Anaerobic respiration, RQ and rice
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The teaching part (2 min) is free to watch. A free account opens the whole lecture: 1 worked example, 3 practice questions, 1 real past-paper question with the answer explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- When the oxygen runs out
- Why fermentation is needed
- Lactate and ethanol fermentation
- A past-paper example
- A past-paper example
Read the transcript of the teaching part
Last time
Before we start: last time, in Respirometers and redox indicators, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
When the oxygen runs out
A sprinter, a diving seal and a rice plant with its roots under water all share one problem. Oxygen is short, but they still need A T P. Each has a different way of coping, and the respiratory quotient lets us tell what fuel they are burning. Together these make a favourite exam theme.
Why fermentation is needed
Without oxygen, the electron transport chain stops, so reduced N A D cannot give up its hydrogen. All the N A D becomes reduced. Glycolysis needs oxidised N A D to continue. Fermentation regenerates it, by passing the hydrogen to pyruvate or a product of pyruvate. So glycolysis can go on making a small net gain of two A T P per glucose, even with no oxygen.
Lactate and ethanol fermentation
In mammals, pyruvate accepts hydrogen from reduced N A D directly, using the enzyme lactate dehydrogenase, and becomes lactate. N A D is regenerated. In yeast and some plants, pyruvate is first decarboxylated to ethanal, releasing carbon dioxide. Ethanal accepts hydrogen from reduced N A D and becomes ethanol. Lactate fermentation is reversible, and the lactate can be converted back to pyruvate in the liver when oxygen is available. Ethanol fermentation is not reversible.
A past-paper example
October November 2023, Paper 42, question 1. Fig 1.1 outlines ethanol fermentation and lactate fermentation. Explain how they allow respiration to continue without oxygen. Both processes oxidise reduced N A D back to N A D. So N A D is available for glycolysis to carry on, and a little A T P continues to be made. In ethanol fermentation, carbon dioxide is also released.
A past-paper example
May June 2025, Paper 42, question 10 part a. A grey seal's blood lactate was measured before, during and after a dive. Lactate rises during a long dive because the muscles run short of oxygen and use lactate fermentation. After the dive, lactate falls as the liver converts it back to pyruvate, using oxygen. This is the oxygen debt being repaid.
How often it comes up
The whole chapter: Energy and Respiration.
Chapter 12: Energy and respiration
- ATP, coenzymes and glycolysis 6 min
- The link reaction, Krebs cycle and oxidative phosphorylation 6 min
- Anaerobic respiration, RQ and rice 6 min
- Respirometers and redox indicators 5 min
Also for A Level Biology 9700
Which topics come up most in A Level Biology 9700 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency, Paper 4 topic frequency.