1. Glycolysis – making pyruvate from glucose
  2. Link reaction – converting pyruvate to acetyl coenzyme A
  3. Krebs cycle – producing reduced coenzymes and ATP
  4. Oxidative phosphorylation – producing a large amount of ATP



  • Splitting one glucose (6C) →2 pyruvates (3C)
  • Doesn’t need oxygen to take place – is anaerobic.
  • In cytoplasm of cells.
  • Stage 1 Phosphorylation: Glucose phosphorylated – 2 phosphates added from 2ATP. This creates 2 molecules of triose phosphate and 2ADP
  • Stage 2 Oxidation: Triose phosphate oxidised – loses hydrogen, forming 2 molecules of pyruvate. NAD collects H ions, forming 2reducedNAD. 4ATP produced, but 2 used in phosphorylation, so net gain of 2ATP.
  • The two molecules of reduced NAD go to oxidative phosphorylation.
  • The 2 pyruvate molecules go into the matrix of the mitochondria for link reaction



  • Decarboxylases (removeCO2) pyruvate.
  • Reduces NAD – collects hydrogen from pyruvate, changing pyruvate to acetate.
  • Combines acetate with coenzyme A to form acetyl coenzyme A.
  • No ATP produced
  • In mitochondria
  • Occurs twice for every glucose molecule -2 pyruvate made for every glucose that enters glycolysis. Means link r.&Krebs cycle happen 2x for every glucose
  • For each glucose: 1)Two molecules of acetyl coenzyme A go into Krebs cycle. 2)Two CO2 released 3)Two reduced NAD are formed and go to oxididative phosphorylation



  • Series of oxidation-reduction reactions
  • In the matrix of the mitochondria
  • Happens once for every pyruvate molecule(2x for every glucose)
  1. Acetyl CoA from link reaction combines with oxaloacetate to form citrate. Coenzyme A goes back to link reaction to be used again.
  2. 6C citrate molecule is converted to 5C molecule. When this happens decarboxylation and dehydrogenation occur. The hydrogen is used to produce reduced NAD from NAD.
  3. 5C molecule is then converted into a 4C molecule. Decarboxylation and dehydrogenation occur between intermediate compounds, producing one reduced FAD and two reduced NAD.
  • ATP is produced by the direct transfer of a phosphate group from an intermediate compound to ADP (substrate-level phosphorylation). Citrate has now been converted to oxaloacetate.
  • From Krebs Cycle: 1CoA reused in link reaction,
  • oxaloacetate regenerated for use in Krebs Cycle,
  • 2CO2 released as waste product,
  • 1ATP used for energy,
  • 3reducedNAD and 1reduced FAD used in oxidative phosphorylation



  • Where energy carried by electrons from reduced coenzymes (reduced NAD & reduced FAD) is used to make ATP.
  • Involves two processes – the electron transport chain and chemiosmosis.
  • Hydrogen atoms are released from reduced NAD and reduced FAD as they are oxidised to FAD and NAD. H atoms split into protons (H+) and e-‘s.
  • Electrons move along electron transport chain, losing energy at each carrier.
  • This energy is used by e- carriers to pump protons from mitochondrial matrix to the intermembrane space.
  • Concentration of protons now higher in intermembrane space than in mitochondrial matrix – electrochemical gradient formed (conc. Gradient of ions).
  • Protons move ↓electrochemical gradient, back into mitochondrial matrix, via ATP synthase. This movement drives the synthesis of ATP from ADP and Pi.
  • Chemiosmosis – Movement of H+ ions across a membrane, generating ATP.
  • In the mitochondrial matrix, protons, electrons and O2 (from the blood) combine to form water. O2 said to be final electron acceptor.


32 ATP can be made in total from one glucose molecule with aerobic respiration.

  • In oxidative phosphorylation, ATP made from reduced coenzymes. 2.5ATP made from each reduced NAD and 1.5ATP made from each reduced FAD.
  • Link reaction and Krebs cycle happen 2x for each glucose (2 pyruvate produced)
  • So 8×2.5 (2reducedNAD in link reaction, 6 in Krebs cycle) + 2×1.5 (2reducedFAD in Krebs Cycle) + 2ATP (made in Krebs Cycle) = 25ATP from link reaction & Krebs Cycle alone.
  • + 2ATP and 2×2.5ATP (from reduced NAD) produced in glycolysis = total of 32 ATP.


  Anaerobic respiration doesn’t involve the link reaction, Krebs cycle or oxidative phosphorylation. Products of glycolysis are converted to ethanol(plant, yeast) or lactate(animal).

The production of lactate regenerates NAD, so glycolysis can continue in the absence of oxygen. A small amount of ATP can still be produced.