Answers · Chemical digestion

Three enzymes, three foods — answer key

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

1. EQ1 · Practice

Options as numbered on the pupil sheet: 1 — Tiny droplets of fat; 2 — Fatty acids and glycerol; 3 — Amino acids; 4 — Starch; 5 — Simple reducing sugars.

Model answer. Amylase breaks starch down into simple reducing sugars, such as maltose. Protease breaks protein down into amino acids, the units the protein chain is made of. Lipase breaks fats and oils down into fatty acids and glycerol. Starch is what amylase acts on, not what it makes, and tiny droplets of fat are still fat, so no enzyme made them (Supplement: bile does).

A full answer includes

Amylase: I chose simple reducing sugars.
Protease: I chose amino acids.
Lipase: I chose fatty acids and glycerol.
I did not choose tiny droplets of fat for lipase.

Also accept

amylase: simple reducing sugars; protease: amino acids; lipase: fatty acids and glycerol

Watch for

“Lipase breaks fat into tiny droplets.” — Tiny droplets are still fat: breaking fat up is physical digestion (Supplement: bile does it in the duodenum). Lipase breaks the fat molecules themselves into fatty acids and glycerol, which are new substances.
“Amylase makes starch.” — Starch is the large molecule that amylase breaks down. Its products are the small molecules: simple reducing sugars.

2. EQ2 · Practice

Model answer. (a) One fat molecule gives three fatty acids and one glycerol. So 40 fat molecules give 40 × 3 = 120 fatty acid molecules and 40 × 1 = 40 glycerol molecules. (b) Each fat molecule gave three fatty acids, so 270 fatty acids came from 270 ÷ 3 = 90 fat molecules. Lipase always makes both products: fatty acids and glycerol.

Checked automatically

(a) Fatty acid molecules: 120 (exact value)
- Common wrong answer 40 : That is the number of fat molecules. Each one has three fatty acids, so multiply by 3.
- Common wrong answer 160 : That counts every part together. The question asks for the fatty acids alone: three for each fat molecule.
(a) Glycerol molecules: 40 (exact value)
- Common wrong answer 120 : That is the number of fatty acids. Each fat molecule has only one glycerol.
(b) Fat molecules digested: 90 (exact value)
- Common wrong answer 810 : You multiplied by 3. The fatty acids came from the fat, so there were fewer fat molecules than fatty acids: divide by 3.
- Common wrong answer 270 : That is the number of fatty acids. Each fat molecule gave three of them, so divide by 3.

A full answer includes

(a) I multiplied the number of fat molecules by 3 for the fatty acids.
(a) I wrote 120 fatty acid molecules.
(a) I used one glycerol for each fat molecule.
(a) I wrote 40 glycerol molecules.
(b) I divided the number of fatty acids by 3.
(b) I wrote 90 fat molecules.
I did not give lipase only one product.

Also accept

(a) 120 fatty acids and 40 glycerol; (b) 90

Watch for

“Lipase breaks fat down into glycerol only.” — Each fat molecule is one glycerol joined to three fatty acids, so lipase releases both: three fatty acids and one glycerol for every fat molecule.
“One fat molecule gives one fatty acid.” — A fat molecule has three fatty acids joined to its glycerol, so each one digested gives three fatty acids.

3. EQ3 · Practice

Model answer. F is the pancreas, the long, flat organ below the stomach. It is the only gland that secretes lipase, and its juice flows along the pancreatic duct into the duodenum, where the lipase acts. C, the liver, makes bile and D, the gall bladder, stores it, but bile is not an enzyme. E, the stomach, secretes protease.

A full answer includes

I chose F.
I did not choose C or D: they make and store bile, which is not an enzyme.

Also accept

F (the pancreas)

Watch for

“D, the gall bladder, because bile works on fat.” — Bile is not an enzyme, so it breaks no fat molecules down. The gall bladder only stores bile. The lipase that does break fat down comes from the pancreas, F.
“C, the liver, because the liver deals with fat.” — The liver makes bile, which is not an enzyme. No enzyme for fat comes from the liver: lipase comes from the pancreas.
“E, because the stomach digests every kind of food.” — The stomach secretes protease, which acts only on protein. Lipase comes from the pancreas.

4. EQ4 · Practice

Options as numbered on the pupil sheet: 1 — Lipase only; 2 — None of these three; 3 — Protease only; 4 — Amylase only; 5 — Amylase, protease and lipase.

Model answer. The salivary glands secrete amylase, into the mouth. The stomach wall secretes protease. The pancreas secretes all three: amylase, protease and lipase, and its juice flows along the pancreatic duct into the duodenum. The liver makes bile, which is not an enzyme, so it secretes none of the three.

A full answer includes

Salivary glands: I chose amylase only.
Stomach: I chose protease only.
Pancreas: I chose amylase, protease and lipase.
Liver: I chose none of these three.
I did not give the liver lipase.

Also accept

salivary glands: amylase; stomach: protease; pancreas: amylase, protease and lipase; liver: none

Watch for

“The liver secretes lipase, because the liver deals with fat.” — The liver makes bile, which is not an enzyme. Lipase comes from the pancreas.
“The stomach secretes all three enzymes, because it digests the food.” — The stomach secretes only protease. Starch digestion starts in the mouth, and fat is not digested until it meets lipase from the pancreas.

5. EQ5 · Practice

Options as numbered on the pupil sheet: 1 — Bile; 2 — Amylase; 3 — Protease; 4 — Lipase.

Model answer. Rice and noodles are mostly starch, so amylase digests them into simple reducing sugars. Chicken breast and white fish are mostly protein, so protease digests them into amino acids. Sunflower oil and coconut oil are fats and oils, so lipase digests both into fatty acids and glycerol. Bile is not an enzyme and makes no new substance (Supplement: it breaks fats into droplets).

A full answer includes

Rice and noodles: I chose amylase.
Chicken breast and white fish: I chose protease.
Sunflower oil and coconut oil: I chose lipase.
I did not choose bile for any food.

Also accept

rice, noodles: amylase; chicken breast, white fish: protease; sunflower oil, coconut oil: lipase

Watch for

“Coconut oil is digested by bile.” — Bile is not an enzyme, so it breaks no fat molecules down. The enzyme that does is lipase.
“Coconut oil needs a different enzyme from sunflower oil, because it is solid when cool.” — Fats and oils are built the same way, from glycerol and three fatty acids. Lipase digests both.

6. EQ6 · Practice

Model answer. C. The salivary glands secrete amylase into the mouth, and it starts digesting starch there. In row A, the liver makes bile, not lipase. In row B, the pancreas does make protease, but none of its enzymes acts in the pancreas: they flow along the pancreatic duct into the duodenum. In row D, lipase from the pancreas acts in the small intestine, not in the stomach.

A full answer includes

I chose row C.
I did not choose row B: the pancreas makes protease, but protease does not act there.

Also accept

Watch for

“Row A: lipase is secreted by the liver.” — The liver makes bile, which is not an enzyme. Lipase is secreted by the pancreas.
“Row B: protease acts in the pancreas, where it is made.” — The pancreas makes protease, but no food passes through the pancreas. Its juice flows along the pancreatic duct, so the protease acts in the small intestine.
“Row D: lipase from the pancreas acts in the stomach.” — Pancreatic juice enters the gut at the duodenum, after the stomach. Fat first meets lipase in the small intestine.

7. EQ7 · Practice

Model answer. Protein is digested by protease. Protease is secreted by the stomach, E, and acts there, and by the pancreas, F, whose juice flows into the duodenum. A, the salivary glands, secrete amylase, which does not act on protein. C, the liver, makes bile and D, the gall bladder, stores it; bile is not an enzyme. H, the colon, absorbs water and does not secrete these enzymes.

A full answer includes

I chose E.
I chose F.
I did not choose A: saliva has amylase, not protease.

Also accept

E and F (the stomach and the pancreas)

Watch for

“A, the salivary glands, because digestion starts in the mouth.” — The digestion of starch starts in the mouth, but the enzyme in saliva is amylase, which acts only on starch. Protein digestion starts in the stomach.
“C, the liver, because it is the largest gland.” — The liver makes bile, which contains no enzymes. Size does not decide it.
“D, the gall bladder, because its juice flows into the duodenum.” — The gall bladder only stores bile, and bile is not an enzyme. The juice with protease that flows into the duodenum comes from the pancreas.
“H, the colon, because food stays there a long time.” — The colon absorbs water; it does not secrete digestive enzymes. Protein is digested before food reaches it.
“Only E: protein is digested in the stomach.” — The stomach starts protein digestion, but the pancreas also secretes protease, and protein digestion is finished in the small intestine.

8. EQ8 · Practice

Model answer. The pupil has mixed up where lipase is made and where it acts. The pancreas secretes lipase, but food never passes through the pancreas. Pancreatic juice flows along the pancreatic duct into the duodenum, so the fat is broken down by lipase into fatty acids and glycerol in the small intestine. A correct statement is: "Lipase is secreted by the pancreas and digests fat in the small intestine."

A full answer includes

I wrote that the pancreas secretes lipase.
I wrote that no fat is digested in the pancreas.
I wrote that the juice flows along the pancreatic duct.
I wrote that lipase digests fat in the small intestine (duodenum).
I did not write that the liver digests fat.

Also accept

lipase is made in the pancreas but acts in the small intestine (duodenum), after flowing along the pancreatic duct

Watch for

“An enzyme always works in the organ that makes it.” — Many enzymes are made in a gland and carried to the gut in a juice. The pancreas makes lipase, but the lipase works in the small intestine.
“The statement is wrong because the liver digests fat.” — The liver makes bile, which is not an enzyme and does not break the fat molecules down. Lipase does that, and it comes from the pancreas.

9. ET1 · Apply

Model answer. In tube 1, lipase breaks the fat in the milk down into fatty acids and glycerol. Fatty acids are acids, so as they are made the pH falls and the indicator turns from blue to yellow-green. In tube 2, amylase acts only on starch, and the milk contains none. Amylase leaves the fat and the protein unchanged, so no acid is made and the pH stays the same.

A full answer includes

Tube 1: I wrote that lipase breaks the fat in the milk down.
Tube 1: I wrote that the products are fatty acids and glycerol.
Tube 1: I wrote that the fatty acids lower the pH.
Tube 1: I wrote that the falling pH changes the colour of the indicator.
Tube 2: I wrote that amylase acts only on starch.
Tube 2: I wrote that the milk contains no starch.
Tube 2: I wrote that no fatty acids are made.
Tube 2: I wrote that the pH stays the same.
I did not write that amylase would digest the fat if given more time.

Also accept

tube 1: lipase breaks fat into fatty acids and glycerol, and the fatty acids lower the pH; tube 2: amylase acts only on starch, the milk has none, so nothing is broken down and the pH stays the same

Watch for

“Amylase digests the fat too, but more slowly.” — An enzyme fits only one kind of molecule. However long it has, amylase leaves the fat unchanged, so it never makes fatty acids.
“The glycerol made by lipase is what makes the milk acidic.” — Glycerol is not an acid. The pH falls because of the other product, the fatty acids.

10. ET2 · Apply

Model answer. Very little starch will be digested in the mouth, because the salivary glands secrete the amylase that acts there. But the pancreas also secretes amylase. Pancreatic juice flows along the pancreatic duct into the duodenum, so the starch is still broken down into simple reducing sugars in the small intestine.

A full answer includes

I wrote that the starch is still digested.
I wrote that the pancreas also secretes amylase.
I wrote that this amylase digests the starch in the small intestine.
I wrote that little starch is digested in the mouth.
I did not write that the stomach digests the starch instead.

Also accept

still digested, in the small intestine, by amylase from the pancreas; little digestion in the mouth

Watch for

“Without saliva, starch cannot be digested at all.” — The salivary glands are only one source of amylase. The pancreas secretes amylase too, and it acts in the small intestine.
“The stomach will digest the starch instead.” — The stomach secretes protease, which acts only on protein. The starch is digested by amylase from the pancreas, in the small intestine.

11. ET3 · Apply

Model answer. Egg is rich in protein, and grease is fat. Protease breaks the protein of the egg stain down into amino acids, and lipase breaks the fat of the grease down into fatty acids and glycerol. The large molecules stuck to the cloth do not dissolve, but the products are small soluble molecules, so the washing water carries them away and the stain is gone.

A full answer includes

I wrote that protease breaks down the protein in the egg stain.
I wrote that the products are amino acids.
I wrote that lipase breaks down the fat in the grease.
I wrote that the products are fatty acids and glycerol.
I wrote that the products are small soluble molecules.
I wrote that the washing water carries the products away.
I did not write that the enzymes kill germs in the stain.

Also accept

protease: protein of the egg into amino acids; lipase: fat of the grease into fatty acids and glycerol; small soluble products wash away

Watch for

“The enzymes kill the germs in the stain.” — Enzymes are not alive and do not kill anything: they break large molecules down into small ones.
“Lipase breaks the grease into tiny droplets.” — Tiny droplets are still fat: breaking fat up is physical digestion (Supplement: bile does it in the duodenum). Lipase makes new substances: fatty acids and glycerol.

12. ET4 · Apply

Model answer. (a) The salivary glands secrete amylase into the mouth. Amylase breaks the starch in the rice down into a simple reducing sugar, maltose, and sugar tastes sweet. Starch itself does not taste sweet. (b) Fat is digested by lipase, and saliva contains no lipase. The amylase in saliva acts only on starch, so it leaves the fat unchanged. The fat is digested later, in the small intestine.

A full answer includes

(a) I wrote that the salivary glands secrete amylase into the mouth.
(a) I wrote that amylase breaks the starch in the rice down.
(a) I wrote that the product is a sugar (maltose).
(a) I wrote that the sugar tastes sweet.
(b) I wrote that fat is digested by lipase.
(b) I wrote that saliva contains no lipase.
(b) I wrote that amylase acts only on starch, not on fat.
I did not write that chewing alone makes the rice sweet.

Also accept

(a) amylase from the salivary glands breaks the starch down into sugar (maltose), which tastes sweet; (b) fat needs lipase, and there is no lipase in saliva; amylase acts only on starch

Watch for

“Chewing alone makes the rice sweet, by breaking it into small pieces.” — Chewing is physical digestion: the pieces get smaller, but they are still starch, which does not taste sweet. The sugar is made by amylase in the saliva.
“The amylase in saliva digests the fat too, but more slowly.” — An enzyme fits only one kind of molecule. However long it has, amylase leaves the fat unchanged: fat needs lipase.