Answers · Absorption

Where food is absorbed — answer key

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

1. AQ1 · Practice

Model answer. G is the ileum, most of the small intestine. The small intestine is the region where nutrients, the small soluble molecules made by digestion, are absorbed into the blood.

A full answer includes

I chose G, the ileum.
I did not choose H: the colon absorbs mainly water.

Also accept

Watch for

“E, the stomach, absorbs the food.” — The stomach churns food and starts to digest protein, but very little is absorbed there.
“H, the colon, absorbs the nutrients.” — The colon absorbs some water. By the time food reaches it, the nutrients have already been absorbed in the small intestine.
“F, the pancreas, absorbs the nutrients.” — The pancreas makes a digestive juice that flows into the small intestine. Food never passes through the pancreas, so nothing is absorbed there.

2. AQ2 · Practice

Model answer. Nutrients are absorbed in the small intestine. Absorption is the movement of nutrients from the intestines into the blood. Only small soluble molecules, such as glucose and amino acids, can be absorbed.

A full answer includes

I wrote small intestine (or ileum) in (1).
I wrote blood in (2).
I wrote soluble in (3).
I did not write stomach in (1).

Also accept

(1) small intestine (or ileum); (2) blood; (3) soluble

Watch for

“Absorption is taking food into the mouth.” — That is ingestion. Absorption is the movement of nutrients from the intestines into the blood.
“Nutrients are absorbed in the stomach.” — The stomach absorbs very little. Nutrients are absorbed in the small intestine.

3. AQ5 · Practice

Model answer. Most of the water entering the gut is absorbed from the small intestine. Some is also absorbed from the colon, and only a little leaves in the faeces.

A full answer includes

I chose: most from the small intestine, some also from the colon.
I did not choose the statement that most water is absorbed from the colon.

Also accept

most from the small intestine, some from the colon

Watch for

“Most water is absorbed from the colon.” — The colon does absorb water, but by the time food reaches it most of the water has already been absorbed from the small intestine.
“Most water is absorbed from the stomach.” — Very little is absorbed through the wall of the stomach. Most water is absorbed from the small intestine.
“No water is absorbed from the colon.” — The colon does absorb some water. That is why faeces are much drier than the contents of the small intestine.

4. AQ3 · Practice

Options as numbered on the pupil sheet: 1 — absorption; 2 — digestion.

Model answer. Breaking larger molecules into smaller ones is digestion: protease on protein, maltase on maltose. Small molecules moving from the intestine into the blood is absorption: the mineral ions, and the water taken up from the colon.

A full answer includes

I matched the protease to digestion.
I matched the mineral ions to absorption.
I matched the maltase to digestion.
I matched the water leaving the colon to absorption.
I did not match the water to digestion: water is absorbed without being broken down.

Also accept

protease, maltase: digestion; mineral ions, water into the blood: absorption

Watch for

“Water moving into the blood is not absorption, because water is not food.” — Absorption is movement from the intestines into the blood. Water moving from the colon into the blood is absorption.
“Maltase making glucose is absorption, because glucose is what gets absorbed.” — Making glucose from maltose breaks a molecule down: that is digestion. The glucose moving into the blood afterwards is absorption.

5. AQ4 · Practice

Model answer. Starch molecules are large and insoluble, too big to pass through the wall of the small intestine into the blood. Digestion, by amylase and then maltase, breaks starch down into glucose, a small soluble molecule, which can be absorbed.

A full answer includes

I wrote that starch molecules are large.
I wrote that starch is insoluble.
I wrote that only small soluble molecules can pass through the wall of the small intestine.
I wrote that digestion breaks starch into glucose, which can be absorbed.
I did not write that starch is digested so that it can be stored.

Also accept

starch is large and insoluble; glucose is small and soluble and can be absorbed

Watch for

“Starch is digested so that it can be stored.” — Digestion makes small soluble molecules so that they can be absorbed into the blood; starch itself cannot get through the wall.

6. AQ6 · Practice

Model answer. Water entering the gut = 2.8 L + 6.9 L = 9.7 L. All of it is absorbed or leaves in the faeces, so the small intestine absorbed 9.7 L − 1.2 L − 0.2 L = 8.3 L: most of the water.

Checked automatically

Water absorbed from the small intestine: 8.3 L (within ±0.05 L)
- Common wrong answer 8.5 L: Take away the 0.2 L in the faeces as well: that water was not absorbed.
- Common wrong answer 9.5 L: Take away the 1.2 L absorbed from the colon as well.
- Common wrong answer 9.7 L: That is all the water entering the gut. Take away what the colon absorbed and what left in the faeces.

A full answer includes

I added the water going in: 2.8 L + 6.9 L = 9.7 L.
I took away the colon and the faeces: 9.7 L − 1.2 L − 0.2 L.
I wrote 8.3 L.
I did not leave out the 0.2 L in the faeces.

Also accept

8.3 L

Watch for

“The small intestine absorbed 8.5 L.” — You forgot the 0.2 L that leaves in the faeces: it was never absorbed.
“The small intestine absorbed 9.5 L.” — You took away only the faeces. The colon absorbed 1.2 L of the water too.

7. AQ7 · Practice

Model answer. Percentage = 7.8 L ÷ 9.2 L × 100 = 84.8 %, which is 85 % to the nearest whole number. Most of the water is absorbed from the small intestine.

Checked automatically

Percentage absorbed from the small intestine: 85 % (within ±0.5 %)
- Common wrong answer 118 %: You divided the whole by the part. Work out 7.8 ÷ 9.2 × 100.
- Common wrong answer 15 %: That is the water the small intestine did not absorb. Use the 7.8 L it did absorb.

A full answer includes

I divided 7.8 L by 9.2 L and multiplied by 100.
I wrote 85 %.
I did not divide 9.2 by 7.8.

Also accept

85 %

Watch for

“9.2 ÷ 7.8 × 100 = 118 %.” — The part goes on top: the small intestine absorbs 7.8 L out of 9.2 L. A part can never be more than 100 % of the whole.
“(9.2 − 7.8) ÷ 9.2 × 100 = 15 %.” — That is the percentage not absorbed from the small intestine. The question asks for the part it does absorb.

8. AT1 · Apply

Model answer. (a) The small intestine is where nutrients are absorbed: the glucose and amino acids can pass through its wall into the blood straight away. (b) Glucose and amino acids are already small and soluble, so they can be absorbed without any digestion. Starch and protein are large insoluble molecules that would first have to be broken down.

A full answer includes

(a) I wrote that the small intestine is where nutrients are absorbed.
(a) I wrote that the glucose and amino acids pass into the blood there.
(b) I wrote that glucose and amino acids are small and soluble.
(b) I wrote that they can be absorbed without being digested.
(b) I wrote that starch and protein would have to be digested first.
I did not write that the small intestine digests the solution.

Also accept

(a) where nutrients are absorbed; (b) already small and soluble, absorbed without digestion

Watch for

“The solution goes into the small intestine because that is where it is digested.” — Glucose and amino acids need no digesting: they are the products of digestion. The small intestine is where they are absorbed.

9. AT2 · Apply

Model answer. (a) The faeces become very watery. (b) Normally most of the water is absorbed from the small intestine, and the colon absorbs only some. With the small intestine absorbing far less, litres more water than usual reach the colon. The colon absorbs some of it, but far less than reaches it, so the rest leaves the body in the faeces.

A full answer includes

(a) I wrote that the faeces become very watery.
(b) I wrote that the small intestine normally absorbs most of the water.
(b) I wrote that the colon absorbs only some of the water.
(b) I wrote that far more water now reaches the colon than it can absorb.
(b) I wrote that the rest of the water leaves the body in the faeces.
I did not write that the colon normally absorbs most of the water.

Also accept

(a) very watery; (b) the small intestine normally absorbs most of the water and the colon only some, so most of the extra water is lost

Watch for

“The colon takes up all the extra water, because the colon is where water is absorbed.” — The colon absorbs only some of the water. Most is normally absorbed in the small intestine, so when that fails, far more reaches the colon than it can take up.
“The faeces become drier, because the colon works harder.” — The colon does absorb some of the extra water, but far more arrives than it can take up, so the faeces become watery.

10. AT3 · Apply

Model answer. (a) The colon absorbs some of the water from the gut contents. Without it, that water is not absorbed, so the contents of the bag are more watery than normal faeces. (b) Glucose and amino acids are absorbed in the small intestine, and all of the small intestine is still there. The colon absorbs mainly water, so removing it does not reduce the nutrients absorbed.

A full answer includes

(a) I wrote that the contents are more watery than normal faeces.
(a) I wrote that the colon, which absorbs some water, is missing.
(b) I wrote that the small intestine, where nutrients are absorbed, is still there.
(b) I wrote that the colon absorbs mainly water, not glucose or amino acids.
I did not write that no water at all can be absorbed.

Also accept

(a) more watery: no colon to absorb some water; (b) the small intestine, where nutrients are absorbed, is still there

Watch for

“Without the colon, no water can be absorbed at all.” — Most water is absorbed from the small intestine, which is still there. Only the part the colon absorbs is lost.
“Without the colon, less glucose is absorbed.” — Glucose and amino acids are absorbed in the small intestine, before the gut contents reach the colon. The colon absorbs mainly water.