How is the chemical energy in glucose released and used to make ATP?
Respiration: glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation; the role of NAD and FAD; anaerobic respiration; and respiratory substrates.
A focused answer to the Eduqas Component 1 statement on respiration. Covers glycolysis, the link reaction, the Krebs cycle, oxidative phosphorylation and chemiosmosis, the role of NAD and FAD, anaerobic respiration, and respiratory substrates.
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What this dot point is asking
Eduqas wants you to describe the four stages of aerobic respiration, explain the role of NAD and FAD as coenzymes, explain chemiosmosis and oxidative phosphorylation, describe anaerobic respiration, and compare respiratory substrates. This is the core of Component 1's energy theme.
Glycolysis (in the cytoplasm)
Glycolysis does not need oxygen and occurs in the cytoplasm:
- Glucose is phosphorylated using 2 ATP, then split into two molecules of triose phosphate.
- Each triose phosphate is oxidised to pyruvate, producing 4 ATP (a net gain of 2 ATP) and 2 reduced NAD.
The link reaction and Krebs cycle (in the matrix)
In the link reaction, each pyruvate enters the mitochondrial matrix and is decarboxylated (losing carbon dioxide) and oxidised (reducing NAD), forming acetyl coenzyme A.
In the Krebs cycle, acetyl coenzyme A combines with a 4-carbon molecule to form a 6-carbon molecule (citrate). Through a series of reactions per turn this releases 2 carbon dioxide, 3 reduced NAD, 1 reduced FAD and 1 ATP (by substrate-level phosphorylation), regenerating the 4-carbon molecule. The cycle turns twice per glucose.
Oxidative phosphorylation (on the inner membrane)
The full aerobic yield is around 30 to 32 ATP per glucose.
Anaerobic respiration
Without oxygen, the electron transport chain stops, so reduced NAD cannot be reoxidised there. Only glycolysis continues, and NAD is regenerated by reducing pyruvate:
- In animals, pyruvate is reduced to lactate (lactate fermentation).
- In yeast, pyruvate is decarboxylated and reduced to ethanol and carbon dioxide.
This regenerates NAD so glycolysis can keep producing a net 2 ATP per glucose, far less than aerobic respiration.
Respiratory substrates
Glucose is the usual substrate, but lipids and proteins can also be respired. Lipids yield the most ATP per gram (they are highly reduced, providing many hydrogens for the electron transport chain), which is why they are good energy stores. The respiratory quotient () indicates which substrate is being used (about 1.0 for carbohydrate, 0.7 for lipid).
Examples in context
Example 1. Why muscles ache during intense exercise. When oxygen supply cannot meet demand, muscle respires anaerobically, producing lactate; this contributes to fatigue and must later be oxidised when oxygen is available (repaying the oxygen debt).
Example 2. Brewing and baking with yeast. Yeast respiring anaerobically produces ethanol (used in brewing) and carbon dioxide (which makes bread rise), a classic Eduqas application linking anaerobic respiration to microbiology.
Try this
Q1. State the net ATP yield and the products of glycolysis from one glucose molecule. [2 marks]
- Cue. A net 2 ATP, plus 2 pyruvate and 2 reduced NAD.
Q2. State the role of oxygen in aerobic respiration. [1 mark]
- Cue. It is the final electron acceptor at the end of the electron transport chain, forming water.
Q3. Explain why lipids release more energy per gram than carbohydrates when respired. [2 marks]
- Cue. Lipids are more reduced (have more hydrogen), so respiring them provides more reduced NAD and FAD to the electron transport chain, yielding more ATP.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20196 marksDescribe oxidative phosphorylation and explain how it produces most of the ATP in aerobic respiration.Show worked answer →
Reduced NAD and reduced FAD from the earlier stages are oxidised, releasing electrons (and protons) to the electron transport chain on the inner mitochondrial membrane.
The electrons pass along a series of carriers, releasing energy at each transfer; this energy is used to pump protons from the matrix into the intermembrane space, building up a proton gradient.
The protons flow back into the matrix through ATP synthase, and this flow (chemiosmosis) drives the synthesis of ATP from ADP and inorganic phosphate.
Oxygen is the final electron acceptor: it accepts electrons and protons to form water, keeping the chain running.
Markers reward electrons from reduced NAD/FAD passing along the chain, protons pumped to form a gradient, ATP made by chemiosmosis through ATP synthase, and oxygen as the final electron acceptor forming water.
Eduqas 20214 marksExplain why far more ATP is produced in aerobic respiration than in anaerobic respiration in a human muscle cell.Show worked answer →
In aerobic respiration, glucose is fully oxidised through glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation, producing many reduced NAD and FAD that drive the electron transport chain to make a large amount of ATP (around 30 to 32 ATP per glucose).
In anaerobic respiration, only glycolysis occurs, producing a net 2 ATP per glucose; pyruvate is then reduced to lactate to regenerate NAD so glycolysis can continue.
Because there is no electron transport chain or Krebs cycle in anaerobic respiration, the large ATP yield of oxidative phosphorylation is not obtained.
Markers reward aerobic respiration fully oxidising glucose with oxidative phosphorylation giving a large yield, anaerobic respiration being limited to glycolysis with a net 2 ATP, and lactate formation regenerating NAD.
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Sources & how we know this
- Eduqas A Level Biology Specification (A400) — Eduqas (2015)