Exam questions · Biology · Respiration
Core Practical: Investigating the Rate of Respiration
- 8 exam questions
- 26 marks
- 8 quick checks
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1 Explain [2 marks]
Explain why soda lime is placed in the tube of a respirometer.
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Model answer
The soda lime absorbs the carbon dioxide that the organisms make. This means that the only change in the volume of gas is the oxygen that is used up, so the movement of the drop shows the oxygen consumption.
Mark scheme
- It absorbs the carbon dioxide — 1 mark
- So only the oxygen used changes the volume of gas — 1 mark
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2 Name [3 marks]
The diagram shows a respirometer. Name the parts labelled A, B and C.
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Model answer
A is the (drop of) coloured liquid. B is the soda lime. C is the germinating seeds.
Mark scheme
- A: coloured liquid (drop) — 1 mark
- B: soda lime — 1 mark
- C: (germinating) seeds — 1 mark
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3 Explain [3 marks]
In a respirometer the drop of coloured liquid moves towards the tube containing the organisms. Explain why.
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Model answer
The organisms use oxygen in aerobic respiration. The carbon dioxide they make is absorbed by the soda lime. The volume of gas in the tube therefore falls, which lowers the pressure and pulls the drop of liquid towards the tube.
Mark scheme
- Oxygen is used by respiration — 1 mark
- The carbon dioxide is absorbed by the soda lime — 1 mark
- The volume (pressure) of gas falls, so the drop is drawn towards the tube — 1 mark
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4 Explain [2 marks]
A second tube is set up in the same way but contains glass beads instead of seeds. Explain the purpose of this control tube.
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Model answer
It shows what happens when there is no respiration. If the drop in the control tube does not move, the movement of the drop in the seed tube is caused by respiration. If it does move, the cause is a change in temperature or air pressure, and the results can be corrected.
Mark scheme
- It contains no living organisms, so there is no respiration — 1 mark
- It shows that movement is caused by respiration, not by changes in temperature or pressure — 1 mark
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5 Calculate [3 marks]
A student measured the distance moved by the drop at 30 degrees Celsius in 5 minutes three times: 22 mm, 26 mm and 24 mm. Calculate the mean rate of movement of the drop in mm per minute.
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Model answer
Mean distance = \((22 + 26 + 24) \div 3 = 24\) mm. Rate = \(24 \div 5 = 4.8\) mm per minute.
Mark scheme
- Mean distance \(= 24\) mm — 1 mark
- Divide by the time, \(24 \div 5\) — 1 mark
- 4.8 mm per minute (with the unit) — 1 mark
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6 Calculate [3 marks]
The capillary tube in the respirometer has an internal radius of 0.5 mm. The drop moved 24 mm in 5 minutes. Calculate the volume of oxygen used per minute. Use volume = \(\pi r^2 h\). Give your answer to 2 significant figures.
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Model answer
Volume in 5 minutes = \(\pi \times 0.5^2 \times 24 = 18.85\) mm³. Volume per minute = \(18.85 \div 5 = 3.77\), which is 3.8 mm³ per minute.
Mark scheme
- \(\pi \times 0.5^2 \times 24 = 18.85\) mm³ — 1 mark
- Divide by 5 minutes — 1 mark
- 3.8 mm³ per minute — 1 mark
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7 Explain [4 marks]
The graph shows the distance moved by the drop in a respirometer containing germinating seeds at five temperatures. (a) Describe the effect of increasing the temperature from 10 degrees Celsius to 40 degrees Celsius. (2) (b) Explain why the distance moved is less at 50 degrees Celsius than at 40 degrees Celsius. (2)
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Model answer
(a) The distance moved increases from 5 mm to 28 mm, so the rate of respiration increases as the temperature rises. (b) At 50 degrees Celsius the enzymes that control respiration are denatured, because their shape changes. The seeds respire more slowly and use less oxygen, so the drop moves less far.
Mark scheme
- (a) Distance (rate) increases as temperature increases — 1 mark
- (a) Data quoted, such as 5 mm to 28 mm — 1 mark
- (b) Enzymes are denatured at 50 degrees Celsius — 1 mark
- (b) Less respiration, so less oxygen is used and the drop moves less — 1 mark
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8 Describe [6 marks]
Describe how you would use a respirometer to investigate the effect of temperature on the rate of respiration of germinating seeds. Include one safety precaution in your answer.
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Model answer
Put soda lime in the bottom of a boiling tube, add cotton wool and a measured mass of germinating seeds, and fit a bung with a capillary tube so that it is airtight. Set up a control tube with glass beads instead of seeds. Place the tubes in a water bath at the first temperature, for example 10 degrees Celsius, and leave them for five minutes. Add a drop of coloured liquid to the capillary tube and record its position. After a set time, such as five minutes, record the position again and calculate the distance moved. Repeat at 20, 30 and 40 degrees Celsius, keeping the mass of seeds, the apparatus and the time the same, and repeat each temperature to calculate a mean. Wear eye protection and gloves because soda lime is corrosive.
Mark scheme
- Soda lime, cotton wool and a measured mass of seeds in a tube with a capillary tube and a drop of coloured liquid — 1 mark
- A control tube with glass beads or boiled seeds — 1 mark
- Tubes placed in a water bath at a measured temperature and left to reach it — 1 mark
- Distance moved by the drop in a set time is measured — 1 mark
- Repeat at several temperatures with the other variables controlled, and calculate means — 1 mark
- A safety precaution, such as eye protection because soda lime is corrosive — 1 mark
Quick check
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1
What is the purpose of the soda lime in a respirometer?
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C: To absorb the carbon dioxide
Soda lime absorbs carbon dioxide, so only the oxygen used changes the volume of gas.
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2
In which direction does the coloured drop move as the organisms respire?
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A: Towards the tube with the organisms
Oxygen is used up and the carbon dioxide is absorbed, so the gas volume falls and the drop is drawn towards the tube.
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3
What is the dependent variable in the temperature experiment?
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D: The distance moved by the drop
The temperature is the independent variable. The distance the drop moves in a set time is what is measured.
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4
What should the control tube contain?
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B: Glass beads or boiled seeds
Glass beads or boiled seeds cannot respire, so any movement shows a change in temperature or pressure.
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5
Why are the tubes left in the water bath for several minutes before starting?
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A: So that the gas inside reaches the water temperature
The gas in the tube has to reach the temperature of the water, otherwise it changes volume and moves the drop.
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6
Why does the distance moved fall at a very high temperature such as 50 degrees Celsius?
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C: The enzymes are denatured
Enzymes that control respiration are denatured at high temperatures, so respiration slows down.
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7
Which is the main safety hazard of soda lime?
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D: It is corrosive
Soda lime is corrosive, so eye protection and gloves are worn.
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8
A drop moved 30 mm in 5 minutes. What is the rate of movement?
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B: 6 mm per minute
Rate is distance divided by time, so 30 divided by 5 gives 6 mm per minute.