Mitochondria

Mitochondria are double-membrane-bound organelles found in the cytoplasm of almost all eukaryotic cells. Often referred to as the “Powerhouse of the Cell,” they are the primary sites of aerobic respiration. They are unique among organelles because they are semi-autonomous, possessing their own genetic material and the machinery to synthesize proteins.

Structural Composition

The mitochondrion exhibits a sophisticated internal architecture designed to maximize the surface area for chemical reactions.

1. Mitochondrial Envelope

• Outer Membrane: Smooth and highly permeable due to the presence of integral proteins called porins.
• Inner Membrane: Deeply folded into finger-like projections called cristae. It is selectively permeable and contains the enzymes for the Electron Transport Chain (ETC) and ATP synthesis.
• Intermembrane Space: The narrow space between the two membranes where protons (H^+) accumulate during respiration.

2. The Matrix

• The dense, semi-fluid central compartment enclosed by the inner membrane.
• It contains the enzymes required for the Krebs Cycle (TCA Cycle).
• It houses a single, circular dsDNA molecule, a few RNA molecules, and 70S ribosomes.

3. F₀-F₁ Particles (Oxysomes)

• These are tennis-racket-shaped particles attached to the inner mitochondrial membrane (cristae).
• The F₁ head acts as an ATP synthase enzyme, catalyzing the production of ATP from ADP and inorganic phosphate.

Key Functions of Mitochondria

• ATP Production: They produce the majority of the cell’s Adenosine Triphosphate (ATP), the universal “energy currency” of the cell.
• Cellular Metabolism: They participate in the breakdown of carbohydrates and fats through the Krebs Cycle and β-oxidation.
• Apoptosis: Mitochondria play a critical role in programmed cell death by releasing Cytochrome c, which triggers cell destruction.
• Calcium Homeostasis: They act as storage sites for calcium ions, helping to regulate the concentration of calcium in the cell signaling pathways.
• Heat Production: In specialized tissues like “Brown Fat,” mitochondria produce heat instead of ATP (thermogenesis) to maintain body temperature in infants and hibernating animals.

Aerobic Respiration: The Three Stages

Genetic and Evolutionary Significance

• Endosymbiotic Theory: Mitochondria are believed to have evolved from an ancient Alpha-proteobacterium that was engulfed by a primitive eukaryotic cell. This is supported by their circular DNA and 70S ribosomes (similar to bacteria).
• Maternal Inheritance: In humans and most animals, mitochondria are inherited exclusively from the mother (via the ovum). This property is used in evolutionary biology to trace maternal lineages (Mitochondrial Eve).
• Self-Replication: Mitochondria do not arise de novo; they reproduce by binary fission, independent of the cell nucleus.

UPSC Prelims Fact File and Trivia

• Mitochondrial DNA (mtDNA): It is small and lacks the protective histones found in nuclear DNA, making it more susceptible to mutations.
• Number of Mitochondria: Varies depending on the metabolic activity of the cell. Heart muscle cells and sperm tail cells contain thousands of mitochondria, while mature human RBCs contain none.
• Mitochondrial Diseases: Defects in mitochondrial genes can lead to disorders like Leber’s Hereditary Optic Neuropathy (LHON).
• Three-Parent Baby: A specialized IVF technique (Mitochondrial Replacement Therapy) where faulty mitochondrial DNA of the mother is replaced by healthy mtDNA from a donor to prevent genetic diseases.
• Vital Stain: Janus Green B is a specific vital stain used to observe mitochondria under a microscope as it remains green in an oxidized state.