Electron Transport Chain - Online Tutor, Practice Problems & Exam Prep

The electron transport chain (ETC) is a crucial step in aerobic respiration, involving a series of redox reactions that transfer electrons from NADH and FADH₂ to oxygen, the final electron acceptor. This process occurs in the inner mitochondrial membrane, creating a proton (H⁺) gradient that drives ATP synthesis via ATP synthase. Key complexes (I, II, III, IV) facilitate electron transfer and proton pumping, while coenzyme Q and cytochrome c serve as electron carriers. The overall reaction culminates in the formation of water from oxygen and protons.

Intro to Electron Transport Chain Concept 1

In this video, we're going to take a look at the electron transport chain. The electron transport chain, abbreviated as ETC, is one of the last steps of aerobic respiration, in catabolism. We're going to say that it is a series of redox reactions that harness the energy of electrons from NADH and FADH₂ coenzymes where they're acting as electron carriers. NADH and FADH₂ are produced within the citric acid cycle.

Here we have our electron transport chain, our cytoplasm, then we have our outer membrane, and within that, our inner membrane. So between them is our intermembrane space. After that, we have our mitochondrial matrix. Within this image, we have these free-floating H⁺ ions within the intermembrane space. Embedded within our inner membrane, we have these structural components representing complexes. So we have complexes 1, 2, 3, and 4. We also have a coenzyme, coenzyme Q, and then we have CYTC, essentially cytochrome c, which acts as a multifunctional protein and another electron carrier within the electron transport chain.

What we have here is the movement of NADH towards complex 1 where it drops off electrons for complex 1. Here we have NAD⁺ being created because NADH has given away its electrons. This also acts as a pump that pumps H⁺ ions to the other side into the intermembrane space. We also have FADH₂ which dumps electrons to complex 2 and becomes FAD. It is next to the coenzyme Q, and it doesn't pump protons into the intermembrane space. Our complex 3 is also pumping H⁺ ions into the intermembrane space.

We have cytochrome c, which is helping to facilitate electron transfer from complex 3 to complex 4. Complex 4 is also pumping H⁺ ions to the intermembrane space and, as these electrons are getting transferred, they are given to O₂ which eventually will get reduced into water. This is essentially a quick overview of the steps of the ETC. We're creating a gradient where there's a buildup of positive charges in the intermembrane space, and then we have these electrons that are traveling along from complex 1 to complex 4, working with this concept of redox reactions, creating different types of charge buildups on different sides. Remember, we're going to talk about how this will lead eventually to the production of ATP.

Intro to Electron Transport Chain Example 1

Which of the following correctly describes the electron transport chain? Here, the harnessing of energy from high-energy electrons from the Krebs Cycle is considered. That's true because, remember, the production of NADH and FADH₂ represent high-energy molecules. They get shuttled over to the electron transport chain, and we harness their energy from their electrons to eventually generate ATP. So this is true.

The breakdown of NADH and FADH₂ to carbon dioxide is incorrect. They just pass along their electrons, become oxidized, and return back to their NAD⁺ and FAD structures. So, they're not being changed into carbon dioxide.

Oxidation of NADH and FADH₂ coenzymes is correct. They give up their electrons, with NADH giving its electrons to Complex 1 to become NAD⁺ again, and FADH₂ hands over its electrons to Complex 2 to become FAD again. Since they are losing electrons, they are being oxidized.

Redox reactions are facilitated by enzyme complexes and electron carriers located in the inner membrane of the cell. So, remember, this happens in different places: it's within the inner membrane, yes, with the embedded enzyme complexes. However, NADH and FADH₂ are part of the mitochondrial matrix. This description does not take that location into account fully. So this is not completely true.

Here, the answer would be options A and option C.

Electron Transport Chain Process Concept 2

Now, the electron transport chain uses energy from electrons to generate H⁺ ion gradient by pumping H⁺ ions into the intermembrane space. Now, here as we're pumping these H⁺ ions into the intermembrane space, we're increasing the concentration of H⁺ ions, which would cause a decrease in our pH.

Now, here, if we take a look and break this up into 7 steps. So in step 1, we're saying that NADH transfers electrons to complex 1. So NADH here is handing over electrons to complex 1, becoming oxidized, and regenerating NAD⁺. In step 2, FADH₂ transfers electrons to complex 2 and it becomes FAD. It's being oxidized as well. Complex 1 and complex 2 will transfer electrons to Coenzyme Q. So they're being transferred here to coenzyme Q. Coenzyme Q then takes those electrons and transfers them to complex 3. Now, complex 3 transfers them to CYTC, which is just cytochrome c. This is just a multi-functional protein that acts as an electron carrier as well within this whole process. Then we're gonna say cytochrome c transfers electrons to complex 4, and then we're gonna say here that, finally, our electrons will be accepted by oxygen or O₂. And we're gonna say here, oxygen acts as the final electron acceptor. And we're gonna have it eventually being reduced into water.

Now, notice we have H⁺ being pumped by complex 1, being pumped by complex 3, being pumped by complex 4. It doesn't happen with complex 2 here. Its complex doesn't move into the intermembrane space like the other complexes. So, note only complexes 1, 3, and 4 pump H⁺ into the intermembrane space. So, basically, this is the chain of the movement of H⁺ ions as well as electrons from complex 1 all the way to complex 4 to eventually having O₂ being the final electron acceptor.

Electron Transport Chain Process Concept 3

Now when it comes to the final electron acceptor, the final molecule that accepts the ETC's electrons is oxygen gas. We're going to say here that oxygen gas interacts with the hydronium ions or H⁺ ions in order to form water. When it comes to the ETC, the summary of the important reactions are, from complex 1, we have NADH plus a hydronium ion plus coenzyme Q. This would give us NAD⁺. Here, NADH has been oxidized, and coenzyme Q gains 2 hydrogens in the process and it becomes coenzyme QH₂. For complex 2, remember this is where FADH₂ drops off its electrons. We're going to say it interacts with Coenzyme Q. We're going to say here when it gives away its 2 hydrogens or electrons here, we're going to have FAD. And then Coenzyme Q again picks up 2 hydrogens. And then finally, for the matrix itself, we have 4 hydronium ions plus 4 electrons interacting with oxygen gas. As a result, at the end we get 2 moles of water. So, these represent the summary of our ETC reactions.

FADH₂ can be oxidized at both complex I and II in the inner membrane of mitochondria. (T/F)

• A. True
• B. False

Write a reaction representing oxidation of NADH coenzyme.

• A. NADH₂ + CoQ → NAD⁺ + CoQH₂
• B. NADH+ H⁺ + Cyt c → NAD⁺ + Cyt cH₂
• C. NADH + H⁺ + CoQ → NAD⁺ + CoQH₂
• D. NAD⁺ + H → NADH + CoQ

Protons are pumped into the intermembrane space due to:

• A. Energy provided by the electrons as they pass through complexes I, III & IV.
• B. Decreasing of the pH in the intermembrane space.
• C. CoQ pumping electrons through the complexes.
• D. Formation of water from final electron acceptor.

The mobile electron carriers of the ETC are:

• A. Complex I & II 
• B. Complex III & IV 
• C. NADH & FADH₂ 
• D. CoQ & Cyt c 

Here’s what students ask on this topic:

What is the electron transport chain and where does it occur?

The electron transport chain (ETC) is a series of redox reactions that transfer electrons from NADH and FADH₂ to oxygen, the final electron acceptor. This process occurs in the inner mitochondrial membrane. The ETC creates a proton (H⁺) gradient across the membrane, which drives ATP synthesis via ATP synthase. Key complexes (I, II, III, IV) facilitate electron transfer and proton pumping, while coenzyme Q and cytochrome c serve as electron carriers. The overall reaction culminates in the formation of water from oxygen and protons.

What are the main steps of the electron transport chain?

The main steps of the electron transport chain are as follows: 1) NADH transfers electrons to Complex I, regenerating NAD⁺. 2) FADH₂ transfers electrons to Complex II, regenerating FAD. 3) Complexes I and II transfer electrons to coenzyme Q. 4) Coenzyme Q transfers electrons to Complex III. 5) Complex III transfers electrons to cytochrome c. 6) Cytochrome c transfers electrons to Complex IV. 7) Complex IV transfers electrons to oxygen, the final electron acceptor, forming water. Complexes I, III, and IV pump H⁺ ions into the intermembrane space, creating a proton gradient.

What role do NADH and FADH₂ play in the electron transport chain?

NADH and FADH₂ are crucial electron carriers in the electron transport chain. NADH donates electrons to Complex I, becoming oxidized to NAD⁺. FADH₂ donates electrons to Complex II, becoming oxidized to FAD. These electrons are then transferred through a series of complexes and carriers, ultimately reaching oxygen, the final electron acceptor. The energy released during these transfers is used to pump H⁺ ions into the intermembrane space, creating a proton gradient that drives ATP synthesis.

How does the electron transport chain contribute to ATP synthesis?

The electron transport chain contributes to ATP synthesis by creating a proton (H⁺) gradient across the inner mitochondrial membrane. As electrons are transferred through the chain, energy is released and used to pump H⁺ ions from the mitochondrial matrix into the intermembrane space. This creates a high concentration of H⁺ ions in the intermembrane space. The resulting electrochemical gradient drives H⁺ ions back into the matrix through ATP synthase, a process known as chemiosmosis. The flow of H⁺ ions through ATP synthase provides the energy needed to convert ADP and inorganic phosphate (P_i) into ATP.

What is the final electron acceptor in the electron transport chain and what is its significance?

The final electron acceptor in the electron transport chain is oxygen (O₂). Oxygen's role is crucial because it accepts electrons at the end of the chain, allowing the entire process to continue. When oxygen accepts electrons, it combines with protons (H⁺) to form water (H₂O). Without oxygen, the electron transport chain would halt, preventing the creation of the proton gradient necessary for ATP synthesis. This is why oxygen is essential for aerobic respiration and energy production in cells.