How do cells release the energy stored in glucose to make ATP?
Aerobic respiration as four stages: glycolysis in the cytoplasm (phosphorylation of glucose, oxidation to pyruvate, net yield of ATP and reduced NAD); the link reaction and the Krebs cycle in the mitochondrial matrix (decarboxylation, dehydrogenation, production of reduced NAD, reduced FAD, ATP and carbon dioxide); oxidative phosphorylation on the inner mitochondrial membrane (the electron transport chain, chemiosmosis, ATP synthase and the role of oxygen as the final electron acceptor); anaerobic respiration in animals (lactate) and in microorganisms and plants (ethanol).
A focused answer to the AQA 3.5 dot point on respiration. Covers glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation by chemiosmosis, the role of oxygen as the final electron acceptor, and anaerobic respiration producing lactate or ethanol.
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What this dot point is asking
AQA wants you to set out the four stages of aerobic respiration in order, state where each happens and what it produces, explain ATP synthesis by chemiosmosis, and describe the two anaerobic pathways.
The answer
Respiration releases energy from organic molecules to synthesise ATP, the universal energy currency. Aerobic respiration has four stages.
Stage 1: Glycolysis (cytoplasm)
Glycolysis happens in the cytoplasm and does not need oxygen.
- Phosphorylation. Glucose (6C) is phosphorylated using 2 ATP, forming a phosphorylated 6-carbon sugar. This makes it more reactive.
- Splitting and oxidation. The 6C sugar splits into two molecules of triose phosphate (3C), which are oxidised to pyruvate (3C). Hydrogen is removed and accepted by NAD, forming reduced NAD.
Per glucose, glycolysis yields 4 ATP (a net of 2 ATP after the 2 used), 2 reduced NAD and 2 pyruvate.
Stage 2: The link reaction (mitochondrial matrix)
Pyruvate is actively transported into the mitochondrial matrix, where for each pyruvate:
- Decarboxylation removes carbon dioxide.
- Dehydrogenation removes hydrogen, reducing NAD.
- The remaining 2-carbon acetyl group joins coenzyme A to form acetyl coenzyme A.
Per glucose (two pyruvate), the link reaction produces 2 acetyl CoA, 2 reduced NAD and 2 CO2.
Stage 3: The Krebs cycle (mitochondrial matrix)
Acetyl CoA delivers its 2-carbon acetyl group to a 4-carbon molecule, forming a 6-carbon compound. The cycle then regenerates the 4-carbon molecule through a series of decarboxylation and dehydrogenation steps. Per turn it produces, by substrate-level phosphorylation and dehydrogenation:
- 2 CO2 (decarboxylation)
- 3 reduced NAD and 1 reduced FAD (dehydrogenation)
- 1 ATP (substrate-level)
The cycle turns twice per glucose (once per acetyl CoA), so the yield doubles.
Stage 4: Oxidative phosphorylation (inner mitochondrial membrane)
This is where most ATP is made, by chemiosmosis on the inner mitochondrial membrane.
- Reduced NAD and reduced FAD are oxidised, donating electrons to the electron transport chain.
- Electrons pass along the carriers, releasing energy that pumps protons from the matrix into the intermembrane space, building an electrochemical gradient.
- Protons flow back into the matrix through ATP synthase, driving ATP synthesis (chemiosmosis).
- Oxygen is the final electron acceptor. It combines with electrons and protons to form water:
Without oxygen, the chain backs up, no more NAD or FAD is regenerated, and aerobic respiration stops. The theoretical maximum yield is about 38 ATP per glucose (often quoted as 30 to 32 in practice once transport costs are counted).
Anaerobic respiration
When oxygen is absent, only glycolysis continues, so cells must regenerate NAD to keep glycolysis going.
- In animals (lactate fermentation). Pyruvate accepts hydrogen from reduced NAD, forming lactate and regenerating NAD. Lactate is later oxidised back to pyruvate in the liver when oxygen returns.
- In plants and microorganisms (ethanol fermentation). Pyruvate is decarboxylated and then accepts hydrogen to form ethanol and carbon dioxide, regenerating NAD. This is irreversible.
Anaerobic respiration yields only the net 2 ATP from glycolysis, far less than aerobic respiration.
Examples in context
Example 1. Yeast in brewing and bread. Yeast respire anaerobically by ethanol fermentation, producing ethanol and carbon dioxide. Brewers exploit the ethanol; bakers exploit the carbon dioxide, which raises dough. The same pathway regenerates NAD so glycolysis, and a slow ATP supply, can continue without oxygen.
Example 2. Muscle fatigue in a sprinter. During a 400 metre sprint, oxygen delivery cannot keep pace with demand, so muscle cells respire anaerobically, producing lactate. The lactate is transported to the liver and oxidised back to pyruvate once oxygen is available, which contributes to the raised breathing rate (the oxygen debt) after the race.
Try this
Q1. State the precise location of each of the four stages of aerobic respiration. [2 marks]
- Cue. Glycolysis: cytoplasm. Link reaction and Krebs cycle: mitochondrial matrix. Oxidative phosphorylation: inner mitochondrial membrane.
Q2. Explain the role of oxygen in oxidative phosphorylation, and predict what happens if oxygen is absent. [3 marks]
- Cue. Oxygen is the final electron acceptor, combining with electrons and protons to form water; without it the electron transport chain stops, reduced NAD and FAD cannot be reoxidised, so no further ATP is made by chemiosmosis and the cell switches to anaerobic respiration.
Q3. Compare the ATP yield of aerobic and anaerobic respiration and explain the difference. [3 marks]
- Cue. Aerobic yields about 38 ATP per glucose; anaerobic yields only the net 2 ATP from glycolysis. The difference is because anaerobic respiration does not use the Krebs cycle or oxidative phosphorylation, which produce most of the ATP.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2017 AQA5 marksDescribe the role of the electron transport chain in the synthesis of ATP during oxidative phosphorylation.Show worked answer →
A 5-mark answer needs chemiosmosis spelled out.
- Reduced NAD and reduced FAD are oxidised, releasing electrons to the chain.
- Electrons pass along the carriers, releasing energy at each transfer.
- This energy is used to pump protons (H plus) from the matrix into the intermembrane space, forming an electrochemical gradient.
- Protons flow back into the matrix through ATP synthase, and this drives the synthesis of ATP (chemiosmosis).
- Oxygen is the final electron acceptor, combining with electrons and protons to form water, which keeps the chain running.
Markers reward proton pumping, ATP synthase, chemiosmosis and oxygen as the final acceptor.
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