Respiration is the enzymatic release of energy from organic compounds in living cells.

1. Glycolysis - the splitting of glucose into pyruvate molecules
2. Link reaction - the conversion of the pyruvate molecules into acetyl coenzyme A
3. Krebs cycle - the conversion of acetyl coenzyme A into ATP, reduced NAD, and reduced FAD
4. Oxidative phosphorylation - the use of the electrons, reduced NAD, and reduced FAD to produce ATP and water

For most organisms, the energy originates in sunlight, which is captured and transferred to chemical energy in organic molecules by photosynthesis.

ATP is made when P_i is combined with ADP, using energy provided by respiration either directly by substrate-linked reactions or by chemiosmosis.

A substrate-linked reaction is one in which ATP is made using energy provided directly by another chemical reaction.

An anabolic reaction is a chemical reaction in which small molecules are built up into larger ones, using an input of energy.

1. Aerobic respiration (requires oxygen)
2. Anaerobic respiration (does not require oxygen)

The main difference is that aerobic respiration requires oxygen while anaerobic respiration does not require oxygen.