Question

In which part of the cell is cytochrome c located? (A) Mitochondrial matrix (B) Outer mitochondrial membrane (C) Inner mitochondrial membrane (D) Cytosol

Short answer

(C) Inner mitochondrial membrane

Step-by-step solution

- Identify the function and location of cytochrome c

Cytochrome c is a crucial protein involved in the electron transport chain, which is a series of reactions that generate ATP, the cell's energy currency.

- Determine where electron transport chain occurs

The electron transport chain takes place in the mitochondria, specifically along the inner mitochondrial membrane, where various enzymes and molecules (including cytochrome c) are embedded.

- Confirm cytochrome c's exact location

Among the options provided, cytochrome c is associated with the inner mitochondrial membrane where it plays a role in transferring electrons between Complex III (cytochrome bc1 complex) and Complex IV (cytochrome c oxidase).

- Select the correct answer

Considering the roles and specific mitochondrial locations of important parts of the electron transport chain, it becomes clear that cytochrome c is located in the inner mitochondrial membrane.

Key concepts

electron transport chain

The electron transport chain (ETC) is a series of protein complexes and other molecules embedded in the inner mitochondrial membrane. This chain is fundamental for cellular respiration. It helps in converting energy from nutrients into adenosine triphosphate (ATP), the cell's main energy currency. The process involves the transfer of electrons from electron donors to electron acceptors through redox reactions. As the electrons move down the chain, protons are pumped across the membrane, creating a proton gradient that powers ATP synthesis.
The key molecules in the ETC include:

• NADH dehydrogenase (Complex I)
• Succinate dehydrogenase (Complex II)
• Cytochrome c reductase (Complex III)
• Cytochrome c oxidase (Complex IV)

Cytochrome c is an essential protein in this chain. It transfers electrons between Complex III and Complex IV, playing a pivotal role in maintaining the chain's efficiency and functionality.

mitochondria

Mitochondria are known as the 'powerhouses' of the cell. They generate most of the cell's ATP through oxidative phosphorylation, which occurs in the inner mitochondrial membrane. Mitochondria have two membranes: an outer membrane that is relatively porous and an inner membrane that is much less permeable and contains numerous folds called cristae.
These folds increase the surface area, allowing for more space for processes like the ETC to occur. Mitochondria also contain their own DNA, which is distinct from the nuclear DNA, and can replicate independently within the cell. They are essential not just for energy production but also for regulating cell death (apoptosis), calcium storage, and other metabolic processes.

inner mitochondrial membrane

The inner mitochondrial membrane is crucial for mitochondrial function. It is the site of the electron transport chain and ATP synthesis. This membrane is unique due to its high protein-to-lipid ratio, which includes all the ETC complexes and ATP synthase. The inner membrane is impermeable to most ions and small molecules, which is essential for maintaining the proton gradient that drives ATP production.
The specific structure of the inner membrane, with its cristae, maximizes surface area, facilitating the numerous chemical reactions necessary for cellular respiration. The membrane's selective permeability is maintained by carrier proteins and channels, which control the movement of metabolites and ions.

cytochrome c function

Cytochrome c is a small heme protein associated with the inner mitochondrial membrane. It plays a pivotal role in the electron transport chain by transferring electrons between Complex III (cytochrome bc1 complex) and Complex IV (cytochrome c oxidase). This transfer is crucial for the continued flow of electrons, which is necessary for ATP production.
Beyond its role in electron transport, cytochrome c is also involved in the intrinsic pathway of apoptosis. During cellular stress or damage, cytochrome c is released into the cytosol from the mitochondria, where it activates caspases to initiate programmed cell death. This dual role makes cytochrome c essential for both energy production and the regulation of cell life cycles.