Most eukaryotic cells have mitochondria, which produce ATP from products of the citric acid cycle, fatty acid oxidation, and amino acid oxidation. At the mitochondrial inner membrane, electrons from NADH and succinate pass through the electron transport chain to oxygen, which is reduced to water.
The maximum diameter of the lens opening is referred to as its maximum aperture or minimum f-number. The minimum f-number is the smallest f-number that can be selected with the lens. The maximum aperture varies from lens to lens and is included in the lens model name.
Aperture, therefore, is just a relative measure for the diameter of your lens opening. Lower f-numbers indicate a wider aperture, with f/2.8 and below being the extremes, while higher ones signify that more light is being blocked. One side effect of a wide-open lens is that it lets in a lot of unfocused light rays.