Question

Put the following substances in the correct order of their action in the electron-transport chain: cytochrome \(c\), coenzyme \(\mathrm{Q},\) and \(\mathrm{NADH}\).

Short answer

\( \mathrm{NADH}, \mathrm{Coenzyme\ Q}, \mathrm{cytochrome\ c} \)

Step-by-step solution

Identify Entry Points

The electron transport chain begins with electron donors. In this case, \( \mathrm{NADH} \) serves as an initial electron donor. Therefore, \( \mathrm{NADH} \) is the first substance to act in the electron-transport chain.

Understand Electron Carriers

After \( \mathrm{NADH} \) donates electrons, these electrons are transferred through various carriers. \( \mathrm{Coenzyme\ Q} \) (also known as ubiquinone) is one of the first major mobile carriers that accepts electrons from the complexes where \( \mathrm{NADH} \) donates them.

Identify Final Electron Carrier in the List

Following \( \mathrm{Coenzyme\ Q} \), electrons are eventually transferred to \( \mathrm{cytochrome\ c} \), which is a soluble electron carrier that passes electrons to the next component of the chain before reaching the final electron acceptor, oxygen.

Determine Order

Based on electron transfer progression, the correct order of action in the electron transport chain is \( \mathrm{NADH}, \mathrm{Coenzyme\ Q}, \mathrm{cytochrome\ c} \).

Key concepts

NADH in the Electron Transport Chain

NADH, or nicotinamide adenine dinucleotide, is a crucial player in the process of cellular respiration. It acts as an electron donor in the electron transport chain (ETC). When molecules like glucose are broken down through glycolysis, NADH is formed as a byproduct. This molecule carries electrons, which are high-energy carriers, necessary for the ETC to proceed.

Here's how NADH works in the ETC:

• NADH is produced in processes such as glycolysis and the citric acid cycle.
• Once NADH reaches the ETC, it donates its electrons to Complex I, also known as NADH dehydrogenase.
• This donation initiates the movement or 'transport' of electrons through the chain, sparking a series of reactions that ultimately lead to ATP production.

The significance of NADH lies in its role as a primary electron donor—it's essentially the "starter" of the ETC. This donation also helps to create the necessary proton gradient across the mitochondrial membrane, which is vital for producing energy-rich ATP molecules through oxidative phosphorylation.

Role of Coenzyme Q

Coenzyme Q, sometimes referred to as ubiquinone, serves as a pivotal electron carrier within the electron transport chain. Located within the mitochondrial inner membrane, it plays a unique role due to its ability to shuttle electrons between different complexes of the ETC.

Important aspects of Coenzyme Q include:

• Coenzyme Q receives electrons from both Complex I (from NADH) and Complex II (from FADH2).
• It is a mobile carrier, meaning it can diffuse freely through the membrane to transfer electrons between complexes.
• This mobility allows Coenzyme Q to deliver electrons to Complex III, where they are further processed and transported along the chain.

Coenzyme Q's role is essential because it connects different segments of the electron transport chain, ensuring continuous flow and eventual transfer of electrons to oxygen. It also participates in the proton pumping across the membrane, contributing to the creation of the electrochemical gradient needed to synthesize ATP.

Understanding Cytochrome c

Cytochrome c is a small, yet significant component of the electron transport chain. It is a highly conserved protein, which means it has retained a similar structure across different species due to its crucial role in cellular respiration. Cytochrome c acts as an intermediary electron carrier after Coenzyme Q.

Here's more about cytochrome c:

• It receives electrons from Complex III, also known as cytochrome bc1 complex.
• As a soluble protein located in the intermembrane space of mitochondria, cytochrome c transfers electrons to Complex IV (cytochrome c oxidase).
• This transfer is pivotal for the final stages of electron passage, leading ultimately to the reduction of oxygen to water.

Cytochrome c's ability to move electrons quickly and its role in linking larger protein complexes are vital. It ensures that the electron transport chain operates efficiently, contributing directly to the energy production in cells by passing electrons towards the terminal electron acceptor, oxygen.