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A2 · Practice questions · Anaerobic respiration, rice and measuring respiration rate

Where does the hydrogen go?

Six original Cambridge-style questions on anaerobic respiration, rice adaptations and planning and analysing investigations into the rate of respiration.

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

Regenerate NAD, measure the rate.

01
Recall
[3 marks]

Outline lactate fermentation in a mammalian muscle cell.

  • Glucose is converted to pyruvate in glycolysis, producing 2 ATP and reduced NAD. ✓
  • Pyruvate accepts hydrogen from reduced NAD and is reduced to lactate, catalysed by lactate dehydrogenase. ✓
  • This regenerates NAD, so glycolysis can continue; lactate is later taken to the liver and converted back. ✓
02
Explain
[3 marks]

Describe how yeast regenerates NAD when respiring in anaerobic conditions.

  • Pyruvate is decarboxylated to ethanal, releasing carbon dioxide (pyruvate decarboxylase). ✓
  • Ethanal accepts hydrogen from reduced NAD and is reduced to ethanol (alcohol dehydrogenase). ✓
  • The NAD released can accept hydrogen in glycolysis again, so glycolysis continues to make 2 ATP per glucose. ✓
03
Explain
[3 marks]

Explain why the energy yield from respiration in aerobic conditions is much greater than in anaerobic conditions.

  • In anaerobic conditions only glycolysis produces ATP: a net 2 ATP per glucose. ✓
  • In aerobic conditions pyruvate is completely oxidised in the link reaction and Krebs cycle, producing many reduced NAD and FAD. ✓
  • These drive oxidative phosphorylation, which makes most of the ATP; anaerobically most of the energy stays in lactate or ethanol. ✓
04
Explain
[3 marks]

Explain how rice is adapted to grow with its roots submerged in water.

  • Aerenchyma in the stems and roots has large air spaces through which oxygen diffuses down to the roots. ✓
  • Root cells carry out ethanol fermentation, and their high levels of alcohol dehydrogenase stop toxic ethanal and ethanol building up, so they tolerate ethanol. ✓
  • Stems grow faster when flooded, keeping leaves above the water for gas exchange and photosynthesis. ✓
05
Calculation
[3 marks]

Yeast suspensions with methylene blue were kept at different temperatures. Times to decolourise: 15 °C 480 s; 25 °C 240 s; 35 °C 125 s; 45 °C 150 s. Calculate the rate (1000 ÷ time, in s−1) at each temperature and describe the trend.

  • Rates: 15 °C 2.1; 25 °C 4.2; 35 °C 8.0; 45 °C 6.7 (all × 10−3 s−1). ✓
  • The rate roughly doubles for each 10 °C rise from 15 to 35 °C, as enzymes and substrates gain kinetic energy and collide more often. ✓
  • The rate falls at 45 °C, as dehydrogenase enzymes begin to denature. ✓
06
Analysis
[4 marks]

Plan an investigation, using a simple respirometer, into the effect of temperature on the rate of respiration of germinating seeds.

  • Place a known mass of germinating seeds on a gauze above potassium hydroxide (to absorb CO2) in a respirometer, with a control tube of glass beads of the same volume. ✓
  • Stand the apparatus in a thermostatically controlled water bath at one temperature, for example 10, 20, 30 and 40 °C, and leave it to equilibrate. ✓
  • Measure the distance moved by the coloured liquid in a fixed time; calculate volume of O2 = πr2h and the rate per gram per minute. ✓
  • Repeat three times at each temperature, calculate means and plot rate against temperature. ✓

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.