Electron transport releases energy that pumps protons from the mitochondrial matrix to the intermembrane space, supporting usable cellular work and measurable biological activity.
Coverage 2021–2021 · Updated 15 Jul 2026
Reduced NAD from glycolysis, the link reaction and the Krebs cycle transfers a pair of electrons to the first carrier of the electron transport chain in the inner mitochondrial membrane.
As electrons pass between carriers, reduced NAD is converted back to NAD and released energy drives proton pumping from the matrix into the intermembrane space. Reduced FAD can also contribute electrons downstream.
The inner membrane's low proton permeability maintains both a concentration difference and electrical potential: an electrochemical proton gradient. Protein-complex names are not required.
Electron donation reoxidizes NAD so earlier respiration stages can continue, while proton pumping stores transferred energy in a gradient ready for ATP synthase.
The electron transport chain builds the gradient; it does not phosphorylate ADP directly. Pumping is matrix → intermembrane space, and chemiosmosis is the return flow.
This objective is assessed through multiple choice.
Build the answer around this relationship: Protons are pumped from the matrix to the intermembrane space.
Representative question
Where are protons pumped, to allow chemiosmosis in aerobic respiration to occur?
From outside the mitochondrion through the double membranes
From carrier to carrier in the inner mitochondrial membrane
From the matrix of the mitochondrion to the space between the membranes
From the space between the membranes to the cytoplasm outside the mitochondrion
In HL respiration, carbon and hydrogen are followed separately. Glycolysis turns glucose into pyruvate, net ATP, and reduced NAD. Without oxygen, pyruvate becomes lactate in humans or ethanol and carbon dioxide in yeast to regenerate NAD. With oxygen, pyruvate enters the link reaction, forming acetyl-CoA, carbon dioxide, and reduced NAD. The Krebs cycle releases more carbon dioxide and reduced coenzymes. Electron transport uses reduced NAD/FAD to pump protons, chemiosmosis through ATP synthase makes ATP, and oxygen accepts electrons and protons to form water. Substrate comparisons depend on ATP yield, oxygen demand, water production, and speed. Regenerated NAD allows glycolysis to continue. Fermentation regenerates NAD for glycolysis and is used in baking and brewing. Electron transfers release energy while coenzymes are reoxidized.
