Cellular respiartion is a basicaly a three step process.
1. Glycolysis, it occurs in the cytoplasm.
2. Krebs cycle, it takes place in the matrix of mitochondria and
3. Electron Transport System (ETS) or Electron Transport Chain (ETC). These are groups or units of electron carriers found embedded in the inner mitochondrial membrane.
Keeping it simple cellular respiration starts with a molecule of glucose, which is a six carbon sugar. It is converted to two molecules of pyruvate (Pyruvic acid) in the first step, with the formation of two molecules of ATP and two molecules of NADH+H.
Both pyruvate molecules are carried to mitochondria to enter the Krebs cycle turn by turn. So for one mlecule of glucose Krebs cycle runs twice. Eight molecules of NADH+H and four molecules of FADHH are formed at the end of Krebs cycle.
In the third step the high energy electron carriers (10 NAD and 4 FAD) are oxidized in the ETS. Both of these high energy reducing agents dump their electrons into the ETS and the two protons (carried by each of them) are transferred into the inter membrane space (space between the inner and outer membranes) . Electrons released by NAD and FAD are transferred down an energy gradient in the ETS to their ground state where oxygen is there to accept them to form water. Energy released by electrons is used by the giant protein ATPase to form the energy currency ATP. A total of 36 or 38 moles of ATP are produced per mole of glucose in aerobic respiration.
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