Glycolysis, the link reaction, the Krebs cycle, oxidative phosphorylation, and anaerobic respiration.

What this dot point is asking

WJEC wants you to describe the four stages of aerobic respiration, where each occurs and what each produces, and to explain anaerobic respiration in animals and in yeast.

Aerobic respiration

In glycolysis, glucose is first phosphorylated using $2$ ATP, then split and oxidised to two pyruvate, yielding $4$ ATP (a net $2$) and $2$ reduced NAD. Pyruvate enters the matrix for the link reaction, losing a carbon as $\text{CO}_2$ and joining coenzyme A. The Krebs cycle then fully oxidises the acetyl group, releasing $\text{CO}_2$ and loading the coenzymes NAD and FAD with hydrogen.

In oxidative phosphorylation, electrons from reduced NAD and FAD pass along the electron transport chain, releasing energy that pumps protons into the intermembrane space to create a gradient. Protons flow back through ATP synthase, making ATP by chemiosmosis. Oxygen is the final electron acceptor, combining with electrons and protons to form water, which keeps the chain clear so it can continue.

Anaerobic respiration

Anaerobic respiration yields far less ATP (a net $2$ per glucose) than aerobic respiration because the link reaction, Krebs cycle and electron transport chain do not run. In animals the lactate is later oxidised back to pyruvate in the liver when oxygen returns, which is the basis of the oxygen debt repaid after exercise.

Examples in context

Example 1. Muscle fatigue in a sprint. During a sprint, muscles outrun their oxygen supply and respire anaerobically, producing lactate. The lactate lowers pH and contributes to fatigue; afterwards, fast breathing repays the oxygen debt by oxidising the lactate. This is the standard WJEC application of anaerobic respiration in animals.

Example 2. Brewing and baking with yeast. Yeast respires anaerobically to make ethanol (used in brewing) and carbon dioxide (which makes bread rise). The same pathway, run for its products rather than its ATP, underpins two major food industries and is a favourite exam context for anaerobic respiration in microbes.

Try this

Q1. State where the Krebs cycle takes place. [1 mark]

• Cue. The matrix of the mitochondrion.

Q2. Explain why anaerobic respiration produces much less ATP than aerobic respiration. [2 marks]

• Cue. Only glycolysis makes ATP; the Krebs cycle and oxidative phosphorylation, which make most of the ATP, do not run without oxygen.

Q3. Aerobic respiration yields about $38$ ATP and anaerobic $2$ ATP per glucose. Calculate the anaerobic yield as a percentage of the aerobic yield. [2 marks]

• Cue. $\frac{2}{38} \times 100 = 5.3\%$ (about $5\%$).