The mitochondria of the eukaryotic cells are the sites of cellular respiration and where most of the steps take place. Cellular respiration allows for the release of energy stored in chemical bonds of glucose (obtained from food) to form adenosine triphosphate, which is the energy currency of the cell.
Cellular respiration occurs in multiple steps. Glucose is first broken down into smaller pyruvate molecules in the cytoplasm. After this step, if the cell does not have oxygen, the pyruvate is fermented in an anaerobic process to release lactate. In the presence of oxygen, aerobic respiration takes place in the mitochondria.
The pyruvate is transported to the mitochondria, where it first enters the tricarboxylic acid cycle, which is also known as Krebs cycle. Pyruvate is oxidized to acetyl CoA, which then enters the TCA cycle, resulting in the production of one ATP molecule and three NADH, or reduced nicotinamide dinucleotide, molecules. The TCA cycle is not a major source for ATP production.
The NADH produced is then cycled through the electron transport along the convoluted membranes of the mitochondria. The oxidation of NADH to NAD+ allows for ATP production. Most of the ATP produced during cellular respiration is produced during the electron transport phase.
Mitochondria are present in all living eukaryotic cells. Some cells, such as fat cells and muscle cells, may have more mitochondria because they either store energy or have a need to respond quickly to move and do work.