Q.29

Cytochrome C is normally found in the inner mitochondrial membrane.
It is released into the cytoplasm during

Options:

(A) Apoptosis

(B) Necrosis

(C) Cell differentiation

(D) Cell proliferation

Cytochrome C serves dual roles: electron transport in mitochondrial respiration and apoptosis executioner. Normally embedded in the inner mitochondrial membrane within the intermembrane space, its release into cytoplasm marks a point-of-no-return in programmed cell death, activating the apoptosome.

The correct answer is (A) Apoptosis, where Bax/Bak pores in outer mitochondrial membrane allow cytochrome C translocation to cytosol, binding Apaf-1 → caspase-9 activation → effector caspases.

Why (A) Apoptosis Triggers Cytochrome C Release

Intrinsic apoptosis pathway (mitochondrial pathway) responds to DNA damage, growth factor withdrawal, or ER stress. Pro-apoptotic Bcl-2 family members (Bax, Bak, Bid, Bim) permeabilize outer mitochondrial membrane, releasing cytochrome C. In cytosol, it oligomerizes Apaf-1 + procaspase-9 + dATP into apoptosome, amplifying death signal.

Explanation of All Options

Each represents distinct cellular processes:

• (A) Apoptosis
  Correct. Cytochrome C release hallmark of intrinsic apoptosis pathway. Triggers caspase cascade via apoptosome formation.

• (B) Necrosis
  Wrong. Uncontrolled cell death from injury/toxins causes membrane rupture, spilling ALL contents indiscriminately. Cytochrome C release occurs passively during lysis, not as regulated signaling event.

• (C) Cell differentiation
  Incorrect. Terminal differentiation (e.g., myogenesis, neurogenesis) involves gene expression changes, not mitochondrial permeabilization or cytochrome C release.

• (D) Cell proliferation
  No. Cell division requires mitochondrial ATP production; cytochrome C stays membrane-bound for electron transport, not released.

Quick Process Comparison Table

Biotech relevance: Cytochrome C release assays measure apoptosis in drug screening. Cancer cells often resist via Bcl-2 overexpression, making BH3 mimetics (Venetoclax) key therapies. Memory trick: “C for Cytochrome, C for Caspase activation in Cell death (Apoptosis).”