RP9 Respiration in single-celled organisms Questions

Specification route: RP9
Question bank: 14 questions
Course stage: Year 13 / A-level only

Sample questions

QUESTION 1 · RP9 · LEVEL 2

When investigating the rate of anaerobic respiration in yeast, a layer of liquid paraffin (oil) is placed over the surface of the yeast suspension. What is its purpose?

To prevent oxygen from the air diffusing into the suspension, ensuring the conditions remain anaerobic.
To trap the carbon dioxide produced so the bubbles can be counted accurately.
To act as an alternative lipid respiratory substrate for the yeast.
To prevent the yeast cells from evaporating out of the test tube.

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Anaerobic means 'without oxygen'.

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Answer: To prevent oxygen from the air diffusing into the suspension, ensuring the conditions remain anaerobic.

Required practical 9. The oil layer acts as a physical barrier to oxygen, stopping it from dissolving into the yeast suspension and forcing the yeast to respire anaerobically.

QUESTION 2 · RP9 · LEVEL 2

In an experiment investigating respiration rates using methylene blue, the yeast suspension is placed in a water bath for 5 minutes before adding the indicator. Why?

To allow the yeast suspension to equilibrate and reach the target temperature before the reaction begins.
To give the yeast time to undergo mitosis and double the population size.
To completely denature any competitive inhibitors present in the mixture.
To boil off any dissolved oxygen from the water.

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Temperature must be the controlled independent variable from the very first second the reaction is timed.

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Answer: To allow the yeast suspension to equilibrate and reach the target temperature before the reaction begins.

Required practical 9. Equilibration ensures the contents of the tube reach the temperature of the water bath. If the indicator is added immediately, the reaction would occur at varying temperatures as it warms up.

QUESTION 3 · RP9 · LEVEL 3

When using a respirometer to measure the oxygen uptake of woodlice, soda lime (or potassium hydroxide) is placed in the bottom of the tube. What is its function?

It absorbs the carbon dioxide produced by the woodlice, so any change in gas volume is entirely due to the consumption of oxygen.
It absorbs the oxygen produced by the woodlice to ensure the environment remains anaerobic.
It provides a constant source of moisture to prevent the woodlice from desiccating.
It acts as a chemical indicator, turning pink as the rate of respiration increases.

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If a woodlouse takes in $1\ cm^3$ of $O_2$ and breathes out $1\ cm^3$ of $CO_2$, the volume doesn't change unless you remove the $CO_2$.

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Answer: It absorbs the carbon dioxide produced by the woodlice, so any change in gas volume is entirely due to the consumption of oxygen.

Required practical 9. In a respirometer, volume changes move a fluid bubble. For aerobic respiration, volume decreases only if the $CO_2$ exhaled is absorbed by soda lime/KOH, leaving the $O_2$ consumed as the only factor changing the volume.

QUESTION 4 · RP9 · LEVEL 2

During an experiment investigating yeast respiration with TTC or methylene blue, the test tubes are placed in a water bath and left for $5$ minutes before the indicator is added. Why is this necessary?

To allow the yeast suspension to equilibrate and reach the exact temperature of the water bath before the timed reaction starts.
To allow the yeast time to completely consume any dissolved oxygen in the water.
To denature any competitive inhibitors present in the yeast cells.
To give the yeast cells time to settle at the bottom of the tube for a clear reading.

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If you add the indicator immediately, the reaction will start while the liquid is still warming up.

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Answer: To allow the yeast suspension to equilibrate and reach the exact temperature of the water bath before the timed reaction starts.

Required practical 9. Equilibration is essential to ensure that the independent variable (temperature) is acting accurately from time zero.

QUESTION 5 · RP9 · LEVEL 2

When investigating anaerobic respiration in yeast, the suspension is covered with a layer of liquid paraffin (oil). What is the exact biological reasoning for this?

To act as a barrier to prevent oxygen from diffusing into the solution from the air, ensuring the yeast is forced to respire anaerobically.
To trap the carbon dioxide so it dissolves into the water and forms a weak acid.
To act as an alternative lipid food source for the yeast once the glucose runs out.
To keep the yeast cells physically pushed down to the bottom of the tube.

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Anaerobic means 'without oxygen'. Oxygen is abundant in the atmosphere above the tube.

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Answer: To act as a barrier to prevent oxygen from diffusing into the solution from the air, ensuring the yeast is forced to respire anaerobically.

Required practical 9. The oil layer blocks gas exchange with the atmosphere. It prevents $O_2$ from entering the suspension, creating an anaerobic environment.

QUESTION 6 · RP9 · LEVEL 3

When investigating the rate of respiration using a redox indicator like methylene blue or TTC, what does the indicator measure?

It acts as an artificial electron/hydrogen acceptor during the oxidation of respiratory substrates, changing colour as it is reduced.
It measures the exact volume of carbon dioxide produced by turning cloudy.
It acts as a pH indicator to measure the acidity of the lactic acid produced.
It physically binds to the ATP molecules produced, giving them a visible colour.

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Respiration involves removing hydrogen atoms (oxidation) from glucose. This indicator steals them.

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Answer: It acts as an artificial electron/hydrogen acceptor during the oxidation of respiratory substrates, changing colour as it is reduced.

Required practical 9. Dehydrogenase enzymes remove hydrogen (electrons + protons) from respiratory substrates. Redox indicators accept these hydrogens instead of NAD/FAD, becoming reduced and changing colour (e.g., methylene blue goes from blue to colourless).

QUESTION 7 · RP9 · LEVEL 3

If a student uses a respirometer to measure the rate of aerobic respiration in yeast, why must they place a chemical like soda lime or potassium hydroxide inside the apparatus?

To absorb the carbon dioxide produced by the respiring yeast, ensuring that any change in gas volume is solely due to the uptake of oxygen.
To absorb the toxic ethanol produced by the yeast to prevent them from dying.
To act as a catalyst for the Krebs cycle.
To provide a constant supply of oxygen to the sealed container.

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For every molecule of $O_2$ used, a molecule of $CO_2$ is usually produced. If you don't remove the $CO_2$, the overall volume of gas won't change.

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Answer: To absorb the carbon dioxide produced by the respiring yeast, ensuring that any change in gas volume is solely due to the uptake of oxygen.

Required practical 9 / AT f. Soda lime absorbs $CO_2$. Therefore, as the organisms consume $O_2$, the total volume of gas in the sealed system decreases, pulling the liquid in the manometer tube towards the organisms.

QUESTION 8 · RP9 · LEVEL 4

In a respirometer setup containing maggots and soda lime, a drop of coloured liquid in the capillary tube moves towards the maggots. How can the student use this movement to calculate the rate of respiration?

Measure the distance the drop moves over a set time, use the internal radius of the capillary tube to calculate the volume of oxygen consumed ($\pi r^2 \times distance$), and divide by the time.
Multiply the distance the drop moves by the total mass of the maggots.
Simply divide the total time taken by the length of the capillary tube.
Measure the change in mass of the soda lime and divide by the distance the drop moved.

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The tube is a cylinder. The volume of a cylinder is Area of base $\times$ length.

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Answer: Measure the distance the drop moves over a set time, use the internal radius of the capillary tube to calculate the volume of oxygen consumed ($\pi r^2 \times distance$), and divide by the time.

AT f: Use of a respirometer. Rate = Volume / Time. The volume of the cylindrical tube is calculated using $\pi r^2 h$ (where $h$ is the distance the drop moved).

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RP9 Respiration in single-celled organisms

Sample questions

When investigating the rate of anaerobic respiration in yeast, a layer of liquid paraffin (oil) is placed over the surface of the yeast suspension. What is its purpose?

In an experiment investigating respiration rates using methylene blue, the yeast suspension is placed in a water bath for 5 minutes *before* adding the indicator. Why?

When using a respirometer to measure the oxygen uptake of woodlice, soda lime (or potassium hydroxide) is placed in the bottom of the tube. What is its function?

During an experiment investigating yeast respiration with TTC or methylene blue, the test tubes are placed in a water bath and left for $5$ minutes before the indicator is added. Why is this necessary?

When investigating anaerobic respiration in yeast, the suspension is covered with a layer of liquid paraffin (oil). What is the exact biological reasoning for this?

When investigating the rate of respiration using a redox indicator like methylene blue or TTC, what does the indicator measure?

If a student uses a respirometer to measure the rate of aerobic respiration in yeast, why must they place a chemical like soda lime or potassium hydroxide inside the apparatus?

In a respirometer setup containing maggots and soda lime, a drop of coloured liquid in the capillary tube moves towards the maggots. How can the student use this movement to calculate the rate of respiration?

Practise this topic

In an experiment investigating respiration rates using methylene blue, the yeast suspension is placed in a water bath for 5 minutes before adding the indicator. Why?