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A2 · Practice questions · Energy, ATP and respiratory quotients

Count the gases, name the fuel.

Six original Cambridge-style questions on ATP and respiratory substrates, including RQ calculations from equations and from respirometer readings.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Energy, currency, fuel.

01
Recall
[3 marks]

Outline why living organisms need energy. Give one example of each use.

  • Active transport: moving ions or molecules against a concentration gradient, for example the sodium-potassium pump. ✓
  • Movement: for example muscle contraction, or cilia and flagella beating. ✓
  • Anabolic reactions: building large molecules from small ones, for example protein synthesis or DNA replication. ✓
02
Explain
[4 marks]

Describe the features of ATP that make it suitable as the universal energy currency.

  • Hydrolysis of ATP releases energy in small, manageable amounts (about 30.5 kJ mol−1), so little is wasted. ✓
  • Hydrolysis is a single-step reaction catalysed by one enzyme, so energy is released quickly when needed. ✓
  • ATP is small and soluble, so it moves easily within a cell, but cannot leave the cell. ✓
  • ATP is quickly regenerated from ADP and phosphate, and the same molecule is used in all organisms. ✓
03
Explain
[3 marks]

Explain why lipids have a higher energy value per gram than carbohydrates when used as respiratory substrates.

  • Lipids (fatty acids) contain a greater proportion of hydrogen atoms per gram than carbohydrates. ✓
  • Most ATP is made in oxidative phosphorylation, which depends on hydrogen carried by reduced NAD and reduced FAD. ✓
  • More hydrogen gives more reduced coenzymes, more protons pumped and so more ATP per gram (and more oxygen is needed). ✓
04
Calculation
[2 marks]

Tripalmitin is respired according to the equation 2C51H98O6 + 145O2 → 102CO2 + 98H2O. Calculate the RQ and state what it indicates.

  • RQ = 102 ÷ 145 = 0.70. ✓
  • This is the typical RQ of a lipid, so lipid is the respiratory substrate. ✓
05
Data
[4 marks]

A respirometer contained germinating seeds. With potassium hydroxide in the tube, the liquid moved 24 mm in 10 minutes. With water instead, it moved 6 mm in the same direction. The capillary bore has a radius of 0.5 mm. Calculate the volume of oxygen taken in and the RQ, and suggest the substrate.

  • O2 uptake = πr2h = 3.14 × 0.52 × 24 = 18.8 mm3 in 10 minutes. ✓
  • CO2 produced corresponds to 24 − 6 = 18 mm of movement. ✓
  • RQ = 18 ÷ 24 = 0.75. ✓
  • This is close to 0.7, so the seeds are mainly respiring lipid (perhaps with some carbohydrate). ✓
06
Analysis
[3 marks]

Suggest how the reliability and validity of a respirometer experiment with blowfly larvae could be improved.

  • Place the respirometer in a thermostatically controlled water bath and allow time to equilibrate before taking readings. ✓
  • Use a control tube with glass beads of the same volume as the larvae to correct for changes in temperature or atmospheric pressure. ✓
  • Use the same mass of larvae of the same age, repeat at least three times and calculate a mean. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.