7.1.4Unit 1: Lifestyle, Transport, Genes and Health
The Electron Transport Chain — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 28 of 29 Unit quiz
Part 1 of 5Steps of Oxidative Phosphorylation
The process is explained by the chemiosmotic theory, which links electron flow to a proton gradient.
1
Reduced NAD and reduced FAD from glycolysis, the link reaction and the Krebs cycle arrive at the inner mitochondrial membrane and give up their hydrogen, which separates into protons and electrons.
2
The electrons pass along a series of carrier proteins in the cristae, losing a little energy at each transfer as the carriers are alternately reduced and oxidised.
3
This released energy is used to pump H⁺ ions from the matrix across the inner membrane into the intermembrane space, against their concentration gradient.
4
Because the inner membrane is impermeable to H⁺, a steep proton gradient builds up, with a high H⁺ concentration and low pH in the intermembrane space.
5
Protons flow back into the matrix down their gradient by facilitated diffusion through the channel of ATP synthase, and the energy released joins ADP and Pi to make ATP.
6
At the end of the chain oxygen accepts the electrons and combines with H⁺ to form water, which keeps the chain moving.
Part 2 of 5Key Components and Their Roles
Inner membrane
- Folded into cristae to increase surface area, and the site of both the carriers and ATP synthase.
Electron carriers
- Membrane proteins held close together so electrons can pass from one to the next and release energy in small steps.
Proton pumps
- Carriers use the energy from electrons to move H⁺ against its gradient, so the membrane stores energy like a charged battery.
ATP synthase
- A channel enzyme that lets protons through and uses their flow to phosphorylate ADP into ATP.
Intermembrane space
- The narrow gap holding a high concentration of protons, which forms one side of the gradient.
Oxygen
- The final electron acceptor, forming water with the electrons and protons arriving at the end of the chain.
Part 3 of 5ATP Yield and the Need for Oxygen
ATP Yield and the Need for Oxygen
Yield per carrier
- Under the convention used in this course, each reduced NAD gives about 3 ATP and each reduced FAD gives about 2 ATP.
Why oxygen matters
- Without a final acceptor the chain stops, so reduced NAD and FAD cannot be reoxidised and the Krebs cycle also halts.
Coenzyme recycling
- Oxidation of reduced NAD and FAD regenerates the oxidised forms needed to carry more hydrogen from earlier stages.
By-product
- Water is formed when oxygen is reduced, so respiring cells produce metabolic water.
| Stage | Reduced NAD | Reduced FAD | ATP made directly |
|---|---|---|---|
| Glycolysis | 2 | 0 | 2 (net) |
| Link reaction | 2 | 0 | 0 |
| Krebs cycle | 6 | 2 | 2 |
Part 4 of 5Worked Example
Question Using 3 ATP per reduced NAD and 2 ATP per reduced FAD, calculate the ATP made by oxidative phosphorylation from one glucose molecule, given that 10 reduced NAD and 2 reduced FAD are produced in total.
- Reduced NAD10 × 3 = 30 ATP.
- Reduced FAD2 × 2 = 4 ATP.
- Add the two totals30 + 4 = 34 ATP.
Answer 34 ATP (adding 2 from glycolysis and 2 from the Krebs cycle gives the textbook maximum of 38).
Part 5 of 5Exam Checklist & Tips
7.1.4 – Key Points for the Exam
- Oxidative phosphorylation Formation of ATP using energy from the electron transport chain and a proton gradient across the inner mitochondrial membrane.
- Chemiosmosis Movement of protons down a gradient through ATP synthase, driving ATP formation.
- Final electron acceptor Oxygen, which takes up electrons and protons at the end of the chain to form water.
- Cristae Folds of the inner mitochondrial membrane that increase the surface area for the chain and ATP synthase.
- ATP synthase Enzyme that makes ATP as protons pass through it into the matrix.
Common Mistakes
- Saying oxygen accepts only hydrogen; it accepts electrons and combines with protons to form water.
- Placing the chain in the matrix; it is in the inner membrane, and the matrix hosts the Krebs cycle.
- Writing that protons are pumped into the matrix; they are pumped out of it, into the intermembrane space.
- Using hydrogen and hydrogen ions as if they were the same;
- protons are H⁺ and hydrogen atoms carry electrons too.
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