The stages of aerobic respiration (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation), the role of ATP and the mitochondrion, and anaerobic respiration in animals and yeast.

What this dot point is asking

Edexcel wants you to describe the four stages of aerobic respiration, explain the role of ATP and the mitochondrion, and describe anaerobic respiration in animals and yeast. Questions often centre on oxidative phosphorylation and on comparing aerobic with anaerobic ATP yields.

ATP and the mitochondrion

The stages of aerobic respiration

Taking the stages in turn helps you keep them straight:

• Glycolysis does not need oxygen. Glucose (6 carbons) is first phosphorylated using 2 ATP, then split and oxidised into two molecules of pyruvate (3 carbons each). This makes 4 ATP (so a net gain of 2) and reduces 2 NAD to reduced NAD.
• Link reaction. Each pyruvate enters the mitochondrial matrix and is decarboxylated (losing carbon dioxide) and oxidised (reducing NAD) to form a 2-carbon acetyl group, which joins coenzyme A to make acetyl coenzyme A.
• Krebs cycle. Acetyl coenzyme A combines with a 4-carbon compound to form a 6-carbon compound (citrate). Through the cycle, carbon dioxide is released, NAD and FAD are reduced and a little ATP is made by substrate-level phosphorylation. The cycle turns twice per glucose.
• Oxidative phosphorylation. The reduced NAD and reduced FAD donate electrons to the electron transport chain on the cristae; the energy released pumps protons to make a gradient, and protons flowing back through ATP synthase make most of the ATP. Oxygen accepts the spent electrons and protons to form water.

Anaerobic respiration

When oxygen is unavailable, only glycolysis can continue. To keep glycolysis going, the reduced NAD must be reoxidised.

• In animals: pyruvate is converted to lactate. This regenerates NAD but the lactate must later be broken down.
• In yeast and plants: pyruvate is converted to ethanol and carbon dioxide (fermentation).

Anaerobic respiration releases far less ATP than aerobic respiration (a net $2$ ATP from glycolysis, against roughly $38$ aerobically) because the Krebs cycle and oxidative phosphorylation do not run. The reduced NAD must be reoxidised by passing its hydrogen to pyruvate, otherwise glycolysis would halt for lack of NAD.

Examples in context

Example 1. The oxygen debt after sprinting. During a sprint, muscles respire anaerobically because oxygen cannot be delivered fast enough, producing lactate. After the sprint, the runner keeps breathing hard to take in extra oxygen (the oxygen debt or excess post-exercise oxygen consumption) to oxidise the lactate back to pyruvate, which is then respired aerobically or converted to glucose in the liver. This links respiration to the muscle and exercise topics.

Example 2. Yeast in brewing and baking. Yeast respiring anaerobically converts sugars to ethanol and carbon dioxide. Brewers use the ethanol; bakers use the carbon dioxide to make bread rise. The same fermentation pathway has two commercial uses, a common applied context, and contrasts with the lactate pathway in animals.

Try this

Q1. State where glycolysis takes place and one product it makes. [2 marks]

• Cue. In the cytoplasm; it produces pyruvate (and a net 2 ATP and reduced NAD).

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

• Cue. Only glycolysis runs; the Krebs cycle and oxidative phosphorylation, which produce most of the ATP, do not occur.