Skills applied: CP15 yeast respiration, CP16 respirometers and RQ

CP15: yeast and a redox indicator

Method recap (see Core practical 15, Unit 5): yeast suspension with glucose and a redox indicator such as methylene blue or TTC, in a water bath. Dehydrogenase enzymes remove hydrogen from respiratory substrates, which reduces the indicator: methylene blue changes from blue to colourless (TTC from colourless to red).

• Rate = 1 / time to decolourise (often x 1000 for convenient numbers).
• Controls: same yeast concentration (stirred suspension), same glucose and indicator volumes, equilibrate in the water bath, compare with a colour standard (boiled yeast with indicator), do not shake (shaking re-oxidises the indicator).
• IV examples: temperature, substrate type (glucose, sucrose, lactose).

CP16: respirometer

Method recap (see Core practical 16, Unit 5): organisms (germinating seeds, maggots) in a sealed tube with potassium hydroxide or soda lime to absorb CO₂. As O₂ is used, the fluid in the capillary moves towards the organisms.

• Volume of O₂ = πr²l.
• A control tube with glass beads of equal volume compensates for temperature and pressure changes.
• Equilibrate in a water bath before closing the tap.

RQ = CO₂ produced ÷ O₂ consumed: run once with KOH (gives O₂ uptake) and once with water in place of KOH (movement = O₂ uptake - CO₂ output).

RQ about 1.0 for carbohydrates, 0.7 for lipids, 0.9 for protein.

Exam angles

• Why wait before starting timing (thermal equilibrium).
• Why the fluid moves towards the seeds.
• Calculate rates per gram of seeds per minute.
• Explain a higher RQ in a seed using carbohydrate, or tetraploid versus diploid seeds (the January 2024 Unit 6 paper).

Worked example

With KOH, the fluid moves 30 mm in 10 minutes. With water instead of KOH, it moves 6 mm towards the seeds in the same time. Calculate RQ.

1. O₂ uptake ∝ 30 mm
2. With water, net decrease = O₂ - CO₂ = 6 mm, so CO₂ ∝ 30 - 6 = 24 mm
3. RQ = 24 / 30 = 0.8

Answer: RQ = 0.8 (the same capillary, so distances are proportional to volumes)

Check yourself

1. Methylene blue decolourised in 125 s. Calculate the rate as 1000 / time.
   Show answer

   8.0. 1000 / 125 = 8.0, so the rate is 8.0 x 10⁻³ s⁻¹ (the x 1000 just gives a convenient number).

2. In a respirometer with a capillary of radius 0.5 mm, the fluid moves 24 mm in 10 minutes. Calculate the rate of oxygen uptake in mm³ min⁻¹ to 3 significant figures.
   Show answer

   1.88 mm³ min⁻¹. π x 0.25 x 24 = 18.85 mm³; ÷ 10 = 1.88 mm³ min⁻¹.

3. Seeds of mass 4.0 g take up 30 mm³ of oxygen in 5 minutes. Calculate the rate per gram per minute.
   Show answer

   1.5 mm³ g⁻¹ min⁻¹. 30 / 4.0 / 5 = 1.5 mm³ g⁻¹ min⁻¹.

4. A seed takes up 25 cm³ of O₂ and gives out 18 cm³ of CO₂. Calculate RQ to 2 decimal places.
   Show answer

   0.72. 18 / 25 = 0.72.

5. An RQ of 0.72 suggests which respiratory substrate?
   • Carbohydrate
   • Protein only
   • Lipid
   • No substrate
   Show answer

   Lipid. Lipids give an RQ near 0.7.

6. What is the role of potassium hydroxide in the respirometer?
   • To absorb carbon dioxide so fluid movement shows oxygen uptake
   • To supply oxygen
   • To kill the seeds
   • To keep the temperature constant
   Show answer

   To absorb carbon dioxide so fluid movement shows oxygen uptake. With CO₂ removed, volume falls as O₂ is used.

7. Why should tubes of methylene blue and yeast not be shaken during CP15?
   • Shaking kills the yeast
   • Shaking warms the tube
   • Shaking adds oxygen, which re-oxidises the indicator back to blue
   • Shaking makes the yeast settle
   Show answer

   Shaking adds oxygen, which re-oxidises the indicator back to blue. Oxygen reverses the colour change.

8. What is the purpose of the control tube with glass beads?
   • To absorb oxygen
   • To provide extra respiration
   • To measure carbon dioxide
   • To compensate for changes in temperature and pressure that would move the fluid
   Show answer

   To compensate for changes in temperature and pressure that would move the fluid. Equal volume, no respiration.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018): Unit 3, Unit 6 and Appendix 6 mathematical skills
• Pearson Examiners' Report WBI16/01, January 2024

Last updated 8 November 2026. Spotted a mistake? Tell us.