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.
A 100mm lens has to have a maximum aperture of 50 mm to be an f/2, and if it has a maximum of 25mm it is a f/4. So if you divide the focal length with the f number you get the aperture. And an aperture that changes to half its size will let in a quarter of the light amount.