Answers · Absorption

Absorption — final check key

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

1. AK1 · Final check

Model answer. (a) Absorption: the small intestine is the region where nutrients are absorbed. (b) Starch is digested to glucose, but glucose is absorbed in the small intestine. With its lining damaged, less glucose passes into the blood, and the rest leaves in the faeces. The healthy stomach and colon cannot take over: they do not absorb nutrients like this.

A full answer includes

(a) I wrote absorption.
(b) I wrote that glucose is absorbed in the small intestine.
(b) I wrote that with the lining damaged, less glucose passes into the blood.
I did not write that the stomach or the colon can absorb the glucose instead.

Also accept

(a) absorption; (b) glucose is absorbed in the small intestine, so less reaches the blood

Watch for

“Digestion in the stomach is affected.” — The stomach is healthy. The damaged part is the small intestine, where nutrients are absorbed.
“The colon absorbs the glucose instead.” — The colon absorbs mainly water. The glucose that the small intestine misses leaves in the faeces.

2. AK2 · Final check

Model answer. Water absorbed = water entering − water passing on. Stomach: 4.0 L − 3.9 L = 0.1 L. Small intestine: 8.6 L − 1.8 L = 6.8 L. Colon: 1.8 L − 0.2 L = 1.6 L. So most of the water is absorbed from the small intestine, and some from the colon.

A full answer includes

I took the water passing on from the water entering, for each part.
I found 6.8 L absorbed from the small intestine.
I found 1.6 L absorbed from the colon.
I chose: most from the small intestine, some from the colon.
I did not read the water passing on as the water absorbed.

Also accept

most from the small intestine, some from the colon

Watch for

“Most from the colon: only 0.2 L passes on from it.” — Little passing on does not mean much absorbed. The colon received only 1.8 L and absorbed 1.8 − 0.2 = 1.6 L; the small intestine absorbed 8.6 − 1.8 = 6.8 L.
“Most from the stomach: the most water passes on from it.” — The water passing on is the water that was not absorbed. The stomach absorbed only 4.0 − 3.9 = 0.1 L.
“Some from the stomach.” — The stomach absorbed only 0.1 L, far less than the 1.6 L absorbed from the colon.

3. VK1 · Final check

Model answer. (a) Microvilli are far too small to be seen one by one with a light microscope, so the damage needs an electron microscope. (b) The microvilli increase the surface area of every lining cell. With fewer, shorter microvilli the internal surface area is smaller, so fewer nutrient molecules are absorbed in the time the food takes to pass through.

A full answer includes

(a) I wrote that microvilli are too small to see one by one with a light microscope.
(a) I wrote that an electron microscope is needed to see them.
(b) I wrote that the internal surface area is smaller.
(b) I wrote that fewer nutrient molecules are absorbed in the same time.
I did not write that the villi are damaged.

Also accept

(a) too small for a light microscope; (b) smaller surface area, less absorbed

Watch for

“The person absorbs less because the villi are damaged.” — The villi look normal. It is the smaller surface of each lining cell, lost with its microvilli, that reduces absorption.

4. VK2 · Final check

Model answer. (a) Total surface = 7.0 mm² × 20 = 140 mm². (b) New surface = 5.0 mm² × 20 = 100 mm². (c) Absorption is proportional to surface area, so before ÷ after = 140 ÷ 100 = 1.4: the patch absorbed glucose 1.4 times faster before the medicine.

Checked automatically

(a) Total surface before: 140 mm² (within ±1 mm²)
- Common wrong answer 27 mm²: You added. The microvilli multiply the surface of the villi: 7.0 × 20.
- Common wrong answer 20 mm²: That is the microvilli's factor alone. Multiply it by the 7.0 mm² from the villi.
(b) Total surface after: 100 mm² (within ±1 mm²)
- Common wrong answer 25 mm²: You added. Multiply the new surface of the villi by 20: 5.0 × 20.
(c) Times faster before: 1.4 (within ±0.05)
- Common wrong answer 40 : That is the surface lost, 140 − 100. "How many times" needs a division: 140 ÷ 100.
- Common wrong answer 0.71 : You divided the smaller surface by the larger. The answer is more than 1: 140 ÷ 100.

A full answer includes

(a) I multiplied 7.0 mm² by 20.
(a) I wrote 140 mm².
(b) I multiplied 5.0 mm² by 20.
(b) I wrote 100 mm².
(c) I divided 140 by 100.
(c) I wrote 1.4.
I did not subtract 100 from 140 for (c).

Also accept

(a) 140 mm²; (b) 100 mm²; (c) 1.4

Watch for

“The villi and microvilli add their surfaces: 7.0 + 20 = 27 mm².” — The microvilli multiply the surface of the villi: 7.0 mm² × 20.
“The patch absorbs 40 times less after the medicine.” — That is the surface lost, 140 − 100 mm². "How many times" needs a division: 140 ÷ 100.

5. LK1 · Final check

Options as numbered on the pupil sheet: 1 — wrong; 2 — correct.

Model answer. Correct: covering cells side by side, with no cell on top of another, make an epithelium one cell thick; the lacteal, a lymph vessel, is closed at its tip. Wrong: the capillary network lies just under the epithelium and the lacteal runs up the middle, not the other way round; a villus is made of many cells, not one; the lacteal carries lymph out at the base and holds no red blood cells.

A full answer includes

I marked the covering cells drawn side by side, none on top of another, as correct.
I marked the lymph vessel with a sealed end near the tip as correct.
I marked the lacteal drawn just under the covering cells as wrong.
I marked the villus drawn as one large cell as wrong.
I marked the lacteal joined to the venule as wrong.
I did not mark the sealed end of the lymph vessel as wrong.

Also accept

correct: covering cells side by side; lymph vessel sealed near the tip. Wrong: lacteal under the covering with capillaries in the middle; one large cell; lacteal joined to the venule

Watch for

“The lymph vessel should be open at its tip.” — The lacteal is closed at its tip; lymph leaves it at the base of the villus.
“It does not matter which vessel is in the middle.” — The capillary network lies just under the epithelium, so absorbed glucose has only a short way to go to the blood; the lacteal runs up the middle.
“A villus is one large cell with microvilli.” — A villus is a fold of the lining made of many cells. Microvilli are projections of each of its epithelial cells.

6. LK2 · Final check

Model answer. A. Glucose, water and mineral ions pass into the blood capillaries and are carried away in the blood. Fatty acids and glycerol pass into the lacteal and are carried away in the lymph, which later joins the blood.

A full answer includes

I chose row A.
I did not choose row D: fatty acids and glycerol pass into the lacteal first.

Also accept

Watch for

“Row B: water and mineral ions pass into the lacteal.” — Water and mineral ions are small and soluble, like glucose: they enter the blood capillaries. The lacteal takes the products of fat digestion.
“Row C: glucose into the lacteal, fatty acids and glycerol into the capillaries.” — The vessels are swapped: glucose enters the capillaries, fatty acids and glycerol the lacteal.
“Row D: everything passes into the capillaries.” — Fatty acids and glycerol do reach the blood, but later: first they enter the lacteal and travel in the lymph.