Answers · Chemical digestion

Through the wall of the gut — answer key

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

1. BQ1 · Practice

Model answer. Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules. In bag A, the large molecules were starch. Amylase, an enzyme, broke them down into a sugar, maltose, whose small molecules passed through the tubing into the water outside.

A full answer includes

(1) I wrote large.
(2) I wrote soluble.
(3) I wrote starch.
(4) I wrote sugar (or maltose).
(5) I wrote amylase.
I did not write that chemical digestion only breaks food into smaller pieces.

Also accept

(1) large; (2) soluble; (3) starch; (4) sugar (maltose, a reducing sugar); (5) amylase

Watch for

“Chemical digestion breaks food into smaller pieces.” — That is physical digestion, as in chewing: the pieces get smaller, but the molecules stay the same. Chemical digestion breaks the molecules themselves, so new, small soluble molecules are made.

2. BQ2 · Practice

Model answer. Iodine turned blue-black outside bag C from 10 minutes on, so starch was in the water outside. Starch molecules are far too large to pass through the holes in whole tubing: outside bag A, iodine stayed yellow-brown every time. So bag C probably has a hole in its wall or a leaking knot. Amylase breaks starch down; it never makes it, and sugar does not join back into starch in the water.

A full answer includes

I chose: starch got out, so bag C probably has a hole or a leak.
I used the iodine results: blue-black means starch is there.
I did not choose that starch molecules are small enough to pass through the tubing.

Also accept

starch got out, so bag C probably has a hole or a leak

Watch for

“Amylase got out of bag C and made starch in the water outside.” — Amylase breaks starch down; it never makes it. Amylase is also a protein, a large molecule, so it cannot get through whole tubing either.
“Sugar got out of bag C and turned back into starch in the water.” — Nothing in the water outside could join sugar molecules back into long starch chains. The blue-black colour means starch itself was there.
“Starch molecules are small enough to pass through the tubing.” — If they were, iodine would have turned blue-black outside bag A too, and it never did. Something must be different about bag C: a hole lets the large starch molecules out.

3. BQ3 · Practice

Model answer. Starch, protein and fat are large insoluble molecules. They cannot pass through the wall of the small intestine, so enzymes must first break them down into small soluble molecules: sugars, amino acids, and fatty acids and glycerol. Glucose, vitamin C, salt and water are already small and soluble, so they are absorbed as they are.

A full answer includes

I chose starch.
I chose protein.
I chose fat.
I did not choose glucose, vitamin C, salt or water: they are already small and soluble.

Also accept

starch, protein, fat

Watch for

“Glucose must be digested, because all food has to be digested.” — Glucose is already a small soluble molecule: it is what starch is digested into. It is absorbed as it is.
“Vitamin C must be digested, because it is a nutrient.” — Being a nutrient does not decide it; size does. Vitamin C is a small soluble molecule, so it is absorbed without digestion.
“Fat need not be digested, because it is a liquid at body temperature.” — Being a liquid is not what counts: fat molecules are large and do not dissolve in water. Lipase must break them into fatty acids and glycerol first.

4. BQ4 · Practice

Options as numbered on the pupil sheet: 1 — Each maltose molecule is split into two glucose molecules; 2 — Glucose passes through the lining of the small intestine into the blood; 3 — Pancreatic juice in the duodenum turns the remaining starch into maltose; 4 — Saliva mixed with the chewed potato starts turning some starch into maltose; 5 — Maltase in the saliva splits maltose into glucose in the mouth; 6 — Maltose meets maltase held on the cells lining the small intestine.

Model answer. Saliva brings amylase from the salivary glands, so in the mouth some of the starch starts to be broken down into maltose. In the duodenum, pancreatic juice brings more amylase, which turns the remaining starch into maltose. The maltose then meets maltase, which is held on the membranes of the epithelium, the cells lining the small intestine. Maltase splits each maltose molecule into two glucose molecules, and the glucose passes through the lining into the blood. The step with maltase in the saliva never happens: maltase is not in any juice, so no maltose is split in the mouth.

A full answer includes

Step 1: I put saliva starting to turn some of the starch into maltose.
Step 2: I put pancreatic juice turning the remaining starch into maltose.
Step 3: I put maltose meeting maltase held on the cells lining the small intestine.
Step 4: I put each maltose molecule being split into two glucose molecules.
Step 5: I put glucose passing into the blood.
I left out the step with maltase in the saliva.
I did not put any maltase step before an amylase step.

Also accept

saliva starts turning starch into maltose; pancreatic juice turns the rest into maltose; maltose meets maltase on the lining of the small intestine; maltose split into two glucose; glucose into the blood; the step with maltase in the saliva left out

Watch for

“Amylase breaks starch all the way down into glucose.” — Amylase stops at maltose, two glucose units joined together. Maltase, on the membranes of the epithelium lining the small intestine, splits maltose into glucose.
“Maltase in the saliva splits maltose into glucose in the mouth.” — Maltase is not poured into the gut in a juice at all: it stays on the cell membranes of the epithelium lining the small intestine, so maltose is split only there.

5. BQ5 · Practice

Options as numbered on the pupil sheet: 1 — Inside the bag and in the water outside; 2 — Only inside the bag.

Model answer. Only small molecules pass through the holes in the tubing. Starch is a large insoluble molecule, and amylase is a protein, also a large molecule, so both are found only inside the bag. Glucose is a small, soluble molecule: it was put into the bag, but it passes through the holes too, so it ends up inside and outside. Amylase breaks the starch down into maltose, a small soluble sugar, which also passes through the wall, so it is found on both sides as well.

A full answer includes

Starch: I chose only inside the bag.
Amylase: I chose only inside the bag.
Glucose: I chose inside the bag and in the water outside.
Maltose: I chose inside the bag and in the water outside.
I did not put amylase outside the bag because it is dissolved.

Also accept

starch, amylase: only inside; glucose, maltose: inside and outside

Watch for

“Amylase passes out, because it is dissolved in the water.” — Being dissolved is not enough: a molecule must be small enough for the holes. Amylase is a protein, a large molecule, so it stays inside.
“Glucose stays in the bag, because it was not made by digestion.” — Size decides whether a molecule gets out, not where it came from. Glucose is small and soluble, so it passes through the holes.
“Maltose stays in the bag, because that is where it was made.” — Maltose is small and soluble, so it passes out, whatever made it.

6. BQ6 · Practice

Model answer. (a) Bag Q holds the same volume of the same gelatine suspension, with the same volume of water instead of the protease (boiled protease, which no longer works, also does). It is made from the same tubing, hangs in the same volume of water and stays at 37 °C for the same time. (b) The protease is then the only difference between the bags. If amino acids appear in the water outside bag P but not outside bag Q, gelatine does not break down on its own, so the protease must have made them.

A full answer includes

(a) I put the same volume of gelatine suspension in bag Q.
(a) I put water (or boiled protease) in bag Q instead of the protease.
(a) I kept bag Q at 37 °C.
(a) I left bag Q for the same time as bag P.
(b) I wrote that bag Q shows whether gelatine breaks down on its own.
(b) I wrote that if amino acids appear outside P but not outside Q, the protease made them.
I did not leave the gelatine out of bag Q.

Also accept

(a) the same gelatine suspension, water or boiled protease instead of protease, the same conditions; (b) the protease is the only difference, so amino acids outside P only show that the protease made them

Watch for

“Bag Q should hold protease and no gelatine.” — That bag would not show whether gelatine breaks down without the enzyme. The enzyme is what is being tested, so it is the one thing to leave out.
“Bag Q is a repeat of bag P, to make the results more reliable.” — A repeat keeps the protease in. A control leaves out the one thing being tested, the protease, to show what happens without it.

7. BQ7 · Practice

Model answer. (a) A protein molecule is a long chain of amino acids: a large, insoluble molecule. It is too big to pass through the wall of the small intestine, so it cannot be absorbed into the blood. (b) Chemical digestion breaks the protein down into amino acids, small soluble molecules, which can be absorbed through the wall of the small intestine into the blood.

A full answer includes

(a) I wrote that a protein is a large molecule.
(a) I wrote that it is too big to pass through the wall of the small intestine.
(b) I wrote that chemical digestion produces small soluble molecules (amino acids).
(b) I wrote that these can be absorbed into the blood.
I did not write that chewing makes the protein small enough to be absorbed.

Also accept

(a) a large, insoluble molecule, too big to pass through the wall; (b) it produces small soluble molecules (amino acids) that can be absorbed

Watch for

“Protein is absorbed as it is, because the body is built from protein.” — The body needs protein, but a protein molecule is too large to cross the wall of the small intestine. It is absorbed as amino acids, and the cells build their own proteins from them.
“Protein can be absorbed once chewing has broken the fish into tiny pieces.” — Chewing makes smaller pieces of the same large protein molecules. Only chemical digestion breaks the molecules themselves into amino acids.

8. BQ8 · Practice

Model answer. (a) Maltase is on the cell membranes of the epithelium, the layer of cells lining the small intestine. It breaks each maltose molecule down into two glucose molecules. (b) The glucose is made at the very surface through which it is absorbed, so it can pass into the blood straight away instead of first moving through the food in the gut. The maltase also stays where it is needed instead of being carried along the gut with the food.

A full answer includes

(a) I wrote that maltase is on the cell membranes of the epithelium.
(a) I wrote that the epithelium lines the small intestine.
(a) I wrote that maltase breaks maltose down into glucose.
(b) I wrote that the glucose can be absorbed into the blood straight away.
I did not write that maltase is made in the pancreas.

Also accept

(a) on the membranes of the epithelium lining the small intestine; maltose to glucose; (b) glucose can be absorbed straight away

Watch for

“Maltase is made in the pancreas and poured into the small intestine.” — That is true of amylase, not maltase. Maltase stays on the membranes of the cells lining the small intestine, and maltose is broken down there.
“Maltase breaks starch down into glucose.” — Maltase acts only on maltose. Amylase must first break the starch down into maltose.

9. BQ9 · Practice

Model answer. Each maltose molecule is two glucose units joined together, so 600 ÷ 2 = 300 maltose molecules. One large, insoluble molecule has become 300 small, soluble ones, each small enough to pass through the tubing.

Checked automatically

Number of maltose molecules: 300 (exact value)
- Common wrong answer 600 : That is the number of glucose units. Each maltose molecule is two of them joined together, so divide by 2.
- Common wrong answer 1200 : That would need more glucose units than the chain has. Each maltose molecule takes two units, so divide 600 by 2.

A full answer includes

I divided 600 by 2, the number of glucose units in one maltose molecule.
I wrote 300.
I did not write 600.

Also accept

Watch for

“Breaking the starch down makes 600 maltose molecules, one for each glucose unit.” — A maltose molecule is a piece two glucose units long, so each one uses two units: 600 ÷ 2 = 300.

10. BT1 · Apply

Model answer. (a) Lipase. Glycerol is made only when fats and oils are digested, and lipase is the enzyme that digests them. Benedict's solution stayed blue, so no reducing sugar was made: the starch was not digested, so the enzyme was not amylase. (b) Lipase broke the oil molecules down into fatty acids and glycerol. Glycerol is a small, soluble molecule, so it passed through the holes in the tubing into the water outside. Starch and oil are large molecules that do not dissolve in water, so they stayed inside the bag, and iodine outside stayed yellow-brown. Lipase is a protein, a large molecule, so it stayed inside too.

A full answer includes

(a) I wrote lipase.
(a) I used the glycerol: it is made only when fats and oils are digested.
(a) I used the Benedict's result: no reducing sugar was made.
(b) I wrote that lipase broke the oil down into fatty acids and glycerol.
(b) I wrote that glycerol is a small soluble molecule.
(b) I wrote that glycerol passes through the holes in the tubing.
(b) I wrote that starch and oil are large molecules that do not dissolve in water.
(b) I wrote that they cannot pass through the holes.
I did not write that the enzyme was amylase.

Also accept

(a) lipase; (b) lipase made glycerol from the oil, a small soluble molecule that passes through the holes; starch and oil are large and insoluble and stay inside

Watch for

“The enzyme was amylase, because the bag held starch.” — Amylase would have broken the starch down into reducing sugar, which would pass out and turn Benedict's solution green or orange. It stayed blue, and glycerol comes from fats and oils, so the enzyme was lipase.
“Oil gets out of the bag because it is a liquid.” — Being a liquid is not what lets a substance through: it must be made of small, soluble molecules. Oil does not dissolve in water and its molecules are large, so it stays inside; only the glycerol and fatty acids made from it get out.
“The starch was digested into glycerol, which passed out.” — Starch is a chain of glucose units, so digesting it gives sugars, not glycerol. Glycerol is part of fat and oil molecules.

11. BT2 · Apply

Model answer. The drip bypasses the gut: it puts the food straight into the blood, where there is no chemical digestion. Normally, digestion breaks starch and protein into small soluble molecules, glucose and amino acids, which are absorbed into the blood and carried to the cells. The drip gives the blood these small molecules directly, in the form the cells can use. Starch and protein are large insoluble molecules; they would not be broken down in the blood, so the cells could not use them.

A full answer includes

I wrote that the drip puts the liquid straight into the blood.
I wrote that there is no chemical digestion in the blood.
I wrote that glucose and amino acids are small soluble molecules.
I wrote that they are what digestion normally makes from starch and protein.
I wrote that starch and protein are large insoluble molecules.
I wrote that they would not be broken down in the blood.
I did not write that glucose and amino acids are easier for the stomach to digest.

Also accept

the drip goes into the blood, where there is no digestion; glucose and amino acids are the small soluble molecules digestion would make and that are absorbed

Watch for

“Glucose and amino acids are given because they are easier for the stomach to digest.” — The liquid never reaches the stomach: it goes into a vein. Glucose and amino acids need no digestion at all; they are already the small soluble molecules that digestion makes.

12. BT3 · Apply

Model answer. Drops of water from outside bag A stay in the pipette and are mixed into the sample from outside bag B. After 10 and 20 minutes that water contains reducing sugar, so Benedict's with the sample from bag B could turn green instead of staying blue. Bag B is the control, and it would then seem to show sugar being made without amylase. She might conclude that starch breaks down on its own in warm water, and she could no longer be sure that the amylase made the sugar in bag A. A separate clean pipette for each beaker, or washing the pipette between samples, avoids this.

A full answer includes

(a) I wrote that drops of water from outside bag A stay in the pipette.
(a) I wrote that these drops mix into the sample from outside bag B.
(a) I wrote that from 10 minutes on this water contains reducing sugar.
(a) I wrote that Benedict's with the sample from bag B could then turn green instead of staying blue.
(b) I wrote that she might conclude that starch breaks down into sugar without amylase.
(b) I wrote that a separate clean pipette for each beaker avoids the problem.
I did not write that the pipette carries amylase into beaker B.

Also accept

sugar carried over from beaker A; Benedict's for bag B could change colour; it looks as if sugar is made without amylase; use a separate clean pipette

Watch for

“It does not matter, because the iodine test would show the mistake.” — Iodine tests for starch, and no starch gets out of either bag. What is carried over is sugar, and only Benedict's would show it, wrongly, in the sample from bag B.
“The pipette would carry amylase into beaker B, and it would digest the starch there.” — The amylase never left bag A: it is too large to pass through the tubing, so there is none in the water outside. Only the small sugar molecules are carried over.

13. BT4 · Apply

Model answer. (a) Maltose. Amylase from the salivary glands and the pancreas does not depend on these cells, so it still breaks the starch down into maltose. The next step needs maltase on the membranes of the epithelium, and there is little of it, so maltose builds up. (b) Little maltose is broken down into glucose, so little glucose is made at the surface of the epithelium, where it would be absorbed. Less glucose is absorbed into the blood.

A full answer includes

(a) I wrote maltose.
(a) I wrote that amylase still breaks starch down into maltose.
(b) I wrote that less maltose is broken down into glucose.
(b) I wrote that less glucose is absorbed into the blood.
I did not write that starch builds up.

Also accept

(a) maltose; (b) less glucose made from maltose, so less glucose absorbed

Watch for

“Starch builds up, because the damaged cells cannot digest it.” — Starch digestion by amylase does not depend on these cells: amylase from the salivary glands and pancreas still breaks starch into maltose. It is the next step, by maltase on the epithelium, that fails, so maltose builds up.
“Glucose absorption is not affected, because amylase makes glucose.” — Amylase makes maltose, not glucose. With little maltase, little glucose is made from the maltose, so little glucose is absorbed.