Online College Courses for Credit

Electron Transport Chain

Electron Transport Chain

Author: Sophia Tutorial

This lesson will describe in detail the processes that occur in the ETC phase of cellular respiration.

See More
Fast, Free College Credit

Developing Effective Teams

Let's Ride
*No strings attached. This college course is 100% free and is worth 1 semester credit.

29 Sophia partners guarantee credit transfer.

312 Institutions have accepted or given pre-approval for credit transfer.

* The American Council on Education's College Credit Recommendation Service (ACE Credit®) has evaluated and recommended college credit for 27 of Sophia’s online courses. Many different colleges and universities consider ACE CREDIT recommendations in determining the applicability to their course and degree programs.


What's Covered

Welcome to today’s lesson on the electron transport chain. In this lesson, you are going to learn the processes that occur in the electron transport chain phase of cellular respiration:

  1. Overview
  2. Electron Transport Chain

1. Overview

Cellular respiration is the process in which cells make ATP, or adenosine triphosphate, which is energy for the cells. The electron transport chain is the third and final step of cellular respiration, and it occurs in the inner membrane of the mitochondria. The stages that come before it are glycolysis and the Krebs cycle.

You have some products from glycolysis being transferred into the electron transport chain, and you have some products from the Krebs cycle also being transferred into the electron transport chain. The chains of reaction occur using energy released by electrons to produce ATP.

The vast majority of ATP is being produced in this third step, and that energy is being produced by electrons. Each chain of reaction that occurs is called an electron transport system.

Term to Know

Electron Transport Chain

The third and final phase of cellular respiration in which electrons are used to produce 32 molecules of ATP.

2. Electron Transport Chain

As mentioned earlier, you have products from glycolysis and from the Krebs cycle that will enter into your electron transport chain.

Step by Step

Step 1: Electrons Transferred

Electrons that were picked up during glycolysis or the Krebs cycle are transferred to the electron transport chain.

Step 2: Hydrogen Release

Hydrogen picked up by molecules in the inner membrane are released to the outer membrane when they pick up and donate electrons. Remember, a mitochondrion has two membranes, an inner and an outer.

Step 3: Hydrogen Gradient

Molecules, when they pick up an electron, will also pick up the hydrogen. That hydrogen will move from the inner membrane to the outer membrane, while the electron will move on to somewhere else. This is going to produce the gradient of hydrogens in the outer membrane.

Step 4: Flow Through Enzymes

Once you have this gradient in the outer membrane, hydrogens are naturally going to start flowing back into the inner membrane to even out that concentration; they're going to flow through enzymes.

Step 5: ATP Formation

These enzymes will catalyze the formation of ATP from ADP and phosphate. We'll have ADP molecules which will then gain a phosphate and turn into ATP, which is energy, as the hydrogen moves through this enzyme.

Step 6: Form H2O

Oxygen molecules outside of the mitochondria will fuse with hydrogen molecules to form H2O. The electron is giving the hydrogen and oxygen energy in order to be able to bond and form H2O, which is water.

Big Idea

In this process, we are actually forming 32 molecules of ATP. If you remember back to glycolysis and the Krebs cycle, each of those is only forming two molecules of ATP. This stage of cellular respiration is forming the vast majority of ATP. If we add the ATP from glycolysis and the Krebs cycle, we would have a net total of 36 ATP produced by the end of cellular respiration.


This lesson has been a brief overview of the process of the electron transport chain, which is the third and final step of cellular respiration.

Keep up the learning and have a great day!


Terms to Know
Electron Transport Chain

The third and final phase of cellular respiration in which electrons are used to produce 32 molecules of ATP.