Lecture Notes: Cells - The Building Blocks of Life

Introduction

• Cells are fundamental units of life.
• The invention of the microscope allowed biologists to study cells, but progress was slow for the first 300 years due to their small size.
• Improvements in optics and the development of the electron microscope opened up new avenues for understanding cell functions.

Understanding Cells

• Cells are complex, organized chemical factories.
• Key questions remain about the origin of cells and their development in early life stages on Earth.

The Origin of Cells

• Prokaryotes: The simplest cells, such as bacteria, which lack a nucleus.
  • Term "prokaryote" means "before nucleus" (pro = before, karyote = nucleus).
  • Discovered fossils of bacteria-like cells in rocks dated to 3.5 billion years ago.
  • Modern bacteria in hot springs resemble these ancient life forms.

Evolution of Life Forms

• Timeline:
  • Earth originated around 4.6 billion years ago.
  • Evidence of life forms (bacteria) appears around 3.5 billion years ago.
  • 1.5 billion years later, signs of oxygen in rocks indicate changes in the atmosphere.
• Blue-Green Algae: New prokaryotic forms, capable of photosynthesis, introduced oxygen into the atmosphere.
  • This process was crucial for developing complex life forms.
  • Oxygen was initially poisonous to anaerobic organisms but beneficial for those that adapted.

Energy Utilization in Cells

• Bacterial cells produce 2 ATP per sugar molecule without oxygen.
• With oxygen, they can produce 18 ATP per sugar, enhancing energy utilization.
• Prokaryotes thrived as oxygen levels increased.

Emergence of Eukaryotic Cells

• Approximately 1 billion years ago, eukaryotic cells with nuclei appeared.
• Eukaryotes: More complex than prokaryotes, with internal structures.
  • Found in ponds today; represent a significant leap in cellular complexity.

Theories on Eukaryotic Evolution

• Chloroplasts: Key players in photosynthesis; can survive outside their host cells.
  • Evidence suggests chloroplasts may have originated from independent photosynthetic prokaryotes through symbiosis.
  • Some sea slugs can incorporate chloroplasts from algae they consume.
• Mitochondria: Energy production units similar to chloroplasts; may also have a symbiotic origin.

Symbiotic Theory

• Suggests that eukaryotic complexity arose from partnerships between different organisms.
  • E.g., primitive nucleated cells engulfing photosynthetic prokaryotes, leading to a mutual relationship.

Consequences of Eukaryotic Evolution

• Development of complex multicellular organisms.
• Humans are a very recent development in this timeline (a few million years ago).

Conclusion

• The story of cellular life is ongoing, with cell biologists continuing to uncover new findings about the origins and functions of cells.