Mitochondria: Structure and Functions

General Overview

• Mitochondria are small, bacteria-sized organelles found in nearly all eukaryotic cells.
• Approximately 2000 mitochondria per cell, occupying around 25% of the cell's volume.

Structure of Mitochondria

Membranes

• Outer Membrane:

  • Smooth, composed of equal parts of phospholipids and proteins.
  • Contains porins which allow molecules ≤ 5000 daltons to pass.
  • Freely permeable to small molecules and ions.

• Inner Membrane:

  • Complex structure, folded into cristae to increase surface area.
  • Contains ATP synthase and transport proteins, impermeable to all but O2, CO2, and H2O.
  • Rich in proteins, contains cardiolipin but little cholesterol.

Other Components

• Intermembrane Space:

  • Composition similar to cytoplasm but different protein content.

• Matrix:

  • Rich in enzymes for ATP production, contains mitochondrial DNA, ribosomes, and tRNAs.

Function of Mitochondria

• Primary function is energy production through ATP synthesis (oxidative phosphorylation).
• Maintain calcium ion concentration.
• Involved in synthesis of certain hormones (e.g., testosterone, estrogen).
• Liver mitochondria detoxify ammonia.
• Play roles in apoptosis (programmed cell death).

Mitochondrial DNA

• Mitochondria have their own DNA, separate from nuclear DNA.
• Human mitochondrial DNA is about 16,500 base pairs, containing 37 essential genes.
• mtDNA is maternally inherited, useful for tracing maternal lineage.
• Mutations in mtDNA can lead to diseases like exercise intolerance.

Mitochondrial Disease

• Results from mitochondrial failure, affecting energy production, leading to cell injury or death.
• Commonly affects brain, heart, liver, muscles, and other systems.

Historical Perspective

• First described as "bioblasts" by Richard Altmann in 1890.
• Term "mitochondrion" coined by Benda in 1897.
• Warburg in the 1920s linked mitochondria to oxidative reactions in cells.

Endosymbiont Theory

• Suggests mitochondria evolved from aerobic bacteria that entered symbiosis with anaerobic eukaryotes.
• Supported by mitochondrial DNA and double membrane structure.

Key Terms

• Cristae: Folds in the inner membrane.
• Cardiolipin: Uncommon lipid found in inner membrane.
• Oxidative Phosphorylation: ATP production process.
• Apoptosis: Programmed cell death.

This document provides a comprehensive overview of mitochondrial structure and function, highlighting its critical role in cellular metabolism and energy production.