Anaerobic respiration and respiratory substratesAQA A-Level Biology: Revision notes

When oxygen is not available, the electron transport chain stops, because oxygen is the final electron acceptor. Reduced NAD can no longer be oxidised there, so the link reaction and Krebs cycle also stop (they need NAD and FAD). Glycolysis, which takes place in the cytoplasm, does not need oxygen and can carry on as long as there is a supply of NAD.

If respiration is only anaerobic, the problem is therefore how to regenerate NAD from reduced NAD. The cell does this by converting pyruvate to ethanol or lactate.

In animal cells and some bacteria, pyruvate is reduced to lactate by the enzyme lactate dehydrogenase, using hydrogen from reduced NAD.

$\text{pyruvate} + \text{reduced NAD} \rightarrow \text{lactate} + \text{NAD}$

The NAD that is regenerated goes back to glycolysis, which makes a net 2 ATP per glucose by substrate-level phosphorylation (4 made, 2 used). Lactate builds up, lowers pH and contributes to muscle fatigue.

In yeast and plant roots in waterlogged soil, pyruvate is first decarboxylated to ethanal, releasing carbon dioxide. Ethanal is then reduced to ethanol by reduced NAD, which is oxidised to NAD.

$\text{pyruvate} \rightarrow \text{ethanal} + \text{CO}_2 \qquad \text{ethanal} + \text{reduced NAD} \rightarrow \text{ethanol} + \text{NAD}$

As with lactate, the yield is only the net 2 ATP from glycolysis, far less than the roughly 32 from aerobic respiration, because the energy still stored in lactate or ethanol is not released.

Glucose is not the only substrate. Other organic molecules can be broken down and their products fed into the Krebs cycle.

• Lipids are hydrolysed to fatty acids and glycerol. Fatty acids are broken down to acetyl coenzyme A, and glycerol is converted to a respiratory intermediate; both enter the Krebs cycle.
• Proteins are hydrolysed to amino acids. These are deaminated and the remaining carbon-containing part is converted to pyruvate, acetyl coenzyme A or a Krebs cycle intermediate, which enters the Krebs cycle.

The Krebs cycle then supplies reduced NAD and reduced FAD to oxidative phosphorylation, so ATP is made aerobically from these substrates.

Cultures of single-celled organisms, such as yeast, can be used to investigate a named variable such as temperature, substrate or concentration of glucose.

• Rate can be measured as volume of CO₂ released (gas syringe or collection over water), as oxygen taken up (respirometer), or by the time for a redox indicator such as methylene blue or DCPIP to decolourise.
• Independent variable: the named variable. Dependent variable: volume of gas per unit time, or time for colour change.
• Control: volume and concentration of glucose and yeast, pH, time, and temperature (except when it is the variable). Equilibrate in a water bath before mixing.
• Use aseptic technique, repeat and calculate a mean.

Worked example. 7.5 cm³ of CO₂ in 5.0 min from 0.50 g yeast: rate = 7.5 ÷ 5.0 = 1.5 cm³ min⁻¹, so 1.5 ÷ 0.50 = 3.0 cm³ min⁻¹ g⁻¹.