Skills applied: CP17 spirometer traces, CP18 amylase in cereal grains

CP17: reading a spirometer trace

Method recap (see Core practical 17, Unit 5): a volunteer breathes from a chamber of medical oxygen through soda lime (absorbs CO₂); the trace records volume against time.

• Tidal volume: height of one normal breath (peak to trough).
• Breathing rate: number of peaks per minute.
• Minute ventilation = tidal volume x breathing rate.
• Vital capacity: from the deepest breath in to the deepest breath out.
• Oxygen consumption: the trace drifts downwards as O₂ is used (CO₂ is absorbed); oxygen consumption rate = fall in the trace ÷ time.

After exercise, tidal volume, breathing rate and oxygen consumption rise.

Ethics and safety: informed consent, medical screening, sterile mouthpiece, fresh soda lime, nose clip, stop if dizzy.

CP18: amylase in germinating grains

Method recap (see Core practical 18, Unit 5): barley grains are cut in half (embryo halves removed) and soaked in different gibberellin concentrations, then placed cut side down on starch agar. After incubation, flood with iodine: clear zones show where amylase has digested starch.

• Area of clear zone = πr² (mean of two diameters).
• Sterilise grain surfaces (dilute bleach) and use aseptic technique, because microorganisms also make amylase.
• Controls: half-grains with no gibberellin; boiled grains.
• Analysis: scatter graph of mean zone area against gibberellin concentration and Spearman's rank.

Exam angles

• Read values from a trace with correct units (dm³ or cm³, per minute).
• Explain why the trace falls (oxygen used; CO₂ absorbed by soda lime).
• Explain why embryo halves are removed in CP18 (the embryo makes its own gibberellin).

Worked example

A spirometer trace shows breaths of 0.5 dm³ with 15 breaths per minute, and the trace falls by 0.9 dm³ over 3 minutes. Calculate minute ventilation and oxygen consumption.

1. Ventilation = 0.5 x 15 = 7.5 dm³ min⁻¹
2. O₂ consumption = 0.9 ÷ 3 = 0.3 dm³ min⁻¹

Answer: 7.5 dm³ min⁻¹ and 0.3 dm³ min⁻¹

Check yourself

1. Tidal volume is 0.65 dm³ and breathing rate is 18 breaths per minute. Calculate minute ventilation.
   Show answer

   11.7 dm³ min⁻¹. 0.65 x 18 = 11.7 dm³ min⁻¹.

2. A trace shows 7 breaths in 30 s. Calculate the breathing rate per minute.
   Show answer

   14 breaths per minute. 7 x 2 = 14 breaths per minute.

3. A spirometer trace falls by 1.2 dm³ over 4 minutes. Calculate the rate of oxygen consumption in dm³ min⁻¹.
   Show answer

   0.3 dm³ min⁻¹. 1.2 / 4 = 0.3 dm³ min⁻¹.

4. A clear zone around a half-grain has diameters of 11 mm and 13 mm. Using the mean diameter, calculate the area in mm² to 3 significant figures.
   Show answer

   113 mm². Mean diameter 12 mm, r = 6 mm; π x 36 = 113 mm².

5. Why does a spirometer trace slope downwards over time?
   • The volunteer breathes out more than in
   • Oxygen is used by the volunteer and the CO₂ breathed out is absorbed by soda lime
   • The chamber leaks oxygen
   • Carbon dioxide builds up
   Show answer

   Oxygen is used by the volunteer and the CO₂ breathed out is absorbed by soda lime. Volume of gas in the chamber decreases.

6. Why are the embryo halves of barley grains discarded in CP18?
   • The embryo contains no starch
   • The embryo is toxic
   • The embryo would grow too quickly
   • The embryo produces its own gibberellin, which would affect the results
   Show answer

   The embryo produces its own gibberellin, which would affect the results. Only added gibberellin should vary.

7. What do clear zones on starch agar flooded with iodine show?
   • Gibberellin has been destroyed
   • Starch has been digested by amylase
   • Glucose has been made into starch
   • Bacteria have died
   Show answer

   Starch has been digested by amylase. No starch means no blue-black colour.

8. Why are barley grains surface-sterilised before CP18?
   • Microorganisms on the grains could produce amylase and digest starch
   • To kill the aleurone cells
   • To add gibberellin
   • To soften the grain
   Show answer

   Microorganisms on the grains could produce amylase and digest starch. Contaminants would add amylase activity.

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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.