In the electron transport chain, where do the electrons come from?
B. Oxygen (O2)
D. NADH and FADH2 produced in glycolysis and the Krebs cycle.
D: In the electron transport chain, electrons come from NADH and FADH2.
NADH or FADH2 gives up its high energy electrons, which moves down the electron transport chain. As it moves down the ETC, it loses some energy which pumps H+ (hydrogen ions) from the mitochondria matrix to the inter membrane space. Oxygen later accepts the electrons at the end of the ETC and two H+ to form H20.
The pumped out H+ which is in the inter membrane space now, creates a concentration gradient of H+ in the mitochondrial matrix and the inter membrane space, aiding diffusion. H+ diffuse down the gradient through ATP synthase, and energy release by the diffusion of H+ is used to synthesize ATP.
The oxidation of 1 molecule of NADH yields 3 ATP molecules whereas the oxidation of 1 molecule of FADH2 yields 2 ATP molecules. This is because NADH enters the ETC at a higher energy potential than FADH2.