Chapter 4,5, & 8: Eukaryotic Organelles: Mitochondria and Chloroplasts

Semiautonomous Organelles in Eukaryotic Cells

• Overview: Eukaryotic cells have organelles termed semiautonomous, primarily mitochondria and chloroplasts.
  • These organelles can grow and divide but rely on cellular components for some internal structures.

Mitochondria

• Function: The primary role of mitochondria is ATP production through the breakdown of organic molecules.
  • Mitochondria convert chemical energy in covalent bonds (sugars, fats, amino acids) into a usable form of energy (ATP).
• Structure:
  • Mitochondria have a double membrane: outer membrane and highly folded inner membrane with folds called cristae.
  • Mitochondrial matrix: The compartment enclosed by the inner membrane where various biochemical reactions occur.
• Composition:
  • Mitochondria contain their own circular DNA (mitochondrial DNA) and ribosomes, providing evidence of their evolutionary origin.
• Importance of Size and Number:
  • Typical cells have hundreds to thousands of mitochondria, particularly muscle cells.
  • Exercise increases the number and size of mitochondria in muscle cells.

Chloroplasts

• Function: Chloroplasts are responsible for photosynthesis, converting light energy into chemical energy stored in organic molecules.
  • Found in plant and algal cells.
• Structure:
  • Enclosed by double membranes with an intermembrane space and a third thylakoid membrane forming thylakoid lumens.
  • Thylakoids stack in structures known as grana, crucial for light reactions of photosynthesis.
  • Stroma: The fluid-filled space between the inner membrane and the thylakoids where the Calvin cycle takes place.
• Types of Plastids:
  • Chloroplasts are a type of plastid; other plastids include chromoplasts (which contain pigments) and leucoplasts (which store starch).

Genetic Material and Reproduction

• Both mitochondria and chloroplasts possess genetic material and reproduce through a process similar to binary fission, akin to bacterial division.
  • This reproduces their circular DNA and separates into two organelles, ensuring proper distribution during cell division.
• Evolutionary Perspective:
  • Mitochondria and chloroplasts originated from an endosymbiotic relationship between early eukaryotic cells and specific bacteria (proteobacteria for mitochondria and cyanobacteria for chloroplasts).

Endosymbiotic Theory

• The presence of distinct genomes in mitochondria and chloroplasts supports their evolutionary origin from free-living bacteria.
• Historical Context:
  • Proposed by Andreas Schimper and later supported by various genetic and morphological studies.
  • Lynn Margulis revitalized this idea, leading to greater acceptance in the scientific community.
• Gene Transfer:
  • Over time, many genes from the organelles have been transferred to the nuclear genome, facilitating better control of metabolism and function since many mitochondrial and chloroplast proteins are now encoded by nuclear DNA.

Membrane Signaling and Transport

• Sorting Signals: Proteins synthesized in the cytosol contain signals that direct them to their proper cellular location, including mitochondria and chloroplasts.
• Cellular Communication (Systems Biology):
  • Cells must communicate and coordinate through signaling pathways to function effectively, especially in multicellular organisms.

Additional Considerations

• Heat Generation: Mitochondria also play a role in thermogenesis in specialized cells (brown fat) to maintain body temperature in certain organisms.
• Diseases: Mutations in mitochondrial DNA can lead to various diseases, especially affecting energy-demanding cells like neurons and muscles, e.g., Leber hereditary optic neuropathy.

Conclusion

• These organelles are vital for energy metabolism and overall cellular function in eukaryotic organisms. Their evolutionary origin underscores the complexity of eukaryotic cell evolution and function, marking significant steps in biological development on Earth.