Answers · Blood

Blood — return questions key

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

1. BR1 · Return

Model answer. The cells that carry oxygen are the red blood cells. When blood is spun, they pack into the dark red layer at the bottom. The body has replaced the plasma but not yet these cells, so each sample has fewer red cells in it, and the dark red layer is thinner than in healthy blood. The plasma at the top is now a larger share of the sample, so its layer is thicker.

A full answer includes

I chose the dark red layer at the bottom.
I did not choose the pale yellow layer: the plasma has been replaced, so it is now a larger share of the blood.

Also accept

the dark red layer at the bottom

Watch for

“The pale yellow layer at the top.” — That layer is the plasma, and the plasma has been replaced. With fewer red cells, the plasma is a larger share of the sample, so its layer is thicker, not thinner.
“The thin pale band in the middle.” — That band holds the white cells and platelets, which do not carry oxygen. The component that has not been replaced is in another layer.
“No layer: all stay the same size.” — The sample has fewer of the cells that carry oxygen than healthy blood, so the layer they pack into must change.

2. BR2 · Return

Model answer. A white blood cell has a nucleus, stained purple; a red blood cell has none. White cells are usually larger. And white cells are far rarer: there are hundreds of red cells for each white cell.

A full answer includes

I compared both kinds of cell in each difference.
I wrote that white cells have a nucleus and red cells do not.
I wrote a second difference: white cells are usually larger (or: white cells are much rarer).
I did not write that red cells look red and white cells look white.

Also accept

two of: nucleus only in white cells; white cells larger; white cells much rarer

Watch for

“Red cells are red and white cells are white.” — In a stained smear the nucleus is what you see: white cells have a purple nucleus; red cells are pink with no nucleus.

3. BR3 · Return

Model answer. (a) A nucleus in lobes and grainy cytoplasm mark a phagocyte. (b) Phagocytes engulf bacteria by phagocytosis and digest them, so with more phagocytes in the blood, more of the bacteria can be destroyed and the infection is fought off sooner.

A full answer includes

(a) I identified a phagocyte.
(a) I did not identify a lymphocyte: a lymphocyte has one large round nucleus.
(b) I wrote that phagocytes engulf bacteria and digest them.
(b) I wrote that more phagocytes can destroy more of the bacteria.

Also accept

(a) phagocyte; (b) phagocytes engulf and digest bacteria, so more can be destroyed

Watch for

“The cells are lymphocytes, because there is an infection.” — Lymphocytes help to fight infections too, by producing antibodies, but a nucleus in lobes and grainy cytoplasm mark a phagocyte.

4. FR1 · Return

Model answer. The plasma carries urea from the liver, where it is made, to the kidneys, which remove it in urine. If the kidneys are not working properly, urea is not removed and builds up in the plasma.

A full answer includes

I chose the kidneys.
I did not choose the lungs: the lungs remove carbon dioxide, not urea.

Also accept

the kidneys

Watch for

“The lungs.” — The lungs remove carbon dioxide from the blood, not urea.
“The small intestine.” — The small intestine absorbs nutrients and ions. Urea is not taken in with food: it is made in the liver.
“The pancreas.” — The pancreas makes insulin and digestive enzymes. It does not remove urea from the blood.

5. FR2 · Return

Model answer. A pathogen is a microorganism that causes disease. (a) Both kinds are white blood cells: white cells defend the body. (b) Taking in a pathogen and digesting it is phagocytosis; the cells that do it are phagocytes. The other kind, lymphocytes, produce antibodies.

A full answer includes

(a) I stated white blood cells.
(a) I did not state platelets: platelets are fragments that help blood to clot.
(b) I stated phagocytosis.
(b) I did not state antibody production: that is the other kind of cell's defence.

Also accept

(a) white blood cells; (b) phagocytosis

Watch for

“(a) Platelets.” — Platelets are fragments of cells that help blood to clot. The cells that defend the body are white blood cells.
“(b) Antibody production.” — Antibody production is the defence of the other kind of cell. Taking in and digesting a pathogen has its own name.

6. FR3 · Return

Model answer. B. Antibodies are stuck to the bacteria and have clumped them, so the lymphocytes, which produce antibodies, are working (not A). The missing step is the next one: phagocytes engulf the clumps by phagocytosis and digest them. So the phagocytes are most likely not working. Phagocytes do not produce antibodies (not C), and platelets help blood to clot; they do not destroy bacteria (not D).

A full answer includes

I chose B.
I used the antibodies stuck to the bacteria: the lymphocytes have done their job.
I used the clumps left in the blood: no cell has engulfed and digested them.
I did not choose C: phagocytes engulf bacteria; they do not produce antibodies.

Also accept

Watch for

“Row A: lymphocytes, because they produce the antibodies.” — The antibodies have been made: they are stuck to the bacteria. So the lymphocytes, which produce them, are working.
“Row C: phagocytes, because they produce the antibodies.” — Phagocytes do not produce antibodies; lymphocytes do. And the antibodies have been made.
“Row D: platelets, because they trap bacteria in a clot.” — Platelets help blood to clot at a cut. They do not destroy bacteria in the blood.

7. CR1 · Return

Model answer. Clotting also prevents pathogens from entering the body. A cut is an opening through the skin, and the clot closes it, so bacteria and other pathogens cannot get in through it.

A full answer includes

I stated that clotting prevents pathogens entering the body.
I explained that the clot closes the opening in the skin.
I did not give stopping blood loss again: the pupil already gave that.

Also accept

prevents the entry of pathogens; the clot closes the opening

Watch for

“The clot kills the bacteria in the cut.” — A clot keeps pathogens out by sealing the opening. Killing bacteria is the work of white blood cells.

8. CR2 · Return

Model answer. The cut keeps bleeding for much longer, because no firm clot forms. Fibrin is made from fibrinogen, so with no fibrinogen there is no fibrin. The platelets still gather at the cut, but without fibrin threads there is no mesh to trap the blood cells and hold the clot together.

A full answer includes

I predicted that the cut keeps bleeding for much longer.
I explained that without fibrinogen, no fibrin can be made.
I explained that without fibrin threads there is no mesh to trap blood cells.
I did not write that the platelets turn into fibrin instead.

Also accept

keeps bleeding; no fibrinogen, so no fibrin; no mesh

Watch for

“The platelets turn into fibrin instead.” — Platelets are fragments of cells; they help start the clot. Fibrin is made only from fibrinogen.