Most of the ATP from a glucose molecule is made on the folded inner membrane of the mitochondrion. Electrons fall down a chain of carriers, the energy pumps protons into a narrow space, and the protons rush back through a molecular turbine.
Reduced NAD and reduced FAD carry hydrogen from glycolysis, the link reaction and the Krebs cycle to carriers in the inner mitochondrial membrane. There each hydrogen atom splits into a proton and an energetic electron. The electrons pass along the electron transport chain, releasing energy that is used to transfer protons from the matrix into the intermembrane space. The protons return to the matrix by facilitated diffusion through ATP synthase, providing the energy to make ATP from ADP and phosphate: this is chemiosmosis. Oxygen is the final electron acceptor: it combines with electrons and protons to form water. Without oxygen the chain stops, and so do the Krebs cycle and link reaction.
Fig. 1: the electron transport chain pumps protons into the intermembrane space; they flow back through ATP synthase
Section 01
Hydrogen to the inner membrane.
Watch reduced NAD deliver its hydrogen with oxygen available: protons are yellow and electrons blue. Read the proton gradient and the ATP made counter. Then untick Oxygen available and watch both again.
NAD and FAD: coenzymes that accept hydrogen from dehydrogenation reactions in glycolysis, the link reaction and the Krebs cycle, becoming reduced. They carry the hydrogen to the electron transport chain and are oxidised back, ready to collect more.
Splitting: at the first carrier each hydrogen atom splits into a proton (H+) and an energetic electron (e−).
Electron transport chain: a series of carrier proteins in the inner membrane. The electrons are passed from carrier to carrier, each time to one at a lower energy level, so energy is released in small steps.
Proton pumping: the released energy is used to transfer protons from the matrix across the inner membrane into the intermembrane space.
Reduced FAD hands its hydrogen to a carrier further along the chain than reduced NAD, so fewer protons are pumped and less ATP is made from it.
Examiner trap
Electrons move along the chain; protons are pumped across the membrane. Do not write that electrons pass through ATP synthase: it is protons that flow through it.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the oxidative phosphorylation sim, what does each hydrogen atom from reduced NAD split into?
Quick check+10 XP
In the sim, where do the carriers move protons to?
Quick check+10 XP
In the sim, you untick Oxygen available. What happens?
Quick check+10 XP
In the sim, by which route do protons return to the matrix?
Section 02
Chemiosmosis: protons flow back.
The pumping builds up stored energy, as a gradient. ATP synthase is the only way back.
Feature
Effect
protons pumped into the narrow intermembrane space
a high proton concentration builds up there: a proton gradient (and an electrical gradient) across the inner membrane
the inner membrane is impermeable to protons
the only route back to the matrix is through the channel in ATP synthase
protons diffuse back through ATP synthase
facilitated diffusion down the gradient releases energy
ATP synthase uses this energy
ADP + Pi → ATP, in the matrix
Making ATP using energy from a proton gradient across a membrane is called chemiosmosis. The same mechanism makes ATP across the thylakoid membranes of chloroplasts in photosynthesis.
Section 03
Why the chain needs oxygen.
Oxygen is used at the very end of aerobic respiration, but everything before it depends on it.
Final electron acceptor: at the end of the chain, oxygen combines with electrons and protons to form water: ½O2 + 2H+ + 2e− → H2O. This keeps the chain clear so electrons can keep flowing.
No oxygen: electrons cannot leave the chain, so the carriers stay reduced, proton pumping stops and ATP synthesis by chemiosmosis stops.
Knock-on effect: reduced NAD and FAD can no longer be oxidised, so there is no NAD or FAD to accept hydrogen in the Krebs cycle and link reaction, and these stop too. Only glycolysis can continue, if NAD is regenerated another way (anaerobic respiration).
Respiratory poisons: cyanide inhibits the last carrier in the chain, with the same effect as removing oxygen.
Examiner trap
Oxygen accepts electrons (and protons) to form water. It does not "make ATP" or "react with glucose" directly, and the CO2 released in respiration does not come from this oxygen.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
What is the role of oxygen in aerobic respiration?
Exam style+20 XP
Cyanide inhibits the last carrier of the electron transport chain. Which effect follows?
Exam style+20 XP
What directly provides the energy used to pump protons across the inner mitochondrial membrane?
Exam style+20 XP
A chemical makes the inner mitochondrial membrane permeable to protons. Mitochondria treated with it keep using oxygen. What else happens?
Skill unlocked
Oxidative phosphorylation, mastered.
This skill is now recorded on your profile. Keep the chain going.