Lecture Notes: Origin and Evolution of Cells

Introduction

• Focus on the origin and evolution of cells.
• Discussion of evidence suggesting prokaryotes were the first cells, followed by eukaryotes.
• Introduction to the Endosymbiosis Theory.

Endosymbiosis Theory

• Definition:
  • Endo: Inside
  • Symbiosis: Working together
• Describes the formation of compartmentalized eukaryotic cells from engulfed prokaryotic cells.
• Focus Organelles: Mitochondria and Chloroplast.

Process of Endosymbiosis

• Early eukaryotes engulfed prokaryotes:
  • Heterotrophic Prokaryotes: Consume nutrients.
  • Autotrophic Prokaryotes: Manufacture own carbon compounds.
• Engulfed prokaryotes evolved into mitochondria and chloroplasts.

Evidence Supporting Endosymbiosis

• Mitochondria and chloroplasts have:
  • Their own circular DNA, similar to prokaryotes.
  • 70S ribosomes, like prokaryotes.
  • Ability to synthesize own proteins.
  • Independent reproduction via binary fission, similar to prokaryotes.
  • Double membrane structure: one from original prokaryote, one from engulfing process.

Cell Differentiation and Multicellularity

• Cell Differentiation: Process in multicellular organisms allowing various cell types.
• Embryonic Development:
  • Stem cells are undifferentiated.
  • Genes expression varies, leading to cell specialization.
  • Some genes are universally expressed (e.g., for ribosome production).

Evolution of Multicellularity

• Pathway: Started with prokaryotes → unicellular eukaryotes → multicellular organisms.
• Multicellularity present in all animals, all plants, some fungi, and algae.

Characteristics of Multicellular Organisms

• Specialized cells performing specific functions.
• Specialized cells cannot survive independently outside the organism.
• Advantages:
  • Longer lifespans.
  • Ability to grow to different sizes and exploit various ecological niches.
  • Efficiency in task accomplishment due to specialized roles.

Unicellularity vs. Multicellularity

• More unicellular organisms exist on Earth than multicellular ones.
• Unicellularity also has advantages, and there are scenarios where it remains more advantageous.

Conclusion

• Demonstration of unity and diversity in biological evolution through the differentiation and specialization of cells.