Chemical digestion — final check key

1. BK1 · Final check

Model answer. Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules, by enzymes. The pupil is wrong. Sugar is already made of small soluble molecules, which is why it dissolves. Dissolving only spreads its molecules out among the water molecules: they are the same molecules as before, no new substance is made and no enzyme is involved. So this is not chemical digestion.

A full answer includes

• I wrote that chemical digestion breaks large insoluble molecules into small soluble molecules.
• I wrote that the pupil is wrong.
• I wrote that sugar is already made of small soluble molecules.
• I wrote that the sugar only dissolves.
• I wrote that its molecules are not broken down.
• I did not write that anything that disappears in water has been digested.

Also accept

• wrong: chemical digestion breaks large insoluble molecules into small soluble ones; the sugar is already small and soluble and only dissolves

Watch for

• “Anything that disappears in water has been digested.” — Dissolving only spreads the molecules out in the water; they are unchanged. Digestion breaks large molecules into new, smaller ones.

2. BK2 · Final check

Model answer. The sports drink. Glucose is already a small soluble molecule, so it can be absorbed through the wall of the small intestine into the blood as soon as it gets there. Starch is a large insoluble molecule and cannot be absorbed. The role of chemical digestion is to produce small soluble molecules that can be absorbed: enzymes must first break the starch down into sugars, and only then can the sugars be absorbed, so the cereal bar is slower.

A full answer includes

• I chose the sports drink.
• I wrote that glucose is already a small soluble molecule.
• I wrote that glucose can be absorbed without digestion.
• I wrote that starch is a large insoluble molecule that cannot be absorbed as it is.
• I wrote that chemical digestion must first break the starch into small soluble sugars.
• I wrote that this digestion takes time.
• I did not write that starch is absorbed first and digested later in the blood.

Also accept

• the sports drink: glucose needs no digestion and is absorbed at once; starch must first be digested into small soluble sugars

Watch for

• “The cereal bar, because starch gives more energy.” — The two give the same energy. What matters is time: starch must be digested before it can be absorbed, and glucose need not be.
• “Both are the same, because both end up as sugar in the blood.” — Both end up in the blood as sugar, but only the starch has to be digested first, so it takes longer.

3. BK3 · Final check

Model answer. (a) Maltase acts only on maltose, two glucose units joined together. It cannot break the long starch chains, so the starch is still there (iodine turns blue-black) and no glucose is made. (b) She should add amylase. Amylase breaks the starch down into maltose, and the maltase already in the tube then breaks each maltose molecule into two glucose molecules. These are the two steps in the body too: amylase in the mouth and small intestine, then maltase on the membranes of the epithelium lining the small intestine.

A full answer includes

• (a) I wrote that maltase breaks down only maltose.
• (a) I wrote that there was no maltose in the tube, only starch.
• (a) I wrote that maltase cannot break down starch.
• (b) I wrote amylase.
• (b) I wrote that amylase breaks starch down into maltose.
• (b) I wrote that maltase then breaks the maltose down into glucose.
• I did not write that maltase only needs more time to digest the starch.

Also accept

• (a) maltase acts only on maltose, not starch; (b) amylase: starch to maltose, then maltase: maltose to glucose

Watch for

• “Maltase can digest starch; it just needs more time.” — Waiting longer will not help: maltase breaks down only maltose. The starch must first be broken down into maltose by amylase.

4. EK1 · Final check

Model answer. In tube 1, amylase breaks the starch down into simple reducing sugars. In tube 2, protease breaks the protein down into amino acids. In tube 3, lipase breaks the oil down into fatty acids and glycerol. In each tube the other two large molecules are left as they were, because each enzyme acts on only one kind of molecule.

A full answer includes

• Tube 1: I wrote that amylase breaks down the starch.
• Tube 1: I wrote simple reducing sugars.
• Tube 2: I wrote that protease breaks down the protein.
• Tube 2: I wrote amino acids.
• Tube 3: I wrote that lipase breaks down the oil.
• Tube 3: I wrote fatty acids and glycerol.
• I did not write that any enzyme breaks down all three molecules.

Also accept

• 1: starch into simple reducing sugars; 2: protein into amino acids; 3: oil into fatty acids and glycerol

Watch for

• “Each enzyme breaks down all three molecules, only more slowly.” — An enzyme fits only one kind of molecule, like a key in one lock. Amylase leaves the protein and the oil unchanged however long it has.

5. EK2 · Final check

Options as numbered on the pupil sheet: 1 — Colon; 2 — Small intestine; 3 — Mouth; 4 — Pancreas; 5 — Stomach.

Model answer. Turn each row into the enzymes that worked. Sample 1 broke down only the starch, so it had amylase alone: saliva, in the mouth. Sample 2 broke down only the protein, so it had protease alone: gastric juice, in the stomach. Sample 3 broke down all three, so it had amylase, protease and lipase: pancreatic juice, which flows into the small intestine. Sample 4 broke down nothing, so it had none of these enzymes: the colon. The pancreas makes all three enzymes, but it is not a place along the alimentary canal.

A full answer includes

• Sample 1: I chose the mouth.
• Sample 2: I chose the stomach.
• Sample 3: I chose the small intestine.
• Sample 4: I chose the colon.
• I did not choose the pancreas: food does not pass through it.

Also accept

• 1: mouth; 2: stomach; 3: small intestine; 4: colon

Watch for

• “Sample 3 came from the pancreas, because it had all three enzymes.” — The pancreas makes all three, but food never passes through it. Its juice flows along the pancreatic duct into the duodenum, so a sample with all three comes from the small intestine.
• “Sample 1 came from the stomach.” — Sample 1 broke down only the starch, so its enzyme was amylase. The stomach secretes protease, which would have made amino acids.
• “Sample 2 came from the small intestine, because protein is digested there.” — Protein is digested there, but the small intestine also has amylase and lipase, so the starch and the fat would have been broken down too. Sample 2 had protease alone.
• “Sample 4 came from the mouth, because food is only chewed there.” — Saliva contains amylase, so a sample from the mouth would break the starch down. Sample 4 broke down nothing.

6. EK3 · Final check

Model answer. (a) Normally all three are secreted by the pancreas. (b) They are carried in pancreatic juice, which flows along the pancreatic duct into the duodenum, the first part of the small intestine, where they act. The capsules put the same enzymes in the same place.

A full answer includes

• (a) I wrote the pancreas.
• (b) I wrote that they are carried in pancreatic juice.
• (b) I wrote that the juice flows along the pancreatic duct.
• (b) I wrote that it enters the duodenum, the first part of the small intestine.
• I did not write that the enzymes normally act in the pancreas.

Also accept

• (a) the pancreas; (b) in pancreatic juice, along the pancreatic duct into the duodenum (small intestine)

Watch for

• “The capsules must work in the pancreas, where the enzymes are normally made.” — The pancreas only makes the enzymes; they act in the small intestine, after flowing along the pancreatic duct. The capsules release them there, where the food is.

7. PK1 · Final check

Model answer. The results show both functions of the acid. First, it kills harmful microorganisms: at pH 7, without acid, bacteria grew and the meat rotted, but at pH 2 they were killed and the meat stayed fresh. Second, it provides an acidic pH for the optimum activity of the stomach's enzyme: pepsin digested the meat at pH 2, its optimum, but hardly at all at pH 7, far from it. In the stomach, the acid in gastric juice does the same two jobs.

A full answer includes

• (a) I wrote that the acid kills harmful microorganisms.
• (a) I wrote that at pH 2 the bacteria did not grow.
• (a) I wrote that the meat at pH 2 did not rot.
• (b) I wrote that the acid provides the acidic pH at which pepsin works best.
• (b) I wrote that pepsin digested the meat at pH 2 but hardly at pH 7.
• I did not write that the acid digested the meat.

Also accept

• (a) kills microorganisms, so the meat did not rot; (b) acidic pH for pepsin, so the meat was digested

Watch for

• “The acid digested the meat at pH 2.” — The acid gave the pH; pepsin did the digesting. The same pepsin at pH 7 hardly digested the meat, because pH 7 is far from its optimum.

8. PK2 · Final check

Model answer. Trypsin, which works in the alkaline duodenum. The most protein, 4.8 mg, was digested at pH 8, so the optimum of this protease is near pH 8, an alkaline pH, and at pH 2 it digested nothing. Trypsin works in the alkaline conditions of the small intestine, whose first part is the duodenum. Pepsin works in the acidic conditions of the stomach, so it would have digested the most at about pH 2.

A full answer includes

• I chose trypsin, which works in the alkaline duodenum.
• I read that the most protein was digested at pH 8.
• I wrote that pH 8 is alkaline.
• I did not choose pepsin.

Also accept

• trypsin, which works in the alkaline duodenum (small intestine)

Watch for

• “Pepsin, which works in the acidic stomach.” — Pepsin works best at about pH 2, but this protease digested nothing at pH 2. Its results peak at pH 8, an alkaline pH.
• “Trypsin, which works in the acidic stomach.” — Trypsin is the right enzyme, but it does not work in the stomach: this protease hardly worked at acidic pH. Trypsin works in the alkaline small intestine.
• “Pepsin, which works in the alkaline duodenum.” — Pepsin works in acid, not in alkali. A protease whose results peak at pH 8 is trypsin.

9. PK3 · Final check

Model answer. A. As bile was added, the pH rose from 2 to 8, so bile is alkaline. Neutral is pH 7, which the mixture reached after 6 cm³ of bile; after 8 cm³ it had passed neutral, to pH 8. Bile neutralises the acid as an alkali does: it contains no enzymes and digests nothing. In the duodenum this gives the enzymes of the small intestine a suitable pH to work at.

A full answer includes

• I chose row A.
• I read that the pH rose as bile was added.
• I used the rise in pH to decide that bile is alkaline.
• I read that the mixture was neutral, pH 7, after 6 cm³.
• I did not choose row C: after 8 cm³ the mixture had passed neutral.

Also accept

• A

Watch for

• “Row B: bile is acidic.” — An acid added to an acid would keep the pH low. The pH rose from 2 to 8, so bile is alkaline.
• “Row C: the mixture is neutral after 8 cm³ of bile.” — After 8 cm³ the pH was 8, just past neutral. Neutral is pH 7.
• “Row D: bile digests the acid.” — Bile contains no enzymes and digests nothing. It is alkaline, so it neutralises the acid.