21: Respiration - Practical

Question 1

21.1 Investigation of oxygen consumption in a grasshopper

A grasshopper is enclosed in a conical flask with a tube of potassium hydroxide solution inside the flask. The flask is stopped and connected to a delivery tube with its other end submerged in a beaker of coloured water.
Explain the purpose of using potassium hydroxide solution. (2)

Answer 1

To absorb carbon dioxide in the flask,
such that the carbon dioxide originally present in the flask and that produced by the grasshopper during respiration is absorbed.

Question 2

21.1 Investigation of oxygen consumption in a grasshopper

A grasshopper is enclosed in a conical flask with a tube of potassium hydroxide solution inside the flask. The flask is stopped and connected to a delivery tube with its other end submerged in a beaker of coloured water.
Explain the working mechanism of this set-up and the expected results. (4)

Answer 2

Oxygen is consumed by the grasshopper for respiration.
Meanwhile, carbon dioxide is produced by the grasshopper by respiration, but it is absorbed by the potassium hydroxide solution.
This causes the number of gas molecules in the conical flask to decrease; thus, the air pressure inside the conical flask and the capillary tube drops and becomes lower than the atmospheric air pressure.
Therefore, the level of the coloured water inside the delivery tube rises due to the pressure difference, and moves towards the conical flask.
(X gas volume decrease because gases occupy the whole volume of the container)

Question 3

21.1 Investigation of oxygen consumption in a grasshopper

A grasshopper is enclosed in a conical flask with a tube of potassium hydroxide solution inside the flask. The flask is stopped and connected to a delivery tube with its other end submerged in a beaker of coloured water.
Explain the control set-up for this investigation.

Answer 3

Set up an identical set-up without putting a grasshopper in the conical flask.
(X the same set-up)

Question 4

21.1 Investigation of oxygen consumption in a grasshopper

A grasshopper is enclosed in a conical flask with a tube of potassium hydroxide solution inside the flask. The flask is stopped and connected to a delivery tube with its other end submerged in a beaker of coloured water.
Explain if the investigation can be carried out for a long period of time.

Answer 4

No. This is because the grasshopper will use up all the oxygen present in the conical flask and die.

Question 5

21.1 Investigation of oxygen consumption in a grasshopper

A grasshopper is enclosed in a conical flask with a tube of potassium hydroxide solution inside the flask. The flask is stopped and connected to a delivery tube with its other end submerged in a beaker of coloured water.
State the word formula for the rate of respiration of the grasshopper.

Answer 5

Rate of respiration = Change in level of coloured liquid in the capillary tube / Time allowed for experiment

Question 6

21.2: Investigation of carbon dioxide production in germinating seeds

Hydrogencarbonate indicator is added to two stoppered test tubes. Wire gauzes and moist cotton wools are added above the indicators in both tubes. Tube A consists of soaked germinating seeds with surface sterilised on the moist cotton wool, while tube B consists of boiled seeds with surface steriliseed.
List 3 important controlled variables of this investigation. (3)

Answer 6

Concentration and volume of the hydrogencarbonate indicator,
mass and species of the seeds,
duration allowed for respiration

Question 7

21.2: Investigation of carbon dioxide production in germinating seeds

Hydrogencarbonate indicator is added to two stoppered test tubes. Wire gauzes and moist cotton wools are added above the indicators in both tubes. Tube A consists of soaked germinating seeds with surface sterilised on the moist cotton wool, while tube B consists of boiled seeds with surface steriliseed.
What is the function of the moist cotton wool?

Answer 7

To provide the necessary amount of water for seed germination.

Question 8

21.2: Investigation of carbon dioxide production in germinating seeds

Hydrogencarbonate indicator is added to two stoppered test tubes. Wire gauzes and moist cotton wools are added above the indicators in both tubes. Tube A consists of soaked germinating seeds with surface sterilised on the moist cotton wool, while tube B consists of boiled seeds with surface steriliseed.
Explain the significance of sterilising the surface of the seeds. (3)

Answer 8

This is to kill the microorganisms on the surface of the seeds.
Otherwise, carbon dioxide released by them during respiration will cause the results to be overestimated,
resulting in a decrease in validity of the experiment.

Question 9

21.2: Investigation of carbon dioxide production in germinating seeds

Hydrogencarbonate indicator is added to two stoppered test tubes. Wire gauzes and moist cotton wools are added above the indicators in both tubes. Tube A consists of soaked germinating seeds with surface sterilised on the moist cotton wool, while tube B consists of boiled seeds with surface steriliseed.
What is the purpose of setting up tube B?

Answer 9

It is a control to show that only living seeds release carbon dioxide.

Question 10

21.2: Investigation of carbon dioxide production in germinating seeds

Hydrogencarbonate indicator is added to two stoppered test tubes. Wire gauzes and moist cotton wools are added above the indicators in both tubes. Tube A consists of soaked germinating seeds with surface sterilised on the moist cotton wool, while tube B consists of boiled seeds with surface steriliseed.
Explain the results and conclusion of this investigation. (3)

Answer 10

The germinating seeds in tube A carry out respiration and carbon dioxide is produced. The carbon dioxide turns the hydrogencarbonate indicator from red to yellow.
The boiled seeds in tube B are dead and do not carry out respiration. No carbon dioxide is produced from them. The hydrogencarbonate indicator remains red.
This shows that the geminating seeds give out carbon dioxide.

Question 11

21.3: Investigation of carbon dioxide production in a living mouse

A suction pump is connected to flasks A to C and a bell jar. (Air flow: flask A -> flask B -> bell jar -> flask C) Potassium hydroxide solution is added in flask A, and lime water is added in flasks B and C. The bell jar is sealed and airtight and contains a living mouse.
What is the function of flask A?

Answer 11

To absorb the carbon dioxide in the incoming air, and make sure that the incoming air towards the living mouse does not contain carbon dioxide.

Question 12

21.3: Investigation of carbon dioxide production in a living mouse

A suction pump is connected to flasks A to C and a bell jar. (Air flow: flask A -> flask B -> bell jar -> flask C) Potassium hydroxide solution is added in flask A, and lime water is added in flasks B and C. The bell jar is sealed and airtight and contains a living mouse.
What is the purpose of using potassium hydroxide solution? (2)

Answer 12

To absorb the carbon dioxide in the incoming air, and make sure that the incoming air towards the living mouse does not contain carbon dioxide.
Any carbon dioxide detected in flask C is therefore released by the living mouse.

Question 13

21.3: Investigation of carbon dioxide production in a living mouse

A suction pump is connected to flasks A to C and a bell jar. (Air flow: flask A -> flask B -> bell jar -> flask C) Potassium hydroxide solution is added in flask A, and lime water is added in flasks B and C. The bell jar is sealed and airtight and contains a living mouse.
What are the functions of the lime water in flasks B and C?

Answer 13

Flask B: To detect whether there is any carbon dioxide in the air entering the bell jar.
Flask C: To detect whether there is any carbon dioxide in the air leaving the bell jar.

Question 14

21.3: Investigation of carbon dioxide production in a living mouse

A suction pump is connected to flasks A to C and a bell jar. (Air flow: flask A -> flask B -> bell jar -> flask C) Potassium hydroxide solution is added in flask A, and lime water is added in flasks B and C. The bell jar is sealed and airtight and contains a living mouse.
Explain the expected results in flask B and flask C after 30 minutes, and the conclusion of the investigation. (3)

Answer 14

The limewater in flask B remains colourless in the experiment. This suggests that carbon dioxide is absent in the air entering the bell jar.
The limewater in flask C turns from colourless to milky after 30 minutes. This suggests that carbon dioxide is given out by the living mouse.
This shows that the living mouse gives out carbon dioxide.

Question 15

21.3: Investigation of carbon dioxide production in a living mouse

A suction pump is connected to flasks A to C and a bell jar. (Air flow: flask A -> flask B -> bell jar -> flask C) Potassium hydroxide solution is added in flask A, and lime water is added in flasks B and C. The bell jar is sealed and airtight and contains a living mouse.
Explain the control set-up for this investigation.

Answer 15

Set up an identical set-up without putting a mouse in the bell jar.
(X the same set-up)

Question 16

21.3: Investigation of carbon dioxide production in a living mouse

A suction pump is connected to flasks A to C and a bell jar. (Air flow: flask A -> flask B -> bell jar -> flask C) Potassium hydroxide solution is added in flask A, and lime water is added in flasks B and C. The bell jar is sealed and airtight and contains a living mouse.
Explain two precautions to be taken if the mouse is replaced by a potted plant. (4)

Answer 16

1. Wrap the pot with a plastic bag.
   Otherwise, carbon dioxide released by the microorganisms in the soil will cause an overestimation of the production of carbon dioxide by the plant. The validity of the investigation will decrease.
2. Cover the bell jar with a piece of black cloth.
   Otherwise, the plant will absorb carbon dioxide for photosynthesis and cause an underestimation of the production of carbon dioxide by the plant through respiration. The validity of the investigation will decrease.

Question 17

21.4: Investigation of heat production in germinating seeds

Two identical vacuum flasks are set up with mouth facing downwards. Cotton wool is plugged at both mouths, and a thermometer is inserted into each flask. Flask A contains soaked germinating seeds with surface sterilised, while flask B contains boiled seeds with surface sterilised.
Explain the significance of sterilising the surface of the seeds. (3)

Answer 17

To kill the microorganisms on the surface of the seeds.
Otherwise, carbon dioxide released by them during respiration will cause an overestimation of the heat production by the germinating seeds.
Thus, the validity of the results decreases.

Question 18

21.4: Investigation of heat production in germinating seeds

Two identical vacuum flasks are set up with mouth facing downwards. Cotton wool is plugged at both mouths, and a thermometer is inserted into each flask. Flask A contains soaked germinating seeds with surface sterilised, while flask B contains boiled seeds with surface sterilised.
Explain three reasons why the vacuum flasks are put in an inverted position. (3)

Answer 18

1. Warm air rises as it is less dense than cold air. Putting the vacuum flasks in an inverted position can trap the warm air and minimize heat loss.
2. Readings of the thermometer can be taken more easily.
3. As elevated concentration of carbon dioxide can reduce the rate of respiration, the inverted vacuum flask that is closed by dry cotton wool allows the escape of carbon dioxide released by the seeds from the flask because carbon dioxide is more dense than air.

Question 19

21.4: Investigation of heat production in germinating seeds

Two identical vacuum flasks are set up with mouth facing downwards. Cotton wool is plugged at both mouths, and a thermometer is inserted into each flask. Flask A contains soaked germinating seeds with surface sterilised, while flask B contains boiled seeds with surface sterilised.
Explain why the vacuum flasks are not completely filled with seeds.

Answer 19

To provide enough air (containing oxygen) for the seeds to carry out respiration.

Question 20

21.4: Investigation of heat production in germinating seeds

Two identical vacuum flasks are set up with mouth facing downwards. Cotton wool is plugged at both mouths, and a thermometer is inserted into each flask. Flask A contains soaked germinating seeds with surface sterilised, while flask B contains boiled seeds with surface sterilised.
Explain the results and conclusion of this investigation. (3)

Answer 20

The germinating seeds in flask A contains enzymes which catalyze the reactions in respiration. Heat is produced during respiration and therefore the temperature inside the flask increases.
The enzymes in the boiled seeds in flask B are denatured and so the boiled seeds do not carry out respiration. No heat is produced from them. Therefore the temperature inside flask B remains unchanged. (The temperature inside flask B may drop due to heat loss to the surroundings)
Therefore, it is concluded that heat is released by germinating seeds.

Question 21

21.4: Investigation of heat production in germinating seeds

Two identical vacuum flasks are set up with mouth facing downwards. Cotton wool is plugged at both mouths, and a thermometer is inserted into each flask. Flask A contains soaked germinating seeds with surface sterilised, while flask B contains boiled seeds with surface sterilised.
Suggest two sources of error and two ways for improvement. (2)

Answer 21

There are variations among the seeds in the two flasks. The experiment can be repeated for a few more times to minimise the effect of individual error, and increase the reliability of the investigation.
There may be transfer of heat in the surrounding environment to the inside of the vacuum flask. Therefore, cotton wool or good heat insulators can be wrapped around the flasks.

Question 22

21.4: Investigation of heat production in germinating seeds

Two identical vacuum flasks are set up with mouth facing downwards. Cotton wool is plugged at both mouths, and a thermometer is inserted into each flask. Flask A contains soaked germinating seeds with surface sterilised, while flask B contains boiled seeds with surface sterilised.
Predict and explain the results if soaked germinating seeds without sterilisation are used. (2)

Answer 22

The temperature increase will be much higher than that caused by sterilized and soaked germinating seeds.
This is because the microorganisms on the seeds also respire and produce heat.

Question 23

21.4: Investigation of heat production in germinating seeds

Two identical vacuum flasks are set up with mouth facing downwards. Cotton wool is plugged at both mouths, and a thermometer is inserted into each flask. Flask A contains soaked germinating seeds with surface sterilised, while flask B contains boiled seeds with surface sterilised.
Predict and explain the results if boiled seeds without sterilisation are used. (2)

Answer 23

At first, the temperature will remain unchanged. When microorganisms start to grow and reproduce on the seeds, the microorganisms respire and produce heat.
This leads to a rapid increase in temperature at the later time of the experiment.

Question 24

21.5: Investigation of heat production in a living mouse

Set up a differential air thermometer with arm A connected to a test tube enclosed in a chamber insulated with cotton wool. Arm B is connected to a test tube enclosed with an insulated chamber, with a living mouse in the chamber.
Explain the function of the cotton wool.

Answer 24

To minimise heat loss from the chambers to the surroundings.

Question 25

21.5: Investigation of heat production in a living mouse

Set up a differential air thermometer with arm A connected to a test tube enclosed in a chamber insulated with cotton wool. Arm B is connected to a test tube enclosed with an insulated chamber, with a living mouse in the chamber. Clips connecting to both arms are opened at the start of the experiment, then they are closed for 10 minutes.
Explain the significance of opening the clips at the start of the investigation.

Answer 25

To equalize the air pressure on both sides of the U-shaped capillary tube. This ensures that the liquid levels on both sides are the same at the start of the experiment.

Question 26

21.5: Investigation of heat production in a living mouse

Set up a differential air thermometer with arm A connected to a test tube enclosed in a chamber insulated with cotton wool. Arm B is connected to a test tube enclosed with an insulated chamber, with a living mouse in the chamber.
Explain why a capillary tube, instead of a tube with a wide bore, is used in the differential air thermometer. (2)

Answer 26

To give a more obvious change in the liquid levels in the U-shaped capillary tube.
This ensures that the readings taken are more precise, thus increasing the accuracy of the experiment.

Question 27

21.5: Investigation of heat production in a living mouse

Set up a differential air thermometer with arm A connected to a test tube enclosed in a chamber insulated with cotton wool. Arm B is connected to a test tube enclosed with an insulated chamber, with a living mouse in the chamber. Clips connecting to both arms are opened at the start of the experiment, then they are closed for 10 minutes.
Explain the changes in the liquid levels in the U-shaped capillary tube (in the differential air themometer) in the two arms, and the conclusion of the investigation. (5)

Answer 27

Heat is released by the mouse by respiration and it warms up the air in the thin-walled test tube above the mouse. The kinetic energy of air molecules increases, causing the air molecules to move and collide with each other more frequently.
The air in the test tube expands and results in an increase in air pressure.
On the other hand, there is no temperature change in the control set-up (the side without the mouse).
The air pressure in tube B and its associated capillary tube becomes higher than that in tube A and its associated capillary tube. Hence, the coloured liquid level in arm B is being pushed downwards, while the liquid level in arm A rises.
Therefore, it is concluded that heat is released by the living mouse.

Question 28

21.5: Investigation of heat production in a living mouse

Set up a differential air thermometer with arm A connected to a test tube enclosed in a chamber insulated with cotton wool. Arm B is connected to a test tube enclosed with an insulated chamber, with a living mouse in the chamber. Clips connecting to both arms are opened at the start of the experiment, then they are closed for 10 minutes.
Explain whether this investigation can be carried out for a long period of time.

Answer 28

No. Since the mouse is kept in an enclosed chamber with no entry or exit of air, the mouse will use up all the oxygen inside the chamber and die.

Question 29

21.5: Investigation of heat production in a living mouse

Set up a differential air thermometer with arm A connected to a test tube enclosed in a chamber insulated with cotton wool. Arm B is connected to a test tube enclosed with an insulated chamber, with a living mouse in the chamber. Clips connecting to both arms are opened at the start of the experiment, then they are closed for 10 minutes.
Predict and explain the results of the investigation if the mouse is replaced by a frog of a similar size. (3)

Answer 29

Both the fall in the liquid level in arm B and the rise in the liquid level in arm A will be smaller.
This is because the frog is an amphibian, which is poikilothermic. Thus, it does not need to spend energy to maintain a constant internal body temperature. Therefore, a lower metabolic rate can support the energy requirement of the mice, so a lower rate of respiration is carried out.
Hence, the rate of heat production by the body of the frog is lower.

Question 30

21.5: Investigation of heat production in a living mouse

Set up a differential air thermometer with arm A connected to a test tube enclosed in a chamber insulated with cotton wool. Arm B is connected to a test tube enclosed with an insulated chamber, with a living mouse in the chamber. Clips connecting to both arms are opened at the start of the experiment, then they are closed for 10 minutes.
Predict and explain the results of the investigation if the test tubes are replaced by bigger boiling tubes. (3)

Answer 30

Both the fall in the liquid level in arm B and the rise in the liquid level in arm A will be smaller. / Longer time is required to obtain the same results.
The volume of air which can be held on each tube becomes larger. More heat is required to warm up the larger amount of air.
Therefore, more heat production by respiration of the mouse (a longer time allowed) is required to reach the same result.

Question 31

21.6: Investigation of the rate of respiration from carbon dioxide

A suction pump is connected to flasks A to C , a U-shaped tube, and a bell jar. (Air flow: flask A -> flask B -> bell jar -> U-tube -> flask C) Potassium hydroxide solution is added in flasks A and C, and lime water is added in flask B. The U-shaped tube is filled with drying agent. The bell jar is sealed and airtight and contains a living mouse. The change in mass of Flask C is measured for both the set-up and the control over 30 minutes.
Explain the purpose of using drying agent.

Answer 31

To dry / remove any water vapour in air entering flask C.

Question 32

21.6: Investigation of the rate of respiration from carbon dioxide

A suction pump is connected to flasks A to C , a U-shaped tube, and a bell jar. (Air flow: flask A -> flask B -> bell jar -> U-tube -> flask C) Potassium hydroxide solution is added in flasks A and C, and lime water is added in flask B. The U-shaped tube is filled with drying agent. The bell jar is sealed and airtight and contains a living mouse. The change in mass of Flask C is measured for both the set-up and the control over 30 minutes.
Calculate the rate of respiration of the mouse (in g CO₂ h^-1) using the data:
M1a: initial mass of flask C in the experimental set-up
M1b: final mass of flask C in the experimental set-up
M2a: initial mass of flask C in the control set-up
M2b: final mass of flask C in the control set-up

Answer 32

[(M1b-M1a)-(M2b-M2a)]/30x60

Question 33

21.6: Investigation of the rate of respiration from carbon dioxide

A suction pump is connected to flasks A to C , a U-shaped tube, and a bell jar. (Air flow: flask A -> flask B -> bell jar -> U-tube -> flask C) Potassium hydroxide solution is added in flasks A and C, and lime water is added in flask B. The U-shaped tube is filled with drying agent. The bell jar is sealed and airtight and contains a living mouse. The change in mass of Flask C is measured for both the set-up and the control over 30 minutes.
Explain a significant assumption made in the experiment.

Answer 33

The rate of evaporation of water from the potassium hydroxide solution in flask C is the same in both set-ups.

Question 34

21.6: Investigation of the rate of respiration from carbon dioxide

A suction pump is connected to flasks A to C , a U-shaped tube, and a bell jar. (Air flow: flask A -> flask B -> bell jar -> U-tube -> flask C) Potassium hydroxide solution is added in flasks A and C, and lime water is added in flask B. The U-shaped tube is filled with drying agent. The bell jar is sealed and airtight and contains a living mouse. The change in mass of Flask C is measured for both the set-up and the control over 30 minutes.
Explain a significant assumption made in the experiment.

Answer 34

The rate of evaporation of water from the potassium hydroxide solution in flask C is the same in both set-ups.

Question 35

21.7 Investigation of the rate of respiration from oxygen consumption

A differential air theremometer is connected to two clipped test tubes (A and B) submerged in a water bath. Both tubes contain soda lime pellets, while tube B contains sterilised germinating seeds un top of a wire gauze. An 1cm³ syringe is connected to tube B. The set-up is first equilibriated, then the syringe is adjusted at one-minute intervals such that the liquid levels in the U-shaped capillary tube remain the same. The readings on the syringe are recorded.
Explain the significance of using a water bath.

Answer 35

To prevent the experimental results from being affected by changes in temperature in the surroundings.

Question 36

21.7 Investigation of the rate of respiration from oxygen consumption

A differential air theremometer is connected to two clipped test tubes (A and B) submerged in a water bath. Both tubes contain soda lime pellets, while tube B contains sterilised germinating seeds un top of a wire gauze. An 1cm³ syringe is connected to tube B. The set-up is first equilibriated, then the syringe is adjusted at one-minute intervals such that the liquid levels in the U-shaped capillary tube remain the same. The readings on the syringe are recorded.
Explain the function of the wire gauze.

Answer 36

To provide support for the germinating seeds and prevent them from contacting the corrosive soda lime pellets.

Question 37

21.7 Investigation of the rate of respiration from oxygen consumption

A differential air theremometer is connected to two clipped test tubes (A and B) submerged in a water bath. Both tubes contain soda lime pellets, while tube B contains sterilised germinating seeds un top of a wire gauze. An 1cm³ syringe is connected to tube B. The set-up is first equilibriated, then the syringe is adjusted at one-minute intervals such that the liquid levels in the U-shaped capillary tube remain the same. The readings on the syringe are recorded.
Explain a significant assumption made in this investigation. (2)

Answer 37

All carbon dioxide (including those produced by respiration and already in tube) is absorbed by the soda lime pellets.
This ensures that any rise in the liquid level in arm B must be due to the oxygen absorbed by the seeds for respiration.

Question 38

21.7 Investigation of the rate of respiration from oxygen consumption

A differential air theremometer is connected to two clipped test tubes (A and B) submerged in a water bath. Both tubes contain soda lime pellets, while tube B contains sterilised germinating seeds un top of a wire gauze. An 1cm³ syringe is connected to tube B. The set-up is first equilibriated, then the syringe is adjusted at one-minute intervals such that the liquid levels in the U-shaped capillary tube remain the same. The readings on the syringe are recorded.
Explain the significance of using germinating seeds without first leaves opened out. (2)

Answer 38

When first leaves open out, seeds carry out photosynthesis, producing oxygen which will cause the rate of oxygen consumption by the germinating seeds to be underestimated ,
which decreases the validity of the results.

Question 39

21.7 Investigation of the rate of respiration from oxygen consumption

A differential air theremometer is connected to two clipped test tubes (A and B) submerged in a water bath. Both tubes contain soda lime pellets, while tube B contains sterilised germinating seeds un top of a wire gauze. An 1cm³ syringe is connected to tube B. The set-up is first equilibriated, then the syringe is adjusted at one-minute intervals such that the liquid levels in the U-shaped capillary tube remain the same. The readings on the syringe are recorded.
Explain the expected results of the investigation. (4)

Answer 39

The respiring seeds consume oxygen and produce carbon dioxide. The carbon dioxide is absorbed by soda lime.
As the number of gas molecules decreases in tube B, the air pressure in tube B and its associated capillary tube decreases and becomes lower than the air pressure in tube A and its associated capillary tube. Thus, the liquid level in arm B rises while the liquid level in arm A falls.
The volume of air injected into tube B to cause the liquid level in arm B to return to its initial level / the same level as in arm A shows the volume of oxygen consumed by the seeds.
The rate of oxygen consumption was calculated in terms of the volume of oxygen consumed per unit time.

Question 40

21.7 Investigation of the rate of respiration from oxygen consumption

A differential air theremometer is connected to two clipped test tubes (A and B) submerged in a water bath. Both tubes contain soda lime pellets, while tube B contains sterilised germinating seeds un top of a wire gauze. An 1cm³ syringe is connected to tube B. The set-up is first equilibriated, then the syringe is adjusted at one-minute intervals such that the liquid levels in the U-shaped capillary tube remain the same. The readings on the syringe are recorded.
Suggest a modification to investigate the effect of temperature on the rate of respiration.

Answer 40

Repeat the experiment with water baths of different temperatures.

Question 41

21.8: Design an investigation of alcoholic fermentation in yeast

What is the aim of the investigation?

Answer 41

To study alcoholic fermentation in yeast.

Question 42

21.8: Design an investigation of alcoholic fermentation in yeast

What is the problem to be investigated?

Answer 42

Does yeast carry out alcoholic fermentation under anaerobic condition?

Question 43

21.8: Design an investigation of alcoholic fermentation in yeast

Explain the biological principle behind the design of the investigation.

Answer 43

To see whether yeast carries out alcoholic fermentation under anaerobic condition, we can prepare a mixture of glucose solution and liquid culture of yeast, test for the products of alcoholic fermentation (e.g. carbon dioxide and ethanol) and compare with a control.

Question 44

21.8: Design an investigation of alcoholic fermentation in yeast

What is the independent variable in the investigation, and how is it manipulated? (2)

Answer 44

Whether the yeast is living or dead,
which is manipulated by boiling some liquid culture and leaving some culture unboiled.

Question 45

21.8: Design an investigation of alcoholic fermentation in yeast

What is the dependent variable in the investigation, and how is it measured? (2)

Answer 45

Production of carbon dioxide. Pass the gas released from the mixture through hydrogencarbonate indicator.
OR
Production of ethanol. Note the smell of the mixture.
OR
Production of heat. Measure the change in temperature of the mixture with a thermometer after the experiment.
(only choose 1 out of the 3)

Question 46

21.8: Design an investigation of alcoholic fermentation in yeast

List 3 controlled variables in the investigation.

Answer 46

pH, volume and concentration of glucose solution, volume and concentration of hydrogencarbonate indicator
(X temperature, because it is supposed to increase)

Question 47

21.8: Design an investigation of alcoholic fermentation in yeast

Explain the effect of the variable “volume and concentration of glucose solution” on the results. (3)

Answer 47

An increased volume or concentration of the glucose solution results in more substrate to collide with the enzymes involving in respiration.
Therefore, there is a higher chance of successful collision between the glucose and the enzymes, creating an enzyme-substrate complex.
So, there is a higher chance of forming products, and a higher rate of respiration of the yeast. The same results will be obtained faster.

Question 48

21.8: Design an investigation of alcoholic fermentation in yeast

Explain if it is necessary to set up a control.

Answer 48

Yes. It is used to confirm that the products of alcoholic fermentation are produced by the living yeast.

Question 49

21.8: Design an investigation of alcoholic fermentation in yeast

Explain a significant assumption made in this investigation.

Answer 49

No oxygen is present in glucose solution.

Question 50

21.8: Design an investigation of alcoholic fermentation in yeast

Explain the results and conclusion of this investigation. (3)

Answer 50

Yeast carries out alcoholic fermentation under anaerobic condition. During the process, carbon dioxide, ethanol and heat are produced.
The carbon dioxide turns the hydrogencarbonate indicator from red to yellow. / The ethanol gives the smell of alcohol in the experimental set-up. / The heat causes the temperature rise of the experimental set-up.
Therefore, it can be concluded that under anaerobic condition, yeast carries out alcoholic fermentation to produce carbon dioxide and ethanol. Heat is also given out.