Answers · Respiration

Yeast, bread and temperature — answer key

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

1. YQ1 · Practice

Model answer. Row C. Anaerobic respiration is the chemical reactions in cells that break down nutrient molecules, such as glucose, to release energy without using oxygen. Without oxygen the glucose is only partly broken down, so it releases much less energy per glucose molecule than aerobic respiration.

A full answer includes

I chose row C.
I checked that no oxygen is used.
I checked that nutrient molecules are still broken down.
I checked that much less energy is released per glucose molecule.
I did not choose row D: anaerobic respiration releases less energy, not more.

Also accept

Watch for

“Row A.” — Anaerobic respiration uses no oxygen. A reaction that uses oxygen is aerobic respiration.
“Row B.” — Anaerobic respiration still breaks down nutrient molecules such as glucose. It is oxygen that it does not use.
“Row D.” — Without oxygen the glucose is only partly broken down, so much less energy is released from each molecule, not more.

2. YQ2 · Practice

Model answer. glucose → alcohol + carbon dioxide. The alcohol is ethanol; the carbon dioxide makes bread dough rise.

A full answer includes

Gap 1: I wrote alcohol (ethanol).
Gap 2: I wrote carbon dioxide.
I did not write lactic acid: that is muscles.

Also accept

alcohol; carbon dioxide (either order)

Watch for

“glucose → lactic acid” — That is anaerobic respiration in muscles. Yeast makes alcohol and carbon dioxide.
“glucose + oxygen → alcohol + carbon dioxide” — Anaerobic respiration does not use oxygen.

3. YQ3 · Practice

Options as numbered on the pupil sheet: 1 — Respires and gives off the gas; 2 — Shows the temperature of the bath; 3 — Stops oxygen from the air dissolving in the solution; 4 — Keeps the tube at the temperature being tested; 5 — Collects the gas and measures its volume.

Model answer. A is the layer of oil: it stops oxygen from the air dissolving back into the solution. B is the yeast in glucose solution: the yeast respires and gives off carbon dioxide. C is the water bath: it keeps the tube at the temperature being tested. D is the gas syringe: the gas pushes its plunger out, and the scale gives the volume in cm³. E is the thermometer: it shows the temperature of the bath, so that you can keep it steady.

A full answer includes

I matched A, the oil, to keeping oxygen from the air out of the solution.
I matched B, the yeast in glucose solution, to respiring and giving off the gas.
I matched C, the water bath, to keeping the tube at the temperature being tested.
I matched D, the gas syringe, to collecting the gas and measuring its volume.
I matched E, the thermometer, to showing the temperature of the bath.
I did not match the oil to food for the yeast.

Also accept

A oil; B yeast in glucose; C water bath; D gas syringe; E thermometer

Watch for

“The oil is food for the yeast.” — The yeast respires the glucose. The oil is a seal that keeps oxygen out.
“The water bath makes the gas.” — The water bath only keeps the temperature steady. The yeast makes the gas.

4. YQ4 · Practice

Model answer. (a) Gas collected = end reading − start reading = 21 − 5 = 16 cm³. (b) Rate = 16 cm³ ÷ 5 minutes = 3.2 cm³ per minute.

Checked automatically

(a) Volume of gas collected: 16 cm³ (within ±2%)
- Common wrong answer 21 : That is the end reading. Subtract the start reading.
- Common wrong answer 26 : Subtract the readings, do not add them.
(b) Rate of gas production: 3.2 cm³ per minute (within ±2%)
- Common wrong answer 4.2 : You divided the end reading. Subtract the start reading first.
- Common wrong answer 16 : That is the gas in 5 minutes. Divide by the time.
- Common wrong answer 80 : Divide by the time, do not multiply.

A full answer includes

Method (a): I subtracted the start reading from the end reading, 21 − 5.
Answer (a): I wrote 16 cm³.
Method (b): I divided the gas collected by the time, 16 ÷ 5.
Answer (b): I wrote 3.2 cm³ per minute.
I did not divide the end reading, 21 cm³, by the time.

Also accept

(a) 16 cm³; (b) 3.2 cm³ per minute

Watch for

“The rate is 21 ÷ 5 = 4.2 cm³ per minute.” — 21 cm³ is where the plunger stopped. Subtract the reading at the start first.
“The rate is the whole volume collected.” — The volume was collected in 5 minutes. A rate is the volume in one minute.

5. YQ5 · Practice

Options as numbered on the pupil sheet: 1 — Stand the tube in the water bath and connect it to the gas syringe; 2 — After 5 minutes, read the syringe again; 3 — Make up a fresh tube of yeast in boiled and cooled glucose solution, with a layer of oil on top; 4 — Leave it for 10 minutes to reach the temperature of the bath; 5 — Note the syringe reading and start the stop-clock.

Model answer. First make up a fresh tube of yeast in boiled and cooled glucose solution, under a layer of oil. Stand it in the water bath and connect it to the gas syringe. Leave it for 10 minutes to reach the temperature of the bath: some of the first gas is air pushed out as the tube warms. Then note the syringe reading and start the stop-clock, and read the syringe again after 5 minutes.

A full answer includes

First: I put making up the tube.
Second: I put standing it in the bath and connecting the syringe.
Third: I put leaving it for 10 minutes.
Fourth: I put noting the reading and starting the clock.
Last: I put reading the syringe again after 5 minutes.
I did not start the clock before the 10 minutes.

Also accept

make up the tube; bath and syringe; 10 minutes; start reading and clock; read again after 5 minutes

Watch for

“Start the clock as soon as the tube goes into the bath.” — The tube is still warming, and some of the first gas is air pushed out. Wait 10 minutes.
“Read the syringe only at the end.” — You need the start reading too, to subtract it.

6. YQ6 · Practice

Model answer. (a) Boiling drives the dissolved oxygen out of the solution; it is cooled so that the heat does not kill the yeast. (b) The layer of oil stops oxygen from the air dissolving back into the solution, so once any oxygen left is used up, the yeast respires anaerobically. (c) The tube needs time to reach the temperature of the bath. As it warms, some air is pushed out into the syringe; that gas did not come from the yeast, so it must not be counted.

A full answer includes

(a) I wrote that boiling drives out the dissolved oxygen.
(a) I wrote that cooling stops the heat from killing the yeast.
(b) I wrote that the oil stops oxygen from the air dissolving back in.
(b) I wrote that the yeast then respires anaerobically.
(c) I wrote that the tube needs time to reach the temperature of the bath.
(c) I wrote that the first gas is air pushed out as the tube warms.
I did not write that the oil is food for the yeast.

Also accept

(a) drive out dissolved oxygen, cool so the heat does not kill the yeast; (b) keep oxygen out; (c) reach the bath temperature, the first gas is air

Watch for

“Boiling the solution kills germs so that the yeast stays safe.” — The point here is a gas dissolved in the water.
“The oil is food for the yeast.” — The oil is a seal. The yeast respires the glucose.

7. YQ7 · Practice

Model answer. (a) Rate = volume ÷ time, so volume = rate × time = 4.0 cm³ per minute × 12 minutes = 48 cm³. (b) Time = volume ÷ rate = 36 cm³ ÷ 2.0 cm³ per minute = 18 minutes. Real yeast at 50 °C would slow down as more of its enzymes are denatured, so (b) is only an estimate.

Checked automatically

(a) Volume of gas at 40 °C in 12 minutes: 48 cm³ (within ±2%)
- Common wrong answer 0.33 : Multiply the rate by the time, do not divide.
- Common wrong answer 3 : That is the time divided by the rate. Multiply the rate by the time.
(b) Time at 50 °C to give off 36 cm³: 18 minutes (within ±2%)
- Common wrong answer 72 : Divide the volume by the rate, do not multiply.
- Common wrong answer 0.056 : Divide the volume by the rate, not the rate by the volume.

A full answer includes

Method (a): I multiplied the mean rate at 40 °C by the time, 4.0 × 12.
Answer (a): I wrote 48 cm³.
Method (b): I divided the volume by the mean rate at 50 °C, 36 ÷ 2.0.
Answer (b): I wrote 18 minutes.
I did not multiply the volume by the rate in (b).

Also accept

(a) 48 cm³; (b) 18 minutes

Watch for

“The volume in 12 minutes is 4.0 ÷ 12 = 0.33 cm³.” — The yeast gives off 4.0 cm³ each minute, so in 12 minutes it gives off more, not less.
“The time is 36 × 2.0 = 72 minutes.” — Each minute gives 2.0 cm³. To find how many minutes it takes to reach the volume, divide.

8. YQ13 · Practice

Model answer. (a) Mean gas at 32 °C = (16 + 17) ÷ 2 = 16.5 cm³ in 5 minutes. Mean rate = 16.5 ÷ 5 = 3.3 cm³ per minute. (b) As the temperature rises from 16 °C to 32 °C, the rate rises, from 0.7 to 3.3 cm³ per minute, the highest in the table. Above 32 °C it falls: 2.6 at 40 °C and 0.9 at 48 °C. (c) The reactions of respiration are controlled by enzymes. Above the optimum, about 32 °C for this yeast, the enzymes start to be denatured: their shape changes and they stop working, so the yeast respires more slowly.

A full answer includes

(a) Method: I found the mean gas of the two runs, (16 + 17) ÷ 2 = 16.5 cm³.
(a) Method: I divided the mean gas by the 5 minutes, 16.5 ÷ 5.
(a) Answer: I wrote 3.3 cm³ per minute.
(b) I wrote that the rate rises from 0.7 cm³ per minute at 16 °C to the highest, 3.3, at 32 °C.
(b) I wrote that the rate then falls, to 0.9 cm³ per minute at 48 °C.
(c) I wrote that enzymes control the reactions of respiration.
(c) I wrote that above the optimum the enzymes are denatured: their shape changes.
I did not write that the enzymes are killed.

Also accept

(a) 3.3 cm³ per minute; (b) rises to a peak at 32 °C, then falls; (c) the enzymes are denatured above the optimum

Watch for

“The mean rate is the mean volume of gas.” — The mean volume was collected in 5 minutes. A rate is per minute.
“The hotter it is, the faster yeast respires.” — Only up to about 32 °C for this yeast. Above that the rate falls.
“Above 32 °C the enzymes are killed.” — Enzymes are molecules, not living things. They are denatured: their shape changes.

9. YQ8 · Practice

Options as numbered on the pupil sheet: 1 — The variable you measure (dependent variable); 2 — Kept the same; 3 — The variable you change (independent variable).

Model answer. The temperature of the bath is the independent variable: you choose it. The volume of gas collected in 5 minutes, which gives the rate, is the dependent variable: you measure it. The mass of yeast, the volume and concentration of the glucose solution and the time of each run are kept the same, so that the test is fair.

A full answer includes

I matched the temperature of the water bath to the variable you change (independent).
I matched the volume of gas collected in 5 minutes to the variable you measure (dependent).
I matched the yeast, the glucose solution and the time of each run to kept the same.
I did not match the volume of gas to the variable you change.

Also accept

temperature: change; volume of gas: measure; yeast, glucose and time: keep the same

Watch for

“The volume of gas is the independent variable.” — You do not choose it; you measure it. It is the dependent variable.
“The time of each run can change.” — Every run lasts 5 minutes, or the volumes could not be compared.

10. YQ9 · Practice

Model answer. Yeast deep in bread dough has used up the oxygen there, so it respires anaerobically. In the last seconds of a 200 m sprint, the heart and lungs cannot bring oxygen to the leg muscles fast enough, so they respire anaerobically as well as aerobically. The heart muscle of a person sitting and reading gets enough oxygen from the blood, and yeast with air bubbled through has plenty of oxygen: both respire aerobically.

A full answer includes

I chose the yeast deep in the dough, after the oxygen is used up.
I chose the leg muscles in the last seconds of the sprint.
I did not choose the heart muscle of the person reading: it gets enough oxygen.
I did not choose the yeast with air bubbled through: it has oxygen, so it respires aerobically.

Also accept

the yeast deep in the dough; the leg muscles at the end of the sprint

Watch for

“Heart muscle never rests, so it always respires anaerobically.” — At rest, the blood brings the heart muscle enough oxygen for aerobic respiration.
“Yeast always respires anaerobically.” — Yeast respires aerobically when it has oxygen and anaerobically when it does not.
“The sprinter's muscles respire only anaerobically.” — They respire aerobically as fast as the oxygen allows, and anaerobically as well.

11. YQ12 · Practice

Model answer. C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Of the 6 carbon atoms of glucose, 4 end up in two ethanol molecules and 2 in two carbon dioxide molecules. The 12 hydrogen atoms all end up in the two ethanol molecules (2 × 6 = 12), and the 6 oxygen atoms are 2 in the ethanol and 4 in the carbon dioxide. No oxygen gas is used.

A full answer includes

Gap 1: I wrote 2, for 2C₂H₅OH.
Gap 2: I wrote 2, for 2CO₂.
I did not copy the 6s from aerobic respiration.

Also accept

2, 2

Watch for

“C₆H₁₂O₆ → 6C₂H₅OH + 6CO₂” — That copies the 6s from aerobic respiration. Count the carbon: 6 × 2 + 6 = 18 on the right, but only 6 in glucose.
“C₆H₁₂O₆ → C₂H₅OH + CO₂” — That is not balanced: there are 6 carbon atoms on the left and only 3 on the right.

12. YQ10 · Practice

Model answer. C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂: each glucose molecule gives 2 ethanol molecules and 2 carbon dioxide molecules. (a) 16 ethanol molecules came from 16 ÷ 2 = 8 glucose molecules. (b) 8 glucose molecules give 8 × 2 = 16 carbon dioxide molecules: the same number as the ethanol molecules. No oxygen is used.

Checked automatically

(a) Glucose molecules broken down: 8 (exact value)
- Common wrong answer 32 : Divide, do not multiply: each glucose molecule gives 2 ethanol molecules.
- Common wrong answer 16 : That is the ethanol. Each glucose molecule gives 2 of them.
(b) Carbon dioxide molecules made: 16 (exact value)
- Common wrong answer 8 : That is the glucose. Each glucose molecule gives 2 carbon dioxide molecules.
- Common wrong answer 48 : Six for each glucose molecule is aerobic respiration. Use the equation for yeast without oxygen.

A full answer includes

Method (a): I used 2 ethanol molecules for each glucose molecule: 16 ÷ 2.
Answer (a): I wrote 8 glucose molecules.
Method (b): I used 2 carbon dioxide molecules for each glucose molecule: 8 × 2.
Answer (b): I wrote 16 carbon dioxide molecules.
I did not use the 6 from the aerobic equation.

Also accept

(a) 8; (b) 16

Watch for

“It broke down 32 glucose molecules: 16 × 2.” — Each glucose molecule gives 2 ethanol molecules, so there are fewer glucose molecules than ethanol molecules. Divide, do not multiply.
“It made 48 carbon dioxide molecules, 6 for each glucose molecule.” — Six carbon dioxide molecules for each glucose molecule is aerobic respiration. Use the equation for yeast without oxygen.

13. YQ11 · Practice

Model answer. (a) Carbon dioxide turns limewater milky, so the gas from the yeast is carbon dioxide, as the word equation says. (b) The tube with no yeast is the control. It is the same in every way except that it has no yeast. It gives no gas, so the gas in the other tubes must come from the yeast respiring, not from the glucose solution or from the tube warming.

A full answer includes

(a) I wrote that the gas is carbon dioxide.
(b) I wrote that the tube with no yeast is the control.
(b) I wrote that the gas comes from the yeast.
(b) I wrote that the gas does not come from the glucose solution or from the tube warming.
I did not write that milky limewater shows oxygen.

Also accept

(a) carbon dioxide; (b) control: the gas comes from the yeast

Watch for

“Milky limewater shows that the gas is oxygen.” — Limewater tests for carbon dioxide.
“The tube with no yeast is a mistake and should be left out.” — It is the control: it shows what happens without yeast.

14. YT1 · Apply

Model answer. Bubbles can be different sizes, and when they come fast some are missed or two join, so the count does not tell you the volume of gas. The gas syringe measures the volume of gas collected in cm³, so it is more accurate and the runs can be compared fairly.

A full answer includes

I wrote that bubbles can be different sizes.
I wrote that bubbles can be missed when they come fast.
I wrote that the gas syringe measures the volume of gas in cm³.
I did not write that the syringe collects more gas.

Also accept

bubbles differ in size and are hard to count; the syringe measures the volume in cm³

Watch for

“Counting bubbles is better because you can see them.” — Seeing is not measuring: bubbles differ in size.
“The syringe collects more gas.” — The yeast makes the same gas either way. The syringe measures its volume.

15. YT2 · Apply

Model answer. Dough rises because the yeast respires the sugar and gives off carbon dioxide, which is trapped as bubbles in the dough. Water at 70 °C is far above the optimum: the enzymes that control the yeast's respiration are denatured and the yeast cells die, so no carbon dioxide is made and the dough does not rise. At 35 °C, close to the optimum, the yeast respires fast and the dough rises well.

A full answer includes

I wrote that dough rises because the yeast gives off carbon dioxide.
I wrote that the carbon dioxide is trapped as bubbles in the dough.
I wrote that at 70 °C the enzymes of the yeast are denatured.
I wrote that the yeast then cannot respire, so no carbon dioxide is made.
I wrote that 35 °C is near the optimum, so the yeast respires fast.
I did not write that the enzymes are killed.

Also accept

70 °C denatures the enzymes of the yeast, so no respiration and no carbon dioxide; 35 °C is near the optimum

Watch for

“The hot water made the gas escape.” — No gas was made: the heat stopped the yeast respiring.
“Hotter water always makes yeast respire faster.” — Only up to the optimum. Far above it, the enzymes stop working.

16. YT3 · Apply

Model answer. With air bubbled through, the yeast respires aerobically, which releases much more energy per glucose molecule than anaerobic respiration. The yeast uses that energy to grow and divide, so the factory gets more yeast. In the brewery, air is kept out so that the yeast respires anaerobically: glucose → alcohol + carbon dioxide. The alcohol is the product that brewing needs.

A full answer includes

I wrote that with air the yeast respires aerobically.
I wrote that aerobic respiration releases much more energy per glucose molecule.
I wrote that the extra energy lets the yeast grow and divide faster.
I wrote that without air the yeast respires anaerobically and makes alcohol.
I did not write that air makes the yeast produce more alcohol.

Also accept

air: aerobic, more energy, faster growth; no air: anaerobic, makes alcohol for brewing

Watch for

“Air makes the yeast produce more alcohol.” — Alcohol is made by anaerobic respiration, without oxygen.
“Air gives the yeast food.” — The food is the sugar. Air supplies oxygen for aerobic respiration.

17. YT4 · Apply

Model answer. The balanced equation is C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. (a) Of the 6 carbon atoms in a glucose molecule, 2 end up in two carbon dioxide molecules and 4 stay in two ethanol molecules. (b) So the yeast breaks the glucose down only partly. Much of the energy that was stored in the glucose is still held in the ethanol, and that is the energy released when ethanol burns in a car engine. In aerobic respiration, C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, all 6 carbon atoms end up in carbon dioxide: the glucose is broken down completely, and much more energy is released from each molecule.

A full answer includes

(a) I wrote that 2 of the 6 carbon atoms end up in carbon dioxide.
(a) I wrote that 4 stay in the two ethanol molecules.
(b) I wrote that the glucose is only partly broken down.
(b) I wrote that much of its energy is still held in the ethanol.
(b) I wrote that aerobic respiration breaks glucose down completely, to carbon dioxide and water.
I did not write that the ethanol has no energy left in it.

Also accept

(a) 2 in carbon dioxide, 4 in ethanol; (b) glucose only partly broken down, much energy left in the ethanol

Watch for

“Anaerobic respiration makes less energy because it has no oxygen to burn.” — Energy is never made. Without oxygen, the glucose is only partly broken down, so less of its energy is released and much stays in the ethanol.
“The ethanol has no energy left in it: it is a waste product.” — Ethanol burns well and is used as a fuel, so it still holds a lot of energy. That is energy the yeast did not release.