Describe ultrastructure of mitochondria

Mitochondria are double-membrane bound organelles often called the powerhouses of the cell. Each mitochondrion consists of the following components:

1. Mitochondrial Membranes

Mitochondria are enclosed by two unit membranes, each about 60–70 Å thick, separated by an intermembrane space of 60–100 Å.

• Outer membrane: Smooth, continuous, and similar in structure to the smooth endoplasmic reticulum. It is freely permeable to many small metabolites and contains various enzymes.
• Inner membrane: Folded into finger-like projections called cristae. These folds increase the surface area and divide the matrix into interconnected compartments.

2. Cristae

The inner membrane forms cristae, whose number and arrangement vary across mitochondria. They may run parallel, perpendicular, or concentrically within the organelle. The size and number of cristae directly influence mitochondrial efficiency.

3. Oxysomes (F₀–F₁ Particles)

Special particles called oxysomes or elementary particles are present on the inner membrane and cristae at intervals of about 100 Å.

• Each mitochondrion has ~10⁴ to 10⁵ oxysomes.
• Each oxysome has three parts: a base, a stalk (50 Å long, 33 Å thick), and a head (~100 Å in diameter).
• They contain ATP synthase (ATPase enzyme) and play a key role in ATP production during the electron transport chain.

4. Mitochondrial Chambers

• Outer chamber (intermembranous space): The space between the two membranes, filled with fluid containing proteins and enzymes.
• Inner chamber: The large central area enclosed by the inner membrane, filled with mitochondrial matrix.

5. Matrix

The mitochondrial matrix contains:

• Proteins, lipids, RNA, and 70S ribosomes (mitoribosomes).
• 2–6 circular DNA molecules (~5 µm long, double-stranded, highly twisted). Mitochondrial DNA is self-replicating and functions like a chromosome.

6. Chemical Composition

Mitochondria are composed of:

• 65–70% proteins
• 25–30% lipids
• 0.5% RNA
• Trace amounts of DNA
• ~60 types of enzymes involved in metabolism and energy production.