Answers · Absorption

Absorption — return questions key

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

1. AR1 · Return

Model answer. A. The capsule passes through the stomach without dissolving, so the acid cannot reach the vitamin. It should open in the small intestine, the region where nutrients are absorbed into the blood. By the colon it would be too late: the colon absorbs mainly water.

A full answer includes

I chose row A.
I did not choose row C: a vitamin is already small enough to be absorbed without digestion.

Also accept

Watch for

“Row B: the colon absorbs most of the nutrients.” — The colon absorbs some water. Nutrients are absorbed in the small intestine, before the food reaches the colon.
“Row C: the vitamin must be digested in the small intestine.” — The small intestine is the right place, but not for that reason. A vitamin is a small molecule: it is absorbed without being digested.
“Row D: food stays longest in the colon, so more is absorbed.” — Food does stay long in the colon, but the colon absorbs mainly water, not nutrients such as vitamins.

2. AR2 · Return

Model answer. Most water is absorbed from the small intestine; some is also absorbed from the colon.

A full answer includes

I chose the small intestine.
I chose the colon.
I did not choose the stomach.

Also accept

small intestine and colon

Watch for

“The oesophagus and the colon.” — Food and drink pass down the oesophagus in a few seconds, so almost no water is absorbed there. The small intestine absorbs most of the water.
“The stomach and the small intestine.” — Very little is absorbed through the wall of the stomach. The second part that absorbs water is the colon.

3. VR1 · Return

Model answer. (a) Factor for the villi = 12 mm² ÷ 2.0 mm² = 6.0. (b) Factor for the microvilli = 300 mm² ÷ 12 mm² = 25. Together they multiply the flat surface by 6.0 × 25 = 150, so far more molecules can be absorbed in the same time than across a flat lining.

Checked automatically

(a) Factor for the villi: 6.0 (within ±0.05)
- Common wrong answer 10 : That is the surface added, 12 − 2.0. "How many times" needs a division: 12 ÷ 2.0.
- Common wrong answer 0.17 : You divided the flat patch by the villi. The factor is more than 1: 12 ÷ 2.0.
(b) Factor for the microvilli: 25 (within ±0.5)
- Common wrong answer 150 : That compares the total with the flat patch, so it includes the villi. Divide by the 12 mm² of the villi.
- Common wrong answer 288 : You subtracted. "How many times" needs a division: 300 ÷ 12.

A full answer includes

(a) I divided 12 mm² by 2.0 mm².
(a) I wrote 6.0.
(b) I divided 300 mm² by 12 mm².
(b) I wrote 25.
I did not divide 300 by 2.0 for (b): that factor includes the villi as well.

Also accept

(a) 6.0; (b) 25

Watch for

“(b) 300 ÷ 2.0 = 150.” — That compares the total with the flat patch, so it includes the villi too. The microvilli multiply the surface of the villi: 300 ÷ 12.
“(b) 300 − 12 = 288.” — "How many times" needs a division, not a subtraction: 300 ÷ 12.

4. VR2 · Return

Model answer. Diameter = 1.6 cm = 0.016 m. Area = 3.14 × 0.016 m × 4.0 m = 0.20 m². The real lining, folded into villi with microvilli on every lining cell, has many times this surface. (For comparison, the folded lining of a human small intestine has tens of square metres.)

Checked automatically

Inner surface area of a smooth tube: 0.20 m² (within ±0.005 m²)
- Common wrong answer 20 m²: Change 1.6 cm to metres first: 1.6 cm = 0.016 m.
- Common wrong answer 2.0 m²: Check the conversion: 1.6 cm = 0.016 m, not 0.16 m.
- Common wrong answer 0.10 m²: You used the radius. Use the diameter, 0.016 m.

A full answer includes

I changed 1.6 cm to 0.016 m.
I multiplied 3.14 × 0.016 m × 4.0 m.
I wrote 0.20 m².
I did not use 1.6 without changing it to metres.

Also accept

0.20 m²

Watch for

“20 m².” — You used the diameter in centimetres. Change 1.6 cm to 0.016 m first (100 cm = 1 m).
“0.10 m².” — You used the radius. The formula here uses the diameter, 0.016 m.

5. LR1 · Return

Model answer. The lacteal is the lymph vessel of a villus: a single tube in its middle, closed at its tip. The capillary network under the epithelium carries blood, and the epithelial cells and their microvilli are not vessels.

A full answer includes

I chose the single tube in the middle, closed at its tip.
I did not choose the network under the epithelium: those vessels carry blood.

Also accept

in the lacteal: the single tube in the middle, closed at its tip

Watch for

“In the network of vessels under the epithelium.” — Those are the capillaries, which carry blood. Lymph is in the lacteal in the middle.
“In the epithelial cells.” — The epithelial cells absorb the food, but they are not vessels and hold no lymph.
“In the microvilli.” — Microvilli are tiny projections of the epithelial cells. They carry no fluid.

6. LR2 · Return

Model answer. A. The microvilli are on the side of each epithelial cell that faces the food. They increase the surface area of the cell, and so the internal surface area across which molecules are absorbed.

A full answer includes

I chose row A.
I did not choose row B: microvilli do not move the food.

Also accept

Watch for

“Row B: microvilli push the food along.” — Microvilli do not move. The food is pushed along by the muscles in the wall of the intestine; microvilli increase the surface area.
“Row C: microvilli face the capillaries.” — Microvilli are on the side facing the food, where molecules enter the cell. Extra surface on the other side would not help molecules get in.
“Row D: microvilli line the lacteal.” — Microvilli are on the epithelial cells, not inside the lacteal. The lacteal carries fatty acids away, but it has no microvilli.

7. LR3 · Return

Options as numbered on the pupil sheet: 1 — the rest of the epithelial cell; 2 — the blood inside the capillary; 3 — the microvilli on the surface of an epithelial cell; 4 — the wall of a capillary.

Model answer. A glucose molecule crosses the surface of the microvilli of an epithelial cell, passes through the cell (one cell thick), then through the thin wall of a capillary into the blood, which carries it away.

A full answer includes

I put the microvilli first.
I put the rest of the epithelial cell second.
I put the wall of a capillary third.
I put the blood last.
I did not put the capillary wall before the epithelial cell.

Also accept

microvilli, epithelial cell, capillary wall, blood

Watch for

“It goes through the lacteal first.” — Glucose passes into the capillaries; the lacteal takes the products of fat digestion.