Oxidative phosphorylation is the third step in cellular respiration. It is composed of two smaller steps (electron transport chain and chemiosmosis). What molecules are donating their energy to power the proteins in the ETC?

NADH and FADH2

Explanation:

Oxidative phosphorylation is the last step of cellular respiration involving two smaller steps; Electron transport chain (ETC) and chemiosmosis.

ETC is a series of proteins and organic molecules present in the cristae of the mitochondria. Electrons are passed from one member of the transport chain to another in a series of reduction-oxidation reactions (redox). Energy released is captured as a proton gradient.

All the electrons that are supplied into the transport chain are donated by NADH and FADH2 molecules (reduced electron carriers) produced in the previous stages of respiration. As the electrons are passed down the chain, they move from a higher to a lower energy level, releasing energy in the process. The released energy is used by several of the protein complexes to pump protons from the matrix of the mitochondria to the intermembrane space, forming a proton gradient.

The electrons donated by NADH are at a higher energy level i. e. NADH is a very good donor of electrons in redox reactions. This attribute allows NADH to transfer its electrons directly to Complex I, where energy is released and the complex uses this energy to pump protons across the membrane.

FADH2, on the other hand, is not a good donor of electrons. Thus, its electrons are at a lower energy level, making it unable to transfer its electrons to ComplexI. It feeds them through Complex II instead, which does not pump protons across the membrane.

This factor (Complex I bypass) causes FADH2 molecules to contribute less to the proton gradient than NADH molecules.

N. B: the energy stored in the proton gradient is subsequently used to produce ATP in Chemiosmosis.