In respiration, the electrons come from the oxidation of organic molecules, and is the final electron acceptor.

Oxygen

Explanation:

Cellular respiration is the process by which ATP is generated. It has three parts: glycolysis, TCA cycle and electron transport chain. Electron transport chain (ETC) is the final step of cellular respiration and takes place in mitochondria.

ETC comprises of four complexes labelled I to IV. NADH and FADH2 which have been generated in previous steps are oxidized here. They give up their electrons which are transferred from complex I to IV generating a proton gradient. This gradient is sued to synthesize ATP by ATP synthase. The final acceptor of electrons is Oxygen which is split and then combined with hydrogen from surrounding medium to form water molecule.