Crash Course Biology: Unicellular Organisms

Introduction

• Discussion on single-celled organisms.
• Single-celled organisms make up two of the three taxonomic domains and one of the four kingdoms.
• Focus on Archaea, Bacteria, and Protists.
• Unicellular organisms are the most abundant and diverse on Earth.
• They have ancient and earliest living lineages.

Importance of Understanding Unicellular Organisms

• Understanding these organisms helps in understanding life on Earth and its origins.
• Some thrive in extreme environments, while others cause diseases.
• Some are essential for processes like nitrogen fixation and digestion.

Unicellular Organism Categories

Archaea

• Oldest known living organisms, over 3.5 billion years old.
• Found in extreme environments such as hydrothermal vents, volcanic springs, and oil wells.
• Methanogens: Archaea that produce methane; found in moderate environments like mud and intestines.
• Extremophiles: Include:
  • Thermophiles: Thrive at high temperatures (e.g., Pyrolobus fumarii).
  • Halophiles: Thrive in high-salt environments.

Bacteria

• Nearly as ancient as Archaea.
• Make up the majority of prokaryotes.
• Capable of horizontal gene transfer, contributing to antibiotic resistance.
• Categories by Cell Membrane:
  • Gram-positive: Thick membranes; include Staphylococcus and Streptococcus.
  • Gram-negative: Thin membranes; include Proteobacteria, Cyanobacteria, Spirochetes, and Chlamydias.

Protists

• Evolved about 1.7 billion years ago.
• More complex, include some multicellular and sexually reproducing members.
• Categories:
  • Protozoa: Animal-like, heterotrophs with movement mechanisms (e.g., flagella, cilia).
  • Algae: Plant-like, photosynthesize using various chlorophylls.
    • Sailor’s Eyeball: A large single-celled algae.
    • Types of algae: red, green, and brown.
  • Fungus-like: Include slime molds, which absorb nutrients and move like amoebae.

Conclusion

• Unicellular organisms, though diverse and sometimes bizarre, are crucial to understanding life.
• They have a variety of roles from causing diseases to making life possible through ecological contributions.

Additional Resources

• For more information, refer to the Crash Course Biology episodes and related resources.