7 min readPathway sheetOften a 10-mark answer

Oxidative phosphorylation

Energy from the electron transport chain pumps protons out; their flow back through ATP synthase makes ATP. This coupling is explained by the chemiosmotic theory.

WhereInner mitochondrial membrane

EnzymeATP synthase (F0F1, Complex V)

TheoryChemiosmotic theory (Peter Mitchell)

P/O ratioNADH 3 or 2.5; FADH2 2 or 1.5

Chemiosmotic theory in six steps

Phosphorylation means adding a phosphate to ADP to make ATP. Mitchell's theory says the ETC and ATP synthase are linked by a gradient of protons (H+ ions) across the inner membrane.

1
Electrons flow through the ETC
Complexes I → IV · energy released in small steps
2★
H+ pumped from matrix → intermembrane space
Complexes I, III, IV · Complex II does not pump
3
Gradient builds: more H+ and more positive charge outside
called the proton-motive force
4
Inner membrane does not let H+ leak back
H+ can return only through ATP synthase
5★
H+ flows back through the F0 channel, turning the rotor
ATP synthase (F0 part)
6★
ADP + Pi → ATP in the matrix
ATP synthase (F1 head) · rotary, binding-change mechanism

★ one-way, controls the pace

P/O ratio: ATP made per oxygen atom used

NADH (older books)3 ATP

NADH (newer books)2.5 ATP

FADH2 (older books)2 ATP

FADH2 (newer books)1.5 ATP

Basis of newer valuesAbout 10 H+ pumped per NADH (6 per FADH2); about 4 H+ needed per ATP

Why FADH2 gives lessIt enters at Complex II and skips the Complex I proton pump

Inhibitors and uncouplers

Agent	Acts on	Effect
Rotenone (fish poison), amobarbital (Amytal)	Complex I	Blocks NADH oxidation; the FADH2 route still works
Malonate	Complex II (succinate dehydrogenase)	Competitive inhibitor; looks like succinate
Antimycin A	Complex III	Stops electron flow from cytochrome b to c1
Cyanide, carbon monoxide, hydrogen sulphide, azide	Complex IV (cytochrome oxidase)	Electrons cannot reach oxygen; cyanide poisoning is rapidly fatal
Oligomycin	ATP synthase (F0)	Blocks the proton channel; no ATP, and electron flow slows too
2,4-Dinitrophenol (DNP), high-dose aspirin	Inner membrane (uncouplers)	Carry H+ back across; oxygen is used but no ATP is made; energy lost as heat
Thermogenin (UCP1)	Brown adipose tissue (natural uncoupler)	Makes heat to keep newborns warm

Inhibitor vs uncoupler

InhibitorBlocks electron flow or ATP synthase. Oxygen use falls and ATP stops. Examples: cyanide, oligomycin.

UncouplerElectron flow and oxygen use continue, even speed up, but the proton gradient leaks away. No ATP is made; the energy is lost as heat. Example: 2,4-DNP.

DNP was sold as a slimming drug in the 1930s. It caused deaths from very high body temperature and was banned.

Nutrition link

ATP works in the body as Mg-ATP, so magnesium is needed wherever ATP is used. Niacin and riboflavin supply the NADH and FADH2 that feed the chain. Brown fat uses natural uncoupling to keep newborns warm, and the slimming drug 2,4-DNP was banned because it wasted food energy as dangerous heat.

Exam angle

10 marks: “Explain oxidative phosphorylation with the chemiosmotic theory. Add a note on inhibitors and uncouplers.” Draw the membrane, explain proton pumping by I, III and IV, the gradient, ATP synthase (F0F1), both P/O values, then list inhibitors and uncouplers.

5 marks: “Write a short note on the chemiosmotic theory.” Mitchell: the ETC pumps H+ out; the proton-motive force drives H+ back through ATP synthase, which makes ATP. The inner membrane must be intact.

2 marks: “What is an uncoupler? Give an example.” A substance that lets protons leak back across the inner membrane, so oxidation continues without ATP synthesis. Example: 2,4-dinitrophenol.

Remember it

Think of a dam: the ETC pumps water (H+) uphill, ATP synthase is the turbine, and an uncoupler is a hole in the dam. Also, Fo is the oligomycin-sensitive part.