Glycolysis is the first series of reactions of respiration. It occurs in the cytosol of the cytoplasm of the cell. The 6-carbon glucose molecule is broken into 2 3-carbon acids (called pyruvic acid). Metabolic energy is produced from breaking one carbon-carbon bond. If no oxygen is present, the 3-carbon acid goes to Anaerobic Fermentation. If oxygen is present (which is most of the time), the 3-carbon acid moves into the matrix of the mitochondrion and enters the Krebs Cycle.
Anaerobic fermentation occurs in the cytosol of the cytoplasm, and only occurs when there is no oxygen present. The 3-carbon acid from glycolysis is broken down into ethanol (a 2-C compound) and CO2; this happens for each of the 2 3-C acids from glycolysis. Some metabolic energy (ATP) is produced from breaking one more carbon-carbon bond. But, there is a carbon-carbon bond left in ethanol that is never broken, thus anaerobic fermentation results in incomplete respiration of the original glucose. This produces enough energy to keep only small organisms (e.g. microorganisms) alive; higher plants/animals die if they only have anaerobic fermentation for extended periods of time.
The Krebs Cycle occurs when oxygen is present and occurs in the matrix of the mitochondrion. The 3-carbon acid from glycolysis (pyruvic acid) loses a CO2, then combines with a 4-carbon acid to produce a 6-carbon acid (citric acid). The 6-carbon acid is broken down into a 5-carbon acid, then a 4-carbon acid, breaking a carbon-carbon bond, releasing CO2 and producing metabolic energy (ATP, NADH and FADH2) with each degradation. The original 4-carbon acid is replenished, and the cycle goes again. The cycle turns 2 times for each glucose (e.g. once for each 3-C acid produced by glycolysis).
The most useful form of metabolic energy is ATP. So the various metabolic energy compounds produced by glycolysis and the Krebs Cycle move to the inner membranes of the mitochondrion. In the inner membrane is an electron transport chain called the cytochrome system, which is very similar to what we saw in Photosynthesis. The metabolic energy compounds (NADH and FADH2) donate their electrons to the electron transport carriers of the electron transport chain, an energy gradient is produced, and an enzyme (ATPase) produces ATP. Oxygen acts as a terminal electron acceptor to keep the chain flowing, and combines with H+ to produce water.
Now the plant has converted all the original energy that was stored in the carbon-carbon bonds of glucose back into the various metabolic energy compounds it needs to power its metabolism. The plant can use the NADH or FADH2 directly, or convert it to ATP for metabolism. Remember, these forms of metabolic energy cannot be stored or transported very easily, so respiration must occur in every cell and it must occur at the exact time the metabolic energy is needed.