Where does the NADH comes from?

good you asked something and i can help you 😀

i do not know much about TCA. 😡

but know one thing oxidative phosphorylation needs oxygen. what happens is NADH give h which later on forms H+ and e-. e- moves in the electron transport chains. giving energy but the h+ goes to the oxygen. to form water.

see NAD takes up the hydrogen. to form NADH but then it needs to dump the hydrogen somewhere. usually its dumped to an oxygen by chemioosmosis during oxidative phosphorylation. during anaerobic respiration only glycolysis takes place.

simple
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jwalin has the key to understanding this. The electrons running through the mitochondrial membrane during chemiosmosis need a terminal electron acceptor which is very stable. Oxygen provides a stable resting place for those electrons, bonding protons to oxygen to form water. You can’t run chemiosmosis without a terminal electron acceptor.

Eukaryotes use oxygen for that, but prokaryotes are more metabolically versatile, using a variety of terminal electron acceptors to run oxidative phsophorylation in anaerobic environments.

Ignoring the requirement for oxygen as a terminal electron acceptor, if you were to use an electron transport chain to regenerate NAD+ for glycolysis then you would run into a problem with transport at the mitochondrial membrane. NADH doesn’t directly enter a mitochondrion. Getting the energy from the NADH into a mitochondrion comes at an energy cost — see the section on the DHAP shuttle (a subsection title) on this page:
http://www.life.illinois.edu/crofts/bio … 14&15.html