Complex III

Definition

Complex III, also known as the cytochrome bc1 complex, is a crucial component of the electron transport chain located in the inner mitochondrial membrane. It plays a vital role in oxidative phosphorylation by facilitating the transfer of electrons from ubiquinol (QH2) to cytochrome c, while simultaneously pumping protons (H+) across the membrane to create a proton gradient. This gradient is essential for ATP synthesis during cellular respiration.

5 Must Know Facts For Your Next Test

1. Complex III contains several key components, including cytochromes b and c1, and an iron-sulfur protein known as the Rieske center.
2. The transfer of electrons through Complex III is coupled with the translocation of protons into the intermembrane space, contributing to the proton motive force necessary for ATP production.
3. Complex III operates through a process called the Q-cycle, which allows for the efficient transfer of electrons and the pumping of protons in two separate phases.
4. Inhibition of Complex III can lead to decreased ATP production and increased generation of reactive oxygen species, which can be damaging to cells.
5. Complex III is essential for cellular respiration, as it connects two major pathways: it receives electrons from both Complex I and Complex II and transfers them to Complex IV.

Review Questions

• How does Complex III facilitate the transfer of electrons within the electron transport chain?
  • Complex III facilitates electron transfer by accepting electrons from ubiquinol (QH2) and transferring them to cytochrome c. This process is critical as it allows for a seamless flow of electrons through the electron transport chain. In doing so, Complex III not only moves electrons but also contributes to creating a proton gradient across the mitochondrial inner membrane by pumping protons into the intermembrane space.
• Discuss the role of the Q-cycle in the function of Complex III and its impact on ATP production.
  • The Q-cycle is an essential mechanism by which Complex III operates, allowing it to efficiently handle electrons from ubiquinol while simultaneously pumping protons. This cycle consists of two phases: one that reduces cytochrome c while releasing protons and another that oxidizes ubiquinol while taking in protons. By maintaining a proton gradient across the inner mitochondrial membrane, the Q-cycle plays a direct role in driving ATP synthesis through ATP synthase, highlighting its significance in energy production.
• Evaluate how malfunctions or inhibition of Complex III can affect cellular respiration and overall cellular health.
  • Malfunctions or inhibition of Complex III can severely impact cellular respiration by reducing ATP production and increasing reactive oxygen species (ROS) levels. Without proper functioning of Complex III, there is a bottleneck in electron transport, leading to insufficient energy for cellular processes. The accumulation of ROS can lead to oxidative stress, damaging cellular components and contributing to various diseases. Thus, maintaining the integrity of Complex III is crucial for optimal cellular function and overall health.

Related terms

Electron Transport Chain: A series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions, creating a proton gradient that drives ATP synthesis.

Ubiquinone:

Also known as coenzyme Q, it is a lipid-soluble molecule that transports electrons between Complex I or II and Complex III in the electron transport chain.

Cytochrome c: A small heme protein found in the intermembrane space of mitochondria that carries electrons from Complex III to Complex IV in the electron transport chain.