Question

Why does FADH, result in LESS ATP formation, even though it donates the same number (2) electrons into the electron transport chain?

Answer 1

The electron transport chain is a collection of membrane-embedded proteins and organic molecules, most of them organized into four large complexes labeled I to IV.

Intermembrane space H HH * out c A 0000000 Complex CooComplex complex XX III XXXX IN Complex Complex A ÎN FADH2 FAD +2H+ NADH 2H*+ 1/20, H2O NAD + H+ Electron Transport Chain

Since the electrons of FADH₂ are at a lower energy level, they are not as good at donating electrons as NADH. Therefore FADH₂ molecule cannot transfer its electrons to complex I.

NADH started with Complex I, it had more chances to pumps more protons across the gradient, which powers the ATP synthase and gives us 3 ATP per molecule of NADH.

FADH₂ produces 2 ATP during the electron transport chain as it gives up its electron to Complex II, bypassing Complex I. By bypassing Complex I, the chance to pump protons across the membrane is missed, hence less protons have been pumped by the time Complex IV is attained.

The main purpose of the electron transport chain is to establish surplus protons in inter membrane space to create an electrochemical proton gradient. This drives the synthesis of adenosine triphosphate (ATP) which is a molecule which stores energy in highly strained bonds form.

Therefore lower contribution to the proton gradient results in lesser ATP molecule generation by FADH₂ molecule in comparison with NADH molecule.