Delivery Guide ‒ Cellular respiration
Learner Resource 5 – Teacher answers
Respirometers
Below is a diagram of a respirometer – used to calculate the gas taken in and gas produced from a
living organism.
Control tube
How does it work?
The seeds in the tube carry out respiration. The oxygen levels will therefore decrease and the level
of CO₂ will increase. The potassium hydroxide solution absorbs any CO₂ produced so the pressure
within the tube falls. The rate at which the pressure falls is a measure of the rate at which the
respiring seeds are taking up oxygen.
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Purpose of the control tube
Two identical closed tubes are used. One contains the seeds under investigation and the other (the
control) either contains no seeds or dead seeds. This is necessary because temperature changes
alter the pressures of the gases. Any differences in the two vessels can be attributed to gas
exchange by the living seeds.
The apparatus was set up and left. After 30 minutes the level in the right arm of the U-tube had
risen by 11 units.
The experiment was repeated with water in the bottom of the boiling tube, instead of the potassium
hydroxide solution. This time the fluid in the right arm of the U-tube rose by 2 units after 30 minutes.
Answer the following questions:1.
Why was KOH solution placed in the bottom of the tubes?
To absorb carbon dioxide
2.
What is the function of the syringe?
It allows the levels in the U tube to be reset ready for fresh readings
3.
Why is the screw clip left open for a short time when the apparatus is assembled?
To allow equilibration
4.
How could the temperature of the apparatus be controlled and why is this important?
Water bath (or suggestions relating to minimising air temperature fluctuations in the lab).
This is important because temperature changes could affect the rate of respiration.
Also if one part of the apparatus warms or cools relative to another it will cause pressure
changes.
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5.
What was the oxygen consumption of the seeds in units per minute?
11 / 30 = 0.37 units per minute
6.
What was the production of CO₂ in units per minute?
(11-2)/30 = 9/30 = 0.30 units per minute
Respirometers can be used to calculate the respiratory quotient or RQ.
RQ = Volume of CO2 produced
Volume of O2 taken in
Calculate the RQ of the seeds in the experiment described.
0.30 / 0.37 = 0.81
RQ varies depending on which substrates are being used for cell respiration and whether aerobic or
anaerobic respiration is occurring.
What do the following tell us?
1.
R.Q. = 1
Could be a mixture of substrates
Could be purely carbohydrate substrate, aerobic respiration
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2.
R.Q. = more than 1
Could be some anaerobic respiration as well as aerobic
Could be all anaerobic respiration
Could be aerobic respiration but a very strange substrate combination
Could be aerobic respiration of carbohydrate combined with synthesis of fat (e.g. in preparation
for hibernation)
3.
R.Q. = 0.7
Could be a mixture of substrates
Could be purely fat/lipid substrate, aerobic respiration
4.
R.Q. = 0.9
Could be a mixture of substrates
Could be purely protein substrate, aerobic respiration
Why is the R.Q. for proteins variable?
Different amino acids (with different R groups) have different R.Q.s
What is the average RQ during rest for humans and why?
Fats are the predominant substrate at rest, with some contribution from carbohydrates, so RQ is
closer to 0.7 than 1.0
Average: 0.8
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