Electron Transport Chain (ETC)

Introduction

The electron transport chain (ETC) is a crucial process in cellular respiration that occurs within the inner mitochondrial membrane of eukaryotic cells. The ETC consists of a series of protein complexes and other molecules. Its primary function is to transfer electrons from electron donors (such as NADH and FADH₂) to electron acceptors (usually molecular oxygen, O₂) via redox reactions. Simultaneously, it couples this electron transfer with the movement of protons (H⁺ ions) across the membrane. In this article we will see the electron transport chain (ETC) and its mechanism.

Table of Contents

The electron transport chain (ETC)

The ETC is a critical process that occurs within the inner mitochondrial membrane of eukaryotic cells. Its primary role is to transfer electrons from electron donors (such as NADH and FADH₂) to electron acceptors (usually molecular oxygen, O₂) via redox reactions. Simultaneously, it couples this electron transfer with the movement of protons (H⁺ ions) across the membrane.

Key Steps and Concepts

Electron Flow

Electrons flow through the ETC from higher to lower energy states.
As they move along the chain, they release energy.

Proton Pumping

The released energy is used to pump protons (H⁺ ions) across the inner mitochondrial membrane, creating an electrochemical proton gradient.
This gradient is essential for ATP synthesis.

ATP Synthesis

The enzyme ATP synthase utilizes the proton gradient to convert adenosine diphosphate (ADP) and inorganic phosphate (Pi) into adenosine triphosphate (ATP).
This process is known as oxidative phosphorylation.

Final Electron Acceptor

In aerobic respiration, the final electron acceptor is molecular oxygen (O₂).
Oxygen combines with electrons and protons to form water (H₂O).

Energy Yield

The ETC generates most of the cell’s ATP.
For every pair of electrons transferred from NADH to oxygen, approximately 3 ATP molecules are produced.
For FADH₂, the yield is slightly lower (around 2 ATP molecules per pair of electrons).

Electron Carriers

The ETC consists of several protein complexes (e.g., Complex I, II, III, IV) and small molecules (such as ubiquinone and cytochrome c).
These carriers shuttle electrons along the chain.

Electron Transport Chain (ETC) source: wikimedia

Pathway Summary

Complex I (NADH Dehydrogenase)

Receives electrons from NADH.
Pumps protons across the membrane.

Ubiquinone (Coenzyme Q)

A mobile electron carrier.
Transfers electrons between Complex I and Complex III.

Complex III (Cytochrome bc₁ Complex)

Receives electrons from ubiquinone.
Pumps more protons.

Cytochrome c

A small protein that shuttles electrons between Complex III and Complex IV.

Complex IV (Cytochrome c Oxidase)

Receives electrons from cytochrome c.
Transfers them to oxygen.
Forms water.

Why Oxygen?

Oxygen is the ideal electron acceptor due to its high electronegativity.
It efficiently captures electrons and allows the ETC to proceed.
Other electron acceptors (like fluoride) are less common and less accessible.

Summary

The electron transport chain (ETC) is a series of protein complexes located in the inner mitochondrial membrane. These complexes transfer electrons from donor molecules (such as NADH and FADH₂) to an acceptor molecule, usually molecular oxygen (O₂). As electrons move along the chain, protons (H⁺ ions) are pumped across the membrane, creating a proton gradient. This gradient drives the synthesis of adenosine triphosphate (ATP), the cell’s energy currency. The ETC plays a vital role in cellular respiration, generating most of the ATP needed for essential metabolic processes.