Oxidative phosphorylationEdexcel A-Level Biology B: Revision notes
Section 1
Overview and location
Oxidative phosphorylation is the final stage of aerobic respiration, and it produces most of the ATP. It takes place on the inner mitochondrial membrane, which contains the electron transport chain (a series of electron carrier proteins) and the enzyme ATP synthase. The reduced coenzymes (NADH and FADH₂) made in glycolysis, the link reaction and the Krebs cycle supply the hydrogen and electrons.
Section 2
The electron transport chain
Reduced NAD gives up its hydrogen, which splits into protons (H⁺) and electrons, so NAD is regenerated and can return to the Krebs cycle. The electrons pass from carrier to carrier along the chain in a series of redox reactions. Each carrier is reduced as it accepts electrons and then oxidised as it passes them on, and energy is released at each step.
This energy is used to pump protons (H⁺) from the matrix, across the inner membrane, into the intermembrane space.
The energy from the electrons does not make ATP directly. It pumps protons, and the proton gradient makes the ATP.
Section 3
Chemiosmosis
Pumping protons produces a proton gradient, with a high concentration in the intermembrane space and a low concentration in the matrix (an electrochemical gradient). The inner membrane is impermeable to protons, so they can only move back into the matrix through a channel in ATP synthase.
As protons flow down their gradient through ATP synthase, the energy released is used to join ADP and inorganic phosphate to make ATP. This method of making ATP is chemiosmosis.
Protons are pumped by the carriers of the chain, but they flow through ATP synthase. Do not mix up the two.
Section 4
Oxygen: the terminal electron acceptor
At the end of the chain the electrons are passed to oxygen, the terminal electron acceptor. Oxygen combines with the electrons and with protons from the matrix to form water: 4H⁺ + 4e⁻ + O₂ → 2H₂O.
Without oxygen the last carrier cannot pass on its electrons, so the whole chain stops, the proton gradient is lost and no ATP is made. NADH is not oxidised back to NAD, so the Krebs cycle also stops.
Section 5
Importance of the mitochondrial membranes
- The inner membrane is folded into cristae, giving a large surface area for many electron transport chains and ATP synthase molecules.
- It is impermeable to protons, so they can only pass through ATP synthase and the gradient is maintained.
- The intermembrane space is narrow, so the proton concentration builds up quickly.
- The matrix next to the membrane contains the Krebs cycle enzymes, supplying reduced NAD.
- Cells with a high demand for ATP, such as heart muscle, have many mitochondria with many cristae.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Oxidative phosphorylation
- Isolated mitochondria are suspended in a buffered medium containing a pH electrode. The outer membranes of the mitochondria are permeable to small ions. When NADH and oxygen are added, the pH of the medium falls, and ATP is formed if ADP and phosphate are also present.Explain why the pH of the medium falls after NADH and oxygen are added.2 marks
- A researcher adds a chemical to isolated mitochondria that makes the inner membrane freely permeable to protons. Electrons continue to pass along the electron transport chain, and oxygen continues to be used at a high rate, but ATP production almost stops.Explain why ATP production almost stops.2 marks
- Cyanide binds to the last protein carrier in the electron transport chain and prevents it from passing electrons to oxygen. Cells exposed to cyanide soon run short of ATP, even though plenty of glucose is available. The Krebs cycle in these cells also stops.Explain why cells exposed to cyanide soon run short of ATP.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).