Question

The correct sequence for the flow of electrons between following components of mitochondrial electron transport chain is:
(A). NADH dehydrogenase
(B). Cytochrome \( bc_1 \) complex
(C). Ubiquinone
(D). Cytochrome C
Choose the correct answer from the options given below:

• A. (A), (B), (C), (D).
• B. (A), (C), (B), (D).
• C. (B), (A), (D), (C).
• D. (C), (B), (D), (A).

Hint:

Mnemonic: Think of the carriers as alternating between large membrane-bound complexes and small mobile shuttles:
Complex I (Stationary) \( \rightarrow \) Ubiquinone (Mobile lipid) \( \rightarrow \) Complex III (Stationary) \( \rightarrow \) Cytochrome C (Mobile protein).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The mitochondrial electron transport chain (ETC) consists of four integral multi-protein complexes and two mobile electron carriers embedded within the inner mitochondrial membrane.
Electrons flow spontaneously through these components in a path determined by their relative standard reduction potentials (\( E_0' \)), moving from carriers with highly negative reduction potentials to those with increasingly positive reduction potentials, until they finally reduce molecular oxygen to water.

Step 2: Detailed Explanation:
The sequential transport of electrons starting from NADH involves the following components:
1. NADH Dehydrogenase (Complex I - Component A): This massive L-shaped protein complex oxidizes cytosolic or mitochondrial \( \text{NADH} \) to \( \text{NAD}^+ \).
The two high-energy electrons extracted from \( \text{NADH} \) are transferred through a flavin mononucleotide (FMN) cofactor and a series of iron-sulfur (Fe-S) clusters within Complex I. This transfer is coupled with the pumping of four protons from the matrix into the intermembrane space.
2. Ubiquinone (Coenzyme Q - Component C): Ubiquinone is a hydrophobic, lipid-soluble benzoquinone derivative that diffuses freely within the lipid bilayer of the inner mitochondrial membrane.
It accepts two electrons from Complex I (and two protons from the matrix), reducing it to ubiquinol (\( \text{QH}_2 \)).
3. Cytochrome \( bc_1 \) Complex (Complex III - Component B): Ubiquinol diffuses through the membrane and delivers its electrons to Complex III.
This complex contains Cytochrome \( b \), Cytochrome \( c_1 \), and a Rieske iron-sulfur protein.
Through the Q-cycle, Complex III transfers electrons while translocation of protons across the membrane takes place.
4. Cytochrome C (Component D): Cytochrome c is a small, water-soluble peripheral membrane protein containing a heme prosthetic group. It is located on the outer surface of the inner mitochondrial membrane (the intermembrane space side).
It accepts a single electron from Complex III and transports it along the membrane surface to Complex IV (Cytochrome c oxidase).
Therefore, the correct chronological and physical sequence of electron transport is:
\[ \text{NADH dehydrogenase (A)} \rightarrow \text{Ubiquinone (C)} \rightarrow \text{Cytochrome } bc_1 \text{ complex (B)} \rightarrow \text{Cytochrome C (D)} \]

Step 3: Final Answer:
The correct sequential flow of electrons is represented by the order (A), (C), (B), (D).