Mitochondria: Structure and Functions

Overview

• Mitochondria are bacteria-sized organelles found in most eukaryotic cells.
• Typically, there are about 2000 mitochondria per cell, making up 25% of cell volume.

Structure of Mitochondria

• Enclosed by two membranes:
  • Outer Membrane: Smooth, has special proteins called porins that allow movement of molecules (5000 daltons or less).
  • Inner Membrane: Folded into cristae, more complex, strictly permeable to oxygen and ATP, involved in ATP production.
• Cristae: Increases surface area inside the organelle to aid ATP production.
• Intermembrane Space: Has a similar composition to the cell's cytoplasm but differs in protein content.
• Matrix: Contains proteins and enzymes essential for ATP synthesis, mitochondrial ribosomes, tRNAs, and mitochondrial DNA.

Mitochondrial Functions

• Produce energy by oxidizing substrates of the Krebs cycle.
• Involved in cellular activities like signaling, cellular differentiation, and cell cycle control.
• Affects human health (e.g., mitochondrial disorders, cardiac dysfunction).

Mitochondrial DNA (mtDNA)

• Contains about 16,500 DNA base pairs, coding for rRNAs, tRNAs, and 13 proteins.
• Essential for mitochondrial function and maternally inherited.
• Able to divide independently from the cell.
• Susceptible to mutations leading to diseases such as exercise intolerance.

Mitochondrial Diseases

• Result from mitochondrial failure, affecting organs like the brain, heart, liver, and muscles.

Functions of Mitochondria in Specific Cells

1. Energy production via oxidative phosphorylation.
2. Maintains concentration of calcium ions within cell compartments.
3. Helps build parts of blood/hormones (e.g., testosterone, estrogen).
4. Liver cell mitochondria contain enzymes to detoxify ammonia.
5. Play a role in apoptosis (programmed cell death).
6. Dysfunction can lead to abnormal cell death, affecting organ function.

Additional Historical Context

• Described by Richard Altmann as "bioblasts" in 1890; term "mitochondrion" coined by Benda in 1897.
• Early 20th-century chemist Warburg identified oxidative reactions in mitochondria.

Evolutionary Origin

• Likely evolved from aerobic bacteria entering into symbiosis with anaerobic eukaryotes (endosymbiont theory).
• Supported by mitochondrial structures, DNA, and ribosomes resembling prokaryotes.