Here’s a simplified Biologists look at glycolysis, the citric acid cycle and oxidative phosphorylation. Three processes involved in ATP production.
Glycolysis involves two stages. All the reactions are catalysed by specific enzymes. First glucose is prepared for catabolism by adding phosphates to its structure. This actually uses some ATP energy. The glucose is then processed and broken down to produce pyruvate.
The pyruvate is passed into the citric acid cycle (also named the Krebs cycle) it undergoes a number of changes involving a different enzymes and metabolites.
Two of these reactions are described by Nelson. Equations: 11.14; 11.15.
These reactions produce reduced NADH and FADH2 molecules. NADH is then used to generate a hydrogen ion gradient in the mitochondrion. The chemiosmotic gradient is generated using four protein complexes. Complex 1 is a NADH dehydrogenase; it catalyses NADH -> NAD+ and pushes 4 protons across the membrane. The electrons captured in this reaction are passed through to the other complexes in succession. Complex 2 is a succinate dehydrogenase; it doesn’t directly transfer hydrogen ions but instead donates further electrons by converting succinate to fumarate. Complex 3 couples the transfer of the electrons (from 1 and 2) with the transport of 4 protons. Complex 4 then reacts the electrons with oxygen to create water and transports a further 2 protons across the membrane. These successive reactions work to fully capture the energy generated from glucose. Without these incremental energy-capturing steps most of this energy would be lost to heat.