Anaerobic respirationEdexcel A-Level Biology B: Revision notes

Anaerobic respiration is the partial breakdown of a hexose sugar such as glucose without oxygen, giving a limited yield of ATP. It occurs in the cytoplasm and consists of glycolysis followed by conversion of pyruvate to either lactate or ethanol.

Glycolysis turns one glucose (6C) into two pyruvate (3C), with a net gain of 2 ATP (2 ATP are used and 4 made) and 2 reduced NAD.

In aerobic respiration the pyruvate goes on to the link reaction, Krebs cycle and oxidative phosphorylation in the mitochondria, giving about 30 ATP per glucose. Anaerobic respiration therefore yields roughly 15 times less ATP, because pyruvate, lactate and ethanol still contain much chemical energy and without oxygen as the final electron acceptor the electron transport chain stops.

Glycolysis needs NAD to accept hydrogen. In aerobic respiration reduced NAD is reoxidised at the electron transport chain, which needs oxygen.

Without oxygen reduced NAD would build up and glycolysis would stop. Anaerobic pathways solve this by passing the hydrogen from reduced NAD to a product of glycolysis. This oxidises reduced NAD to NAD, so glycolysis can carry on and keep making a small amount of ATP.

In mammals pyruvate accepts hydrogen from reduced NAD and becomes lactate, catalysed by lactate dehydrogenase. The reaction is reversible.

Lactate and the hydrogen ions it releases lower the pH of muscle cells. The change in pH alters the tertiary structure of enzymes, slowing glycolysis and ATP production, and interferes with the actin-myosin interaction. Fewer cross-bridges form, so contraction is weaker and the muscle fatigues; cramp may occur.

After exercise lactate is carried by the blood to the liver, where it is converted back to pyruvate and then glucose or glycogen, or oxidised aerobically. The extra oxygen needed is the oxygen debt, which is why breathing stays fast after exercise.

In plants (and yeast) pyruvate is decarboxylated to ethanal, releasing carbon dioxide. Ethanal then accepts hydrogen from reduced NAD and becomes ethanol, which regenerates NAD.

This happens in waterlogged roots and in seeds with a coat that restricts oxygen. The conversion to ethanol is irreversible, and ethanol is toxic at high concentrations, so plants cannot rely on it for long.

A respirometer measures oxygen uptake by living organisms such as germinating seeds, woodlice or yeast. The organisms sit in a sealed tube above potassium hydroxide (or soda lime), which absorbs the carbon dioxide produced, so gas volume falls only because oxygen is used. A coloured liquid droplet in a capillary tube moves towards the organisms.

A control tube holding glass beads or dead organisms of equal volume corrects for temperature and pressure changes. Equilibrate in a water bath, then time the droplet.

Rate = volume ÷ time, where volume = πr² × distance moved. Example: r = 0.50 mm, distance 24 mm in 6.0 min gives 18.8 mm³ ÷ 6.0 = 3.14 mm³ min⁻¹.

Vary one factor, such as temperature or substrate, and control the rest. For safe and ethical use of organisms, handle them gently, keep them out of the potassium hydroxide, return animals to their habitat afterwards and avoid overheating them. Use gloves and eye protection for the caustic solution.