The final acceptor of electrons in the electron transport chain is :

Question

Accepted Answer

The correct option is 2: Oxygen.

The Electron Transport Chain (ETC), which occurs on the inner mitochondrial membrane, is the final stage of aerobic cellular respiration, responsible for generating the majority of the cell’s ATP. This entire process is driven by a series of redox reactions, where electrons are passed from one complex to the next in a sequence of increasing electronegativity. The ETC requires a molecule with a very high affinity for electrons to serve as the powerful "pull" that keeps the chain moving and prevents a bottleneck. Oxygen is that molecule; it acts as the terminal electron acceptor at Complex IV (Cytochrome c oxidase), the last enzyme complex in the chain. When oxygen accepts the spent electrons, it simultaneously combines with protons (hydrogen ions) that have accumulated in the matrix, resulting in the formation of a harmless byproduct: metabolic water ($2H^+ + 2e^- + 1/2 O_2 \rightarrow H_2O$). This final, irreversible step is essential for re-oxidizing the carrier molecules and allowing the entire cycle of cellular respiration to continue.

The incorrect option is 1: ATPase.
ATPase, more formally known as ATP Synthase, is not an electron acceptor but rather an enzyme responsible for synthesizing ATP. It uses the energy stored in the electrochemical proton gradient created by the ETC to drive the phosphorylation of ADP. While the electron transport chain creates the conditions necessary for ATP Synthase to function, the enzyme itself acts as a molecular machine that facilitates the flow of protons, not electrons. Therefore, its role is mechanochemical energy conversion, making it a producer of energy rather than an acceptor of electrons.

The incorrect option is 3: Plastoquinone.
Plastoquinone is a mobile electron carrier, but it primarily functions in the electron transport chain of *photosynthesis*, which takes place in the thylakoid membranes of plant chloroplasts. In the photosynthetic ETC, Plastoquinone transports electrons between Photosystem II and the Cytochrome $b\_6f$ complex. Even where analogous quinones appear in the mitochondrial ETC (such as Ubiquinone/Coenzyme Q), they are *intermediate* carriers, meaning they pass the electrons on to the next complex. They are involved in the middle of the process, not at the end where the final acceptance and permanent reduction occur.

The incorrect option is 4: Cytochrome C.
Cytochrome C is a small, water-soluble protein that plays a vital role as a mobile carrier within the mitochondrial ETC. It is responsible for shuttling electrons between Complex III and Complex IV, residing in the intermembrane space. While it does accept and donate electrons, making it an electron carrier, it is not the terminal acceptor. It passes the electrons it receives to Complex IV, which then catalyzes the final transfer of those electrons to the ultimate destination, which is Oxygen.