Chemical digestion — return questions key

1. BR1 · Return

Model answer. A protease breaking bean protein down into amino acids. Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules: protein molecules are large and insoluble, and amino acids are small and soluble. Grinding a peanut only makes smaller pieces of the same molecules: that is physical digestion. Melting chocolate changes its fat from a solid to a liquid, but the fat molecules stay the same. Glucose passing through the wall of the small intestine is absorption: nothing is broken down.

A full answer includes

• I chose the protease breaking bean protein down into amino acids.
• I used the meaning: large insoluble molecules are broken down into small soluble molecules.
• I did not choose the peanut ground into small pieces.

Also accept

• a protease breaks bean protein down into amino acids

Watch for

• “Teeth grinding a peanut into small pieces is chemical digestion.” — Grinding makes smaller pieces of the same molecules: that is physical digestion. No large molecule is broken down into small ones.
• “Chocolate melting in the mouth is chemical digestion.” — Melting changes the fat in the chocolate from a solid to a liquid, but the fat molecules stay the same size: nothing is broken down.
• “Glucose passing through the wall of the small intestine is chemical digestion.” — That is absorption. Glucose is already a small soluble molecule, and nothing is broken down as it crosses the wall.

2. BR2 · Return

Model answer. It passes along the gut and leaves the body in the faeces. Only small soluble molecules can be absorbed through the wall of the small intestine, and chemical digestion is what makes them. Humans have no enzyme that breaks cellulose down, so it stays a large insoluble molecule and cannot be absorbed. (Bacteria in the colon break down a little of it, but most of it leaves the body as fibre.)

A full answer includes

• I chose: it passes along the gut and leaves the body in the faeces.
• I used the rule: only small soluble molecules can be absorbed.
• I did not choose that chewing turns it into glucose.

Also accept

• it passes along the gut and leaves the body in the faeces

Watch for

• “Cellulose is absorbed into the blood as large molecules.” — Large insoluble molecules cannot pass through the wall of the small intestine. Without an enzyme to break it into small soluble molecules, cellulose cannot be absorbed.
• “Chewing breaks cellulose down into glucose, which is absorbed.” — Chewing is physical digestion: it makes smaller pieces of the same large molecules. Only an enzyme could break cellulose into glucose, and humans make none.
• “Cellulose dissolves in the stomach acid and is absorbed.” — The acid digests nothing, and cellulose does not dissolve in it. It stays a large insoluble molecule.

3. BR3 · Return

Model answer. B. Maltase breaks maltose down into glucose. It is not in any juice: it stays on the cell membranes of the epithelium, the layer of cells lining the small intestine, so the maltose is broken down there and the glucose is absorbed at once. Amylase acts on starch, which it breaks down into maltose; it cannot break maltose down, so rows A and D are wrong.

A full answer includes

• I chose row B.
• I chose maltase, the enzyme that acts on maltose.
• I placed maltase on the membranes of the epithelium.
• I did not choose row C: maltase is not in pancreatic juice.

Also accept

• B

Watch for

• “Row A: amylase in pancreatic juice digests the maltose.” — Amylase breaks starch down into maltose; it does not break maltose down. Maltase does.
• “Row C: maltase in pancreatic juice digests the maltose.” — Maltase is not poured into the gut in a juice. It stays on the membranes of the epithelium lining the small intestine.
• “Row D: amylase on the epithelium digests the maltose.” — Amylase acts only on starch, and it is not held on the epithelium: it arrives in saliva and pancreatic juice.

4. ER1 · Return

Model answer. (a) Amylase. Benedict's solution stayed blue before the enzyme was added, so at the start there was no reducing sugar. After 15 minutes it turned orange, so reducing sugar had been made. Amylase is the enzyme that breaks starch down into simple reducing sugars. (b) Simple reducing sugars, such as maltose. Starch itself does not change the colour of Benedict's solution: the sample taken before the enzyme was added held starch and stayed blue.

A full answer includes

• (a) I wrote amylase.
• (a) I used the Benedict's results: a new substance was made from the starch.
• (b) I wrote simple reducing sugars (such as maltose).
• I did not write that the starch turned the Benedict's solution orange.

Also accept

• (a) amylase; (b) simple reducing sugars (maltose, or glucose)

Watch for

• “The enzyme was protease.” — Protease breaks protein down into amino acids, and Benedict's solution does not detect them. A reducing sugar was made from the starch, so the enzyme was amylase.
• “Benedict's solution turned orange because of the starch.” — Starch does not change Benedict's solution: the sample taken before the enzyme was added held starch and stayed blue. The orange colour shows the sugar made from the starch.

5. ER2 · Return

Model answer. D. Starch is digested by amylase, and the salivary glands secrete amylase into the mouth, so starch digestion starts there. Protein is digested by protease, and the first protease the food meets is secreted by the stomach. Fat is digested by lipase, which comes from the pancreas, so fat digestion starts only when the food reaches the small intestine.

A full answer includes

• I chose row D.
• For the starch, I chose the mouth.
• For the protein, I chose the stomach.
• For the fat, I chose the small intestine.
• I did not choose row A: saliva has no protease.

Also accept

• D

Watch for

• “Row A: the digestion of the protein starts in the mouth.” — Saliva contains amylase, which acts only on starch. There is no protease in the mouth, so protein digestion starts in the stomach.
• “Row B: the digestion of the starch starts in the stomach.” — The stomach secretes protease, not amylase. Starch first meets amylase in the mouth, in saliva.
• “Row C: the digestion of the fat starts in the stomach.” — The stomach secretes protease, which acts only on protein. Fat first meets lipase in the small intestine, in juice from the pancreas.

6. ER3 · Return

Model answer. (a) Protease. (b) Protease breaks protein down into amino acids, so amino acids appeared. Each enzyme acts on only one kind of molecule, so the protease left the starch unchanged: iodine still turns blue-black. Amylase would have broken the starch down into sugars.

A full answer includes

• (a) I wrote protease.
• (b) I wrote that protease broke the protein down into amino acids.
• (b) I wrote that the blue-black colour shows starch is still there.
• (b) I wrote that protease does not act on starch.
• I did not write amylase.

Also accept

• (a) protease; (b) protein broken into amino acids; starch not broken down, because protease acts only on protein

Watch for

• “The enzyme was amylase, because there was starch in the tube.” — Amylase would have broken the starch down into simple reducing sugars, and iodine would no longer turn blue-black. Amino acids come from protein, so the enzyme was protease.

7. PR1 · Return

Model answer. The first part is right: the acid kills harmful microorganisms in food. But the acid also helps digestion. It gives the stomach contents an acidic pH, about pH 2, which is the optimum pH for the stomach's protease, pepsin. Without the acid, the stomach would be near neutral, far from that optimum, and the protease would digest protein only slowly. The acid helps digestion without digesting anything itself.

A full answer includes

• I wrote that the acid does kill harmful microorganisms in food.
• I wrote that the acid also gives the stomach an acidic pH.
• I wrote that this is the optimum pH for the stomach's protease.
• I wrote that without the acid, the protease would digest protein slowly.
• I did not write that the acid digests protein itself.

Also accept

• the acid also provides an acidic pH, the optimum for the stomach's protease, so it helps protein digestion as well as killing microorganisms

Watch for

• “The statement is wrong because the acid digests protein itself.” — The acid digests nothing on its own: protein in acid with no enzyme is not broken down. It helps by giving the stomach's protease its optimum pH.
• “The acid helps digestion by neutralising the food.” — An acid does not neutralise the food: it makes the stomach contents acidic. That acidic pH is what the protease needs.

8. PR2 · Return

Options as numbered on the pupil sheet: 1 — Both; 2 — Pepsin; 3 — Trypsin.

Model answer. Pepsin works in the stomach, where the conditions are acidic, so it digests protein in food that has just reached the stomach, and it would digest protein in dilute hydrochloric acid. Trypsin works in the small intestine, where the conditions are alkaline, so it digests protein in the duodenum, and it would digest protein in a weak alkali. The last statement is true of both: each protease works fastest near the pH of the place where it acts, acidic for pepsin and alkaline for trypsin.

A full answer includes

• Food that has just reached the stomach: I chose pepsin.
• Food that has moved on into the duodenum: I chose trypsin.
• Protein in dilute hydrochloric acid: I chose pepsin.
• Protein in a weak alkali: I chose trypsin.
• Works fastest near the conditions of the place where it acts: I chose both.
• I did not choose trypsin for the acid.

Also accept

• stomach: pepsin; duodenum: trypsin; acid: pepsin; alkali: trypsin; fastest near the conditions where it acts: both

Watch for

• “Both proteases digest protein in food that has just reached the stomach.” — Trypsin reaches the food only in the duodenum, and the stomach is acidic, far from its optimum. Only pepsin works there.
• “Trypsin would digest protein in acid, because the stomach is acidic.” — Trypsin works in alkaline conditions; in acid it hardly works. The protease that works in acid is pepsin.
• “Only pepsin works fastest near the pH of its place.” — Trypsin does too: its optimum is alkaline, close to the conditions of the small intestine.

9. PR3 · Return

Model answer. The mixture of food and gastric juice arriving from the stomach is acidic, about pH 2, where amylase hardly works. Bile is alkaline, so it neutralises the acid and the pH rises towards 7. That is near the optimum pH of amylase, so amylase from the pancreas digests the starch much faster. Bile contains no enzymes and digests nothing itself: it gives the enzyme a suitable pH.

A full answer includes

• I wrote that the mixture arriving from the stomach is acidic.
• I wrote that bile is alkaline.
• I wrote that bile neutralises the acid.
• I wrote that the pH rises towards 7.
• I wrote that pH 7 is near amylase's optimum.
• I wrote that amylase then digests the starch faster.
• I did not write that bile digests the starch.

Also accept

• the acidic mixture from the stomach is neutralised by alkaline bile; the pH rises towards 7, near amylase's optimum, so the starch is digested faster

Watch for

• “Bile digests the starch.” — Bile contains no enzymes and digests nothing. It helps by neutralising the acid, so that amylase has a suitable pH.
• “Bile makes the mixture more acidic, which speeds up amylase.” — Bile is alkaline: it neutralises the acid and raises the pH. Amylase hardly works at pH 2, so a more acidic mixture would slow it down.