Answers · Chemical digestion

Acid and alkali — answer key

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

1. PQ1 · Practice

Model answer. The acid kills harmful microorganisms in food, and it gives the acidic pH at which the stomach's protease works fastest. It does not digest anything itself: tube 3, with acid but no enzyme, stayed cloudy. It does not break fat into droplets (Supplement: bile does that), starch is digested by amylase, and the acid makes the stomach acidic rather than neutral.

A full answer includes

I chose: it kills harmful microorganisms in food.
I chose: it gives the best pH for the stomach's protease.
I did not choose: it digests the protein in food.

Also accept

kills harmful microorganisms; gives an acidic pH for the stomach's enzymes

Watch for

“The acid digests the protein in food into amino acids.” — Tube 3 had acid at pH 2 but no enzyme, and it stayed cloudy: the protein was not digested. The acid gives pepsin the pH it needs; pepsin does the digesting.
“The acid breaks the fat in food into small droplets.” — Breaking fat into droplets is not the acid's job (Supplement: bile does it, in the duodenum). The acid's jobs are killing microorganisms and giving the stomach's enzymes an acidic pH.
“The acid digests the starch in food into sugars.” — Starch is digested by amylase, an enzyme. The acid digests nothing; in fact the acid pH stops salivary amylase working in the stomach.
“The acid neutralises the food so enzymes can work.” — An acid does not neutralise the food: it makes the stomach contents acidic, about pH 2. That acidic pH is what the stomach's protease needs.

2. PQ2 · Practice

Model answer. Tube Y. It had hydrochloric acid at pH 2, as tube W did, but no pepsin, and its cube did not change. So the acid on its own did not digest the protein. In tube W it was pepsin that digested the cube, at the acidic pH the acid provided.

A full answer includes

I chose tube Y.
I used a tube with acid and no enzyme.
I did not choose tube W, which also had pepsin.

Also accept

tube Y

Watch for

“Tube W, because it had acid and the cube got smaller.” — Tube W also had pepsin, so the digestion could be the pepsin's work. Only a tube with acid and no enzyme can show what the acid does on its own.
“Tube X, because the cube did not change.” — Tube X had pepsin but no acid. It shows that pepsin hardly works at pH 7; it says nothing about the acid on its own.
“Tube Z, because it had no enzyme.” — Tube Z had no enzyme, but it had no acid either. It shows that water alone does not digest egg white; the question asks about the acid.

3. PQ3 · Practice

Model answer. Trypsin. At pH 6 the pepsin curve has already fallen to zero, but the trypsin curve is above zero and rising towards its peak at about pH 8. So trypsin can already digest some protein at pH 6, though more slowly than at its optimum. As more bile and pancreatic juice mix in and raise the pH further, trypsin works faster.

A full answer includes

I chose trypsin.
I read that the trypsin curve is above zero at pH 6.
I read that the pepsin curve is at zero at pH 6.
I did not choose pepsin because pH 6 is acidic.

Also accept

trypsin: its curve is above zero (and rising) at pH 6

Watch for

“Pepsin, because pH 6 is still acidic.” — pH 6 is only slightly acidic. Pepsin works only in strongly acidic conditions: its curve is at zero by pH 5.
“Neither, because both curves are at zero at pH 6.” — Read again at pH 6: the pepsin curve is at zero, but the trypsin curve has started to rise.
“Both equally, because pH 6 is between their optimum values.” — Being between the two peaks does not make them equal. At pH 6 pepsin has stopped, while trypsin is already working.

4. PQ4 · Practice

Options as numbered on the pupil sheet: 1 — The enzymes of the small intestine work at a suitable pH; 2 — Bile flows along the bile duct; 3 — Bile enters the duodenum; 4 — The alkaline bile neutralises the acid from the stomach; 5 — Bile is stored in the gall bladder; 6 — Bile is made in the liver.

Model answer. Bile is made in the liver and stored in the gall bladder. It flows along the bile duct into the duodenum, the first part of the small intestine. Bile is alkaline, so it neutralises the acidic mixture of food and gastric juice arriving from the stomach, and the enzymes of the small intestine then have a suitable pH to work at.

A full answer includes

Step 1: I put made in the liver.
Step 2: I put stored in the gall bladder.
Step 3: I put flows along the bile duct.
Step 4: I put enters the duodenum.
Step 5: I put neutralises the acid from the stomach.
Step 6: I put the enzymes of the small intestine work at a suitable pH.
I did not put the gall bladder first.

Also accept

liver, gall bladder, bile duct, duodenum, neutralises the acid, suitable pH for enzymes

Watch for

“Bile is made in the gall bladder.” — The gall bladder only stores bile. The liver makes it.
“Bile neutralises the acid while the food is still in the stomach.” — Bile enters at the duodenum, after the stomach. The stomach stays acidic, which is the pH pepsin needs.

5. PQ5 · Practice

Model answer. The acid's job for digestion is to give the stomach's protease the acidic pH at which it works best, its optimum, about pH 2. pH 1 is further from the optimum, so the shape of the protease changes a little, protein fits it less well and it works more slowly, not faster. The acid itself digests no protein, so more acid does not add any digestion of its own.

A full answer includes

I wrote that the acid gives the acidic pH for the optimum activity of the protease.
I wrote that the optimum is about pH 2.
I wrote that pH 1 is further from the optimum.
I wrote that at pH 1 the protease works more slowly.
I did not write that more acid digests more protein by itself.

Also accept

the acid gives the optimum pH, about 2; pH 1 is further from the optimum, so the protease works more slowly

Watch for

“More acid digests more protein, because the acid breaks protein down.” — The acid on its own does not digest protein: a tube with acid and no enzyme stays unchanged. Its help is giving the protease its optimum pH.
“An enzyme works faster the more acidic its surroundings.” — Each enzyme has one optimum pH. Moving the pH away from it in either direction slows the enzyme down.

6. PQ6 · Practice

Model answer. (a) The three times at pH 3 are 6.0, 9.5 and 6.4 min. 9.5 min is far from the other two, so it is anomalous. Mean = (6.0 + 6.4) ÷ 2 = 12.4 ÷ 2 = 6.2 min. (b) The shortest times, and so the fastest digestion, are at pH 2 (a mean of about 4.2 min). Pepsin works in the stomach, where hydrochloric acid makes the conditions acidic, about pH 2, so the stomach has the pH at which pepsin works best. At pH 5 it never cleared the egg white at all.

Checked automatically

(a) Mean time at pH 3: 6.2 min (within ±0.01 min)
- Common wrong answer 7.3 : That mean includes the anomalous 9.5 min. Leave it out and use only the other two times.
- Common wrong answer 12.4 : That is the total of the two times. Divide it by 2, the number of times you added.
(b) pH of fastest digestion: 2 (exact value)
- Common wrong answer 4 : At pH 4 the times are the longest of all the tubes that cleared, so digestion was slowest there. Choose the shortest times.
- Common wrong answer 5 : At pH 5 the egg white never cleared, so pepsin hardly worked at all. Choose the pH with the shortest times.

A full answer includes

(a) I left out 9.5 min.
(a) I added 6.0 and 6.4 and divided by 2.
(a) I wrote 6.2 min.
(b) I chose the pH with the shortest mean time.
(b) I wrote pH 2.
(b) I wrote that pepsin works in the stomach.
(b) I wrote that the stomach is acidic, about pH 2.
I did not choose the pH with the longest time.

Also accept

(a) 6.2 min; (b) pH 2; pepsin works in the acidic stomach

Watch for

“The fastest digestion is at pH 4, because the times there are the largest.” — A larger time to clear means slower digestion. The fastest digestion has the shortest times, at pH 2.
“The anomalous result should be kept in the mean, because it was measured.” — An anomalous result is far from the other repeats, so something probably went wrong with it. Including it would make the mean misleading.

7. PQ7 · Practice

Model answer. C. Bile is alkaline, so it neutralises the acid in tube X and the pH rises. Trypsin works best in alkaline conditions, so at the higher pH it is nearer to its optimum and digests the protein faster: the egg white in tube X clears sooner. Water only dilutes the acid a little, so tube Y stays far too acidic for trypsin.

A full answer includes

I chose row C.
I did not choose row B: bile needs no enzyme of its own to speed digestion.

Also accept

Watch for

“Row A: bile makes the pH lower.” — Bile is alkaline, so it neutralises the acid and the pH rises. A lower pH would take trypsin further from its optimum.
“Row B: the egg white clears no sooner, because bile contains no enzyme.” — Bile needs no enzyme of its own. The trypsin is already there, and a pH nearer to its optimum makes it digest faster.
“Row D: the pH falls and nothing else changes.” — Bile is alkaline and raises the pH. What speeds the digestion is that change in pH, not anything bile digests.

8. PQ8 · Practice

Model answer. (a) Bile, which flows along the bile duct, and pancreatic juice, which flows along the pancreatic duct. Both are alkaline, so they neutralise the acidic mixture from the stomach. (b) Point Q. On the graph, the trypsin curve is at about zero at pH 4 but is high at pH 7, close to its peak at about pH 8. The neutralised mixture at Q is near the optimum pH of trypsin, so trypsin digests protein much faster there.

A full answer includes

(a) I wrote bile.
(a) I wrote pancreatic juice.
(b) I wrote point Q.
(b) I read that the trypsin curve is at about zero at pH 4.
(b) I read that the trypsin curve is much higher at pH 7.
(b) I wrote that pH 7 is nearer to the optimum of trypsin.
I did not write that bile digests the protein.

Also accept

(a) bile and pancreatic juice; (b) Q: the trypsin curve is near zero at pH 4 and high at pH 7, near its optimum

Watch for

“Bile raises the pH because it digests the acid.” — Bile contains no enzymes and digests nothing. It is alkaline, so it neutralises the acid.
“Trypsin is faster at P, because the acid helps every enzyme.” — The acid helps the stomach's protease, pepsin. Trypsin works in alkaline conditions: at pH 4 its curve is at about zero.

9. PQ9 · Practice

Model answer. (a) Two things change between the tubes: the pH and the temperature. If tube 1 is digested and tube 2 is not, she cannot tell whether the pH or the temperature caused the difference. Both tubes must be at 37 °C, so that only the pH changes. (b) A tube with acid at pH 2 and no pepsin shows what the acid does on its own. If its egg white is not digested, the digestion in tube 1 was done by pepsin, and the acid only provided the pH.

A full answer includes

(a) I wrote that both the pH and the temperature are different.
(a) I wrote that she cannot tell which of the two caused a difference.
(a) I wrote that tube 2 should also be at 37 °C.
(b) I wrote that the tube shows whether the acid alone digests the egg white.
(b) I wrote that if it does not, the pepsin did the digesting in tube 1.
I did not write that the extra tube is a repeat for reliability.

Also accept

(a) the pH and the temperature both change, so the cause is unknown; keep both at 37 °C; (b) to show the acid alone does not digest the egg white, so pepsin did

Watch for

“The extra tube is a repeat, to make the results more reliable.” — A repeat has the same contents. This tube leaves out the pepsin, to show what the acid does on its own: it is a control.
“The temperature does not matter, because pH is what is being tested.” — Enzymes also work faster or slower at different temperatures. If the temperature changes too, it could cause the difference instead of the pH.

10. PT1 · Apply

Model answer. (a) Killed in flask 1: 2400 − 36 = 2364. Percentage killed = 2364 ÷ 2400 × 100 = 98.5 %. (b) Killed in flask 2: 2400 − 2280 = 120. Percentage killed = 120 ÷ 2400 × 100 = 5.0 %. (c) The flasks differed only in the acid, and far more of the bacteria were killed at pH 2. So one function of the hydrochloric acid in gastric juice is to kill harmful microorganisms in food before they can cause illness.

Checked automatically

(a) Percentage killed in flask 1: 98.5 % (within ±0.05 %)
- Common wrong answer 1.5 : That is the percentage still alive. Work out how many were killed first, then the percentage.
- Common wrong answer 2364 : That is the number killed, not a percentage. Divide it by 2400, then multiply by 100.
- Common wrong answer 0.985 : That is the fraction killed. Multiply it by 100 to give a percentage.
(b) Percentage killed in flask 2: 5.0 % (within ±0.05 %)
- Common wrong answer 95.0 : That is the percentage still alive at pH 6. Use the number killed instead.
- Common wrong answer 120 : That is the number killed, not a percentage. Divide it by 2400, then multiply by 100.
- Common wrong answer 0.05 : That is the fraction killed. Multiply it by 100 to give a percentage.

A full answer includes

(a) I worked out the number killed: 2400 − 36 = 2364.
(a) I worked out 2364 ÷ 2400 × 100.
(a) I wrote 98.5 %.
(b) I worked out the number killed: 2400 − 2280 = 120.
(b) I worked out 120 ÷ 2400 × 100.
(b) I wrote 5.0 %.
(c) I wrote that far more bacteria were killed at pH 2 than at pH 6.
(c) I wrote that the acid kills harmful microorganisms in food.
I did not give the percentage still alive.

Also accept

(a) 98.5 %; (b) 5.0 % (5 %); (c) far more were killed at pH 2, so the acid kills harmful microorganisms in food

Watch for

“The acid digested the bacteria.” — The acid digests nothing. It kills the bacteria, because most of them cannot survive at pH 2.
“In flask 1, 1.5 % of the bacteria were killed.” — That is the percentage still alive. The question asks for the percentage killed: start from the number of bacteria that did not survive.

11. PT2 · Apply

Model answer. The statement is incorrect. Bile and pancreatic juice make the conditions in the small intestine alkaline, about pH 8. Pepsin works best at about pH 2, in the acidic conditions of the stomach; on the graph its curve has fallen to almost nothing well before pH 7. So in the small intestine the pH is far from pepsin's optimum: its shape changes, protein no longer fits it well and it hardly works at all. Trypsin digests the protein there: its curve peaks at about pH 8, so it works fastest in the alkaline conditions of the small intestine.

A full answer includes

I wrote that the conditions in the small intestine are alkaline.
I wrote that pepsin's optimum is about pH 2.
I read that the pepsin curve is at about zero at alkaline pH, such as pH 8.
I wrote that pepsin hardly works in the small intestine.
I wrote that trypsin digests protein there.
I did not write that pepsin is killed.

Also accept

incorrect: the small intestine is alkaline, far from pepsin's optimum of about pH 2, so pepsin hardly works there; trypsin digests the protein

Watch for

“Pepsin is killed as soon as the pH changes from 2.” — Enzymes are not alive, and a small change of pH does not stop them: activity falls gradually away from the optimum, as the curve shows. The alkaline small intestine is so far from pH 2 that pepsin hardly works at all.
“Pepsin stops because bile digests it.” — Bile contains no enzymes and digests nothing. Pepsin stops because bile and pancreatic juice make the conditions alkaline, far from its optimum.

12. PT3 · Apply

Model answer. (a) Bile is alkaline, so it would neutralise the hydrochloric acid in the stomach. The pH would rise well above 2, away from the optimum of pepsin, so pepsin would digest protein more slowly. (b) The stomach would no longer be strongly acidic, so fewer harmful microorganisms in food would be killed. (c) In the duodenum, the acid has already done both its jobs in the stomach. Bile entering there neutralises the acidic mixture just where the enzymes of the small intestine, such as trypsin, need an alkaline, suitable pH.

A full answer includes

(a) I wrote that bile would neutralise the acid in the stomach.
(a) I wrote that the pH would rise above the optimum of pepsin.
(a) I wrote that pepsin would digest protein more slowly.
(b) I wrote that fewer harmful microorganisms would be killed.
(c) I wrote that in the duodenum the acid has already done its jobs in the stomach.
(c) I wrote that bile then neutralises the acid where the enzymes of the small intestine need a suitable pH.
I did not write that bile would digest the food in the stomach.

Also accept

(a) bile neutralises the acid, the pH rises from pepsin's optimum, protein digestion slows; (b) fewer microorganisms killed; (c) the acid has done its jobs; bile neutralises it where the small intestine's enzymes need a suitable pH

Watch for

“Bile in the stomach would help digestion, because bile digests fat.” — Bile contains no enzymes and digests nothing. In the stomach it would neutralise the acid that pepsin needs, so digestion would be slower.
“Bile would make the stomach more acidic.” — Bile is alkaline. It would neutralise the acid, so the stomach would be less acidic.

13. PT4 · Apply

Model answer. Set up tubes with the same volume of egg-white suspension and buffers at several pH values close to 8, for example pH 7, 7.5, 8, 8.5 and 9. Warm them in a water bath at 37 °C, then add the same volume of the same trypsin solution to each and start a stop-clock. Measure the time each tube takes to turn clear: the shorter the time, the faster the protein is digested. Keep everything except the pH the same. Set up a tube with water, or boiled trypsin, instead of trypsin at each pH, to show that the buffer alone does not clear the egg white. Repeat each pH and calculate a mean time. The pH with the shortest mean time is closest to the optimum; you could then test values even closer around it.

A full answer includes

(a) I wrote that I would change the pH.
(a) I chose several pH values close to 8.
(b) I kept the volume of egg-white suspension the same.
(b) I kept the volume of trypsin solution the same.
(b) I kept the concentration of trypsin the same.
(b) I kept the temperature the same, for example 37 °C in a water bath.
(c) I wrote that I would measure the time until each tube turns clear.
(d) I used a tube with water (or boiled trypsin) instead of trypsin at each pH.
(d) I wrote that the pH with the shortest time is closest to the optimum.
I did not change the amount of trypsin between tubes.

Also accept

several buffers around pH 8; same volumes, trypsin and 37 °C; time until clear; tubes with water or boiled trypsin instead of trypsin; the shortest time is nearest the optimum
repeats and a mean time are a good addition

Watch for

“Test pH 2 and pH 8 again, but for longer.” — Two pH values only tell you which of the two is better. To find the peak, test several pH values close to 8.
“Use more trypsin at the pH values where it works slowly, so that every tube clears.” — If the amount of trypsin changes as well as the pH, you cannot tell which one caused a difference. Keep the trypsin the same in every tube.