12. Electron transfer from donors such as NADH and FADH₂ to O₂ occurs in
(1) membranes of ER, chloroplast and mitochondria
(2) chloroplast only
(3) mitochondria only
(4) organellar membranes which possess ATP synthase

In cellular respiration, the transfer of electrons from electron donors such as NADH and FADH2 to the final electron acceptor, oxygen (O2), is a crucial step for ATP production. This electron transfer occurs through a series of protein complexes known as the electron transport chain (ETC). Understanding where this process takes place is fundamental to bioenergetics.

Location of Electron Transfer in Eukaryotes

The electron transport chain components that facilitate electron transfer from NADH and FADH2 to oxygen are embedded in the inner mitochondrial membrane. This membrane houses the four main ETC complexes (I-IV) and ATP synthase.

• NADH donates electrons to Complex I.

• FADH2 donates electrons to Complex II.

• Electrons are passed through ubiquinone (CoQ) and cytochrome c between complexes.

• Finally, electrons reduce oxygen to water at Complex IV.

This electron flow drives proton pumping across the inner membrane, creating a proton gradient that powers ATP synthesis.

Why Other Membranes Are Not Involved

• The endoplasmic reticulum (ER) and chloroplast membranes do not participate in this electron transfer process in mitochondria.

• Chloroplast thylakoid membranes conduct electron transfer during photosynthesis, not cellular respiration.

• The plasma membrane in prokaryotes carries out electron transport, but in eukaryotes, this function is localized to mitochondria.

• Organellar membranes with ATP synthase include both mitochondria and chloroplasts, but only mitochondria perform electron transfer from NADH/FADH2 to oxygen.

Summary of Options

Final Answer:

(3) Mitochondria only