12.2.2—Glycolysis pathway

Aerobic respiration can be mapped as four linked stages. The location and broad output of one stage determine what can enter the next, while the final stage uses reducing power and oxygen to support ATP production.

1. Glycolysis — cytoplasm: glucose is split into pyruvate. The stage also provides some ATP and reduced coenzymes. Pyruvate is the carbon product passed towards the mitochondrion.
2. Link reaction — mitochondrial matrix: pyruvate is processed into an acetyl group carried by coenzyme A. Carbon dioxide and reduced coenzyme are produced, and acetyl-CoA is the entry material for the Krebs cycle.
3. Krebs cycle — mitochondrial matrix: acetyl-CoA enters a cyclic sequence of enzyme-controlled reactions. Carbon dioxide is released, a small amount of ATP is made, and more reduced coenzymes carry transferable electrons onward.
4. Oxidative phosphorylation — inner mitochondrial membrane: electrons from reduced coenzymes pass through membrane carriers; the released energy supports ATP synthesis, and oxygen accepts electrons at the endpoint to form water.

Route cue: cytoplasm → mitochondrial matrix → mitochondrial matrix → inner mitochondrial membrane. Carbon skeletons are progressively processed, while reducing power is transferred to the membrane stage for the main ATP-synthesis mechanism. The four-stage map is a scaffold; the detailed chemistry and accounting belong to the following cards.

The four stages are not four separate versions of the same reaction. Glycolysis is outside the mitochondrion; the link reaction and Krebs cycle are in the matrix; oxidative phosphorylation is on the inner membrane. This overview does not supply detailed NAD/FAD accounting or a fixed ATP total.