- Cellular respiration uses energy in glucose to make ATP. Aerobic (“oxygen-using”) respiration occurs in three stages: glycolysis, the Krebs cycle, and electron transport.
- In glycolysis, glucose is split into two molecules of pyruvate. This results in a net gain of two ATP molecules.
- Life first evolved in the absence of oxygen, and glycolysis does not require oxygen. Therefore, glycolysis was probably the earliest way of making ATP from glucose.
- The Krebs cycle and electron transport occur in the mitochondria. The Krebs cycle takes place in the matrix, and electron transport takes place on the inner membrane.
- During the Krebs cycle, pyruvate undergoes a series of reactions to produce two more molecules of ATP and also several molecules of NADH and FADH2.
- During electron transport, energy from NADH and FADH2 is used to make many more molecules of ATP.
- In all three stages of aerobic respiration, up to 38 molecules of ATP may be produced from a single molecule of glucose.
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