Certain bacteria can do this. Hydrogen gas can be liberated directly. Can only happen if H 2 is removed, so its concentration remains low. Fermentations play major role. Many sugars can be fermented.
Respiration Use an external electron acceptor.
Oxygen as prototype. Alternative solution is to use some non-organic molecule that has a low redox potential, can accept electrons and become some reduced molecule. In mitochondria, this is a cytochrome, cyt a. As soon as this is used up, respiration ceases. Although cells could transfer electrons directly from NADH to oxygen, this would liberate all energy in NADH directly as heat.