BIOL 2200 Lecture 15: Living small: unicellular life

Lecture Overview

Learning Objectives

  • Summarize major evolutionary milestones in the history of life on Earth.

  • Describe the cellular structure of prokaryotes.

  • Describe how bacteria generate genetic diversity.

  • Compare prokaryotes to unicellular eukaryotes.

  • Explore ecological roles of unicellular organisms.

Evolutionary Milestones

Timeline of Life on Earth (BYA - billion years ago)

  • ~4.6 BYA: Formation of the solar system.

  • ~3.5 BYA: Conclusive evidence of prokaryotes; fossilized and modern stromatolites in Australia.

  • Before 2.7 BYA: Appearance of photosynthesis.

  • ~1.8 BYA: First eukaryotes; fossil algae discovered in 2022, dated ~1 billion years old.

  • ~1.2 BYA: First multicellular eukaryotes; example: Dickensonia costa, from Ediacaran fauna.

  • 535-525 MYA: Cambrian explosion, marked by a rapid increase in diversity and complexity of life forms.

Prokaryotic Cell Structure

External Structure

  • Cell Wall:

    • Bacteria: made of peptidoglycan.

    • Archaea: composed of polysaccharides and proteins.

  • Pili: Facilitates attachment and genetic exchange, longer hairy strands on outside surface

  • Flagella: Provides mobility, tail at the bottom

  • Fimbrae and Capsule: Assist in adhesion to surfaces

    • Fimbraie: shorter hairy on outside surface that allows bacteria to adhere to host cell

    • Capsule: the outermost layer of the cell wall that protects the structure from phagocytosis and etc.

Internal Structure

  • Aerobic Prokaryotes: Possess a respiratory membrane similar to mitochondria.

  • Photosynthetic Prokaryotes: Have thylakoid membranes similar to chloroplasts.

  • Chromosomes: Typically circular, with less genetic material than eukaryotes.

  • Plasmids: Small, circular DNA that can replicate independently.

Prokaryotic Survivability

  • Prokaryotes can thrive in extreme environments:

  • Archaea

    • Extreme Thermophiles: Live in extremely high temperatures.

    • Extreme Halophiles: Survive in high salt concentrations.

  • Endospores: Form as a bacterial response to harsh conditions; remain dormant until environmental conditions improve.

Genetic Diversity in Prokaryotes

  • Binary Fission: Primary method of reproduction, but does not contribute to genetic diversity.

  • Genetic Diversity Mechanisms:

    • Mutation: Random changes in the DNA sequence.

    • Recombination: Involves several processes:

      • Transformation: Uptake of foreign DNA from the environment.

      • Transduction: Bacteriophage (virus) facilitates DNA transfer between bacterial cells within host cell

      • Conjugation: Direct transfer of DNA via a pilus between cells.

Prokaryotes vs. Eukaryotes

Morphological Differences

  • Prokaryotes lack membrane-bound organelles and a defined nucleus.

    • They’re a lot more simple in structure

  • Eukaryotes possess organized compartments and a flexible or rigid cell wall.

    • A lot more complex and intricate due to membrane bound organelles with specialized functions

Key Innovations in Eukaryotes

  • Separation of cellular functions into organelles.

  • Multicellularity, particularly in protist lineages.

  • Cell walls that can be flexible or rigid

Unicellular Eukaryotes

Protists

  • Diverse group, mainly unicellular, found in various habitats.

  • Exhibit a wide range of mobility and resource acquisition methods.

Fungi

  • Heterotrophic organisms closely related to animals.

  • Exhibit various interaction modes, including mutualism and parasitism.

Ecology of Unicellular Organisms

  • Decomposers: Break down dead organic materials; includes bacteria and slime molds (fungi).

  • Oxygen Production: Cyanobacteria and algae function as primary producers.

  • Nitrogen Fixation: Cyanobacteria and methanogens convert atmospheric nitrogen (N2) into usable forms (e.g., NH3).