagocytes engulf pathogens by phagocytosis: they take them inside and digest them. Lymphocytes produce antibodies, proteins that stick to pathogens. Neither kind makes the threads of a clot: fibrin forms from fibrinogen in the plasma.

A full answer includes

I matched the phagocyte to taking pathogens inside itself and digesting them.
I matched the lymphocyte to releasing proteins (antibodies) that stick to pathogens.
I did not match the lymphocyte to taking pathogens inside itself.

Also accept

phagocyte: engulfs and digests; lymphocyte: antibodies

Watch for

“Lymphocytes engulf bacteria.” — Lymphocytes produce antibodies; phagocytes engulf.
“A white cell forms a mesh of fibrin threads at a cut.” — Fibrin threads form at a cut from fibrinogen, a protein in the plasma. White cells do not make them.

5. FQ5 · Practice

Model answer. C. Haemoglobin is what combines with oxygen, so the cell should be filled with it (not D). With no nucleus there is more room for haemoglobin, so the cell can carry more oxygen (not A). B holds as much haemoglobin as C, but in a thick ball the middle is far from the surface, so oxygen takes longer to diffuse in. A disc that is thinner in the middle keeps every part of the cell close to its surface, so oxygen has only a short way to go and is taken up quickly. Human red blood cells are like C.

A full answer includes

I chose C.
I used the missing nucleus: it leaves more room for haemoglobin.
I used the thin middle: oxygen has only a short way to diffuse in, so it is taken up quickly.
I did not choose A: a nucleus leaves less room for haemoglobin.
I did not choose B: it holds as much haemoglobin, but oxygen has further to diffuse into a thick ball.

Also accept

Watch for

“Row A: a large nucleus; a disc, thinner in the middle; haemoglobin.” — The nucleus takes up room that could hold more haemoglobin, so A carries less oxygen.
“Row B: no nucleus; a thick ball; haemoglobin.” — B holds as much haemoglobin as C, but in a thick ball the middle of the cell is far from its surface. Oxygen has further to diffuse, so it is taken up more slowly.
“Row D: no nucleus; a disc, thinner in the middle; water and enzymes.” — Without haemoglobin there is nothing to combine with the oxygen, so D carries very little.

6. FQ6 · Practice

Options as numbered on the pupil sheet: 1 — oxygen; 2 — adrenaline; 3 — urea; 4 — carbon dioxide; 5 — glucose; 6 — amino acids.

Model answer. During a long run the liver breaks down its store of glycogen to glucose, and the plasma carries the glucose to the brain, which uses it in respiration. Carbon dioxide from respiring muscle is carried to the lungs and breathed out. Adrenaline, a hormone, goes from the adrenal glands to its target organs, such as the heart. Amino acids from digested protein are absorbed in the small intestine and carried to a growing muscle, which builds them into proteins. Urea is made in the liver too, but it is carried to the kidneys, not the brain, and oxygen is carried by the red cells, not the plasma.

A full answer includes

I matched the journey from the liver to the brain to glucose.
I matched the journey from a working leg muscle to the lungs to carbon dioxide.
I matched the journey to the heart to adrenaline.
I matched the journey to a growing muscle to amino acids.
I did not match any journey to oxygen: the plasma does not carry the oxygen.

Also accept

glucose; carbon dioxide; adrenaline; amino acids

Watch for

“The journey from the liver is urea, because urea is made in the liver.” — Urea is made in the liver, but the plasma carries it to the kidneys, not the brain. The brain needs glucose for respiration.
“Oxygen goes from the leg muscle to the lungs.” — Oxygen goes the other way, from the lungs to the muscle, and it is carried by haemoglobin in the red cells, not by the plasma.

7. FQ7 · Practice

Model answer. Calcium ions, urea, vitamin C and carbon dioxide are all carried dissolved in the plasma: an ion, a waste made in the liver, a nutrient absorbed from the gut and a waste gas from respiring cells. Haemoglobin is inside the red blood cells, not dissolved in the plasma. Dietary fibre is not digested, so it stays in the gut and is never absorbed into the blood.

A full answer includes

I chose calcium ions.
I chose urea.
I chose vitamin C.
I chose carbon dioxide.
I did not choose haemoglobin: it stays inside the red cells.
I did not choose dietary fibre: it is never digested or absorbed.

Also accept

calcium ions; urea; vitamin C; carbon dioxide

Watch for

“Haemoglobin.” — Haemoglobin is a protein inside red blood cells. The plasma carries the red cells, but the haemoglobin stays inside them.
“Dietary fibre.” — Fibre is the part of plant food that is not digested. It passes along the gut and is never absorbed, so the blood does not carry it.

8. FQ8 · Practice

Options as numbered on the pupil sheet: 1 — Lymphocytes release antibodies against the bacteria.; 2 — A phagocyte flows round a clump and takes it inside.; 3 — Enzymes inside the phagocyte digest the bacteria.; 4 — Bacteria from the tooth get into the blood.; 5 — Antibodies stick to the bacteria and clump them together..

Model answer. Bacteria from the infected tooth get into the blood. Lymphocytes respond by releasing antibodies against them. The antibodies stick to the bacteria and clump them together. A phagocyte flows round a clump and takes it inside. There the bacteria are digested by enzymes. Phagocytes also engulf single bacteria from the start of an infection; this order follows one clump.

A full answer includes

I put the bacteria getting into the blood first.
I put the lymphocytes releasing antibodies before the antibodies stick to the bacteria.
I put the phagocyte taking in a clump after the antibodies had clumped the bacteria.
I put digestion by enzymes last.
I did not put digestion before the phagocyte took the bacteria inside.

Also accept

bacteria enter; lymphocytes release antibodies; antibodies clump the bacteria; a phagocyte takes in a clump; enzymes digest the bacteria

Watch for

“The phagocyte digests the bacteria first, then takes them in.” — Digestion happens inside the phagocyte, after the bacteria have been taken in.
“The antibodies digest the bacteria.” — Antibodies are proteins that stick to the bacteria. Enzymes inside the phagocyte digest them.
“The phagocyte takes in the bacteria before the antibodies clump them.” — The phagocyte takes in a clump, so the antibodies must have clumped the bacteria first.

9. FQ9 · Practice

Model answer. (a) Lymphocytes: the clumps are made by antibodies in the first person's plasma, and lymphocytes produce antibodies. (b) Antibodies are made against one kind of pathogen. The second person has never had this infection, so their lymphocytes have never made antibodies against these bacteria, and their plasma has none to stick to them and clump them. Plasma is the liquid part of blood, so neither sample contains white cells: the clumping is done by antibodies dissolved in it.

A full answer includes

(a) I identified lymphocytes.
(a) I did not identify phagocytes: phagocytes engulf pathogens; they do not make antibodies.
(b) I wrote that this plasma has no antibodies against these bacteria.
(b) I wrote that this person's lymphocytes have never met these bacteria, so they have not made those antibodies.
(b) I did not write that this plasma lacks lymphocytes: neither sample of plasma contains cells.

Also accept

(a) lymphocytes; (b) no antibodies against these bacteria, because their lymphocytes have never met them

Watch for

“(b) The second person's plasma has no lymphocytes in it.” — Neither sample has lymphocytes in it: plasma is the liquid part of blood, without the cells. What the first sample has is antibodies dissolved in it.
“(a) Phagocytes, because they destroy bacteria.” — Phagocytes engulf bacteria, but they do not release a substance that clumps them. Antibodies come from lymphocytes.

10. FT1 · Apply

Model answer. Each red blood cell carries oxygen combined with haemoglobin. With more red cells there is more haemoglobin in each drop of blood, so the blood can pick up and carry more oxygen from the thinner air, and enough oxygen reaches the tissues for respiration.

A full answer includes

I wrote that more red cells means more haemoglobin.
I wrote that more haemoglobin means the blood can carry more oxygen.
I wrote that this makes up for the lower oxygen in the air.
I wrote that enough oxygen then reaches the tissues for respiration.
I did not write that red cells make oxygen.

Also accept

more red cells, more haemoglobin, more oxygen carried to the tissues

Watch for

“More red cells make more oxygen.” — Red cells do not make oxygen; they carry it. More cells means more haemoglobin to carry the oxygen there is.

11. FT2 · Apply

Model answer. Haemoglobin that has combined with carbon monoxide cannot combine with oxygen, and the carbon monoxide lets go of it only very slowly. So the blood carries less oxygen, and the tissues, such as the brain and heart muscle, do not get enough oxygen for respiration.

A full answer includes

I wrote that haemoglobin combined with carbon monoxide cannot carry oxygen.
I wrote that less oxygen reaches the tissues.
I wrote that the tissues then cannot respire properly.
I did not write that carbon monoxide destroys the red cells.

Also accept

haemoglobin blocked; less oxygen carried; tissues get too little oxygen

Watch for

“Carbon monoxide destroys the red cells.” — The cells are not destroyed: their haemoglobin is taken up by carbon monoxide, so it cannot carry oxygen.

12. FT3 · Apply

Model answer. Phagocytes still engulf bacteria by phagocytosis and digest them. But lymphocytes produce antibodies, so without working lymphocytes the person makes no antibodies, and bacteria are not clumped and marked for destruction; infections are harder to fight.

A full answer includes

(a) I wrote phagocytosis.
(a) I linked it to the phagocytes, which still engulf and digest bacteria.
(b) I wrote antibody production.
(b) I linked it to the lymphocytes, which produce antibodies.
I did not write that they have no defence at all.

Also accept

still: phagocytosis; lost: antibodies (from lymphocytes)

Watch for

“They have no defence at all.” — Phagocytes are normal, so phagocytosis still works. Only antibody production is lost.