Prokaryotes


Prokaryotes Overview

  • Prokaryotes are represented by two distinct domains:

    • Domain: Bacteria

    • Domain: Archaea

  • Size Range: Prokaryotic cells typically range in size from 0.5 to 5 micrometers (µm).

  • Prokaryotes exhibit three main shapes:

    • Spherical (cocci)

    • Rod-shaped (bacilli)

    • Spiral (spirilla)

  • The cell wall plays a crucial role in maintaining the shape and protecting the cell from environmental stresses.

Evolution of the Cell Wall

  • Bacterial Cell Walls: Composed of modified sugars linked to short polypeptides, which together form peptidoglycan.

  • Eukaryotic Cell Walls: Primarily consist of chitin (in fungi) and cellulose (in plants).

  • Archaea Cell Walls: Made of a combination of sugars and proteins, differing significantly from bacterial compositions.

Bacteria: The Gram Stain

  • The Gram Stain Test: A laboratory technique used to classify bacteria based on the structural differences of their cell walls.

    • Gram-positive bacteria retain the crystal violet dye, appearing purple, while Gram-negative bacteria do not retain the dye and appear pink after counterstaining.

Bacteria: Adaptations

  • Many prokaryotes possess additional features for survival and adaptation:

    • Capsule: A layer of polysaccharides and proteins surrounding the cell wall, aiding in attachment to surfaces.

    • Endospore Formation: A survival strategy allowing prokaryotes to endure extreme conditions.

    • Fimbriae: Hair-like projections that enhance attachment to surfaces.

    • Flagella: Structures that enable mobility, allowing prokaryotes to respond to stimuli via taxis (movement towards or away from stimuli).

    • Membrane Infoldings: Internal structures that can perform specialized functions such as respiration and photosynthesis.

Bacteria: Reproduction

  • Asexual Reproduction: Occurs through a process known as binary fission, which produces clones of the parent cell.

  • Sexual Reproduction: Involves the following mechanisms for genetic recombination:

    • Transformation: Uptake of foreign DNA from the environment.

    • Transduction: The transfer of genes between bacteria via bacteriophages (viruses that infect bacteria).

    • Conjugation: Direct transfer of genetic material between two prokaryotes.

  • Prokaryotes also exhibit mutations that contribute to genetic variability.

Sexual Reproduction Mechanisms

  • Transformation:

    • Defined as the uptake of foreign DNA from the surrounding environment.

    • Example: A non-pathogenic strain of bacteria may become pathogenic upon acquiring DNA from a pathogenic strain.

  • Transduction:

    • Defined as the movement of genes between bacteria facilitated by bacteriophages (viruses).

  • Conjugation:

    • Defined as the transfer of genetic material directly between two prokaryotic cells.

    • Fertility Plasmid (F plasmid): A genetic element that promotes the formation of the F+ cell when conjugated with another cell.

    • Hfr Cell: A high-frequency recombination cell that can lead to the formation of a recombinant F- cell when it transfers genes during conjugation.

Bacteria: Nutritional Modes

  • Prokaryotes exhibit various feeding modes that allow them to exploit diverse nutritional resources.

    • Autotrophs: Organisms capable of producing their own organic nutrients.

    • Heterotrophs: Organisms that must consume at least one organic nutrient from other organisms.

Bacteria: Metabolic Adaptations

  • Oxygen-based Metabolism:

    • Prokaryotes can be classified as:

    • Obligate Aerobes: Require oxygen for survival.

    • Obligate Anaerobes: Cannot survive in the presence of oxygen and may use alternative electron acceptors

    • Facultative Aerobes: Can switch between aerobic and anaerobic pathways according to oxygen availability.

  • Nitrogen-based Metabolism:

    • Many prokaryotes perform nitrogen fixation, converting inorganic nitrogen gas

  • Metabolic Cooperation:

    • Some prokaryotes can form filamentous chains, allowing specialized functions in different cells.

    • Heterocysts enable nitrogen fixation that would otherwise be inhibited by oxygen-driven pathways.

Biofilms

  • Prokaryotic communities can form biofilms, surface coatings created by the attachment and interaction of multiple species of prokaryotes.

    • Biofilms illustrate cooperation and communication within and among different species.

    • They are prevalent in natural environments and can have both beneficial and harmful effects.

Prokaryotes: Two Domains

  • Domain Bacteria:

    • Includes five major groupings:

    • Proteobacteria

    • Chlamydias

    • Spirochetes

    • Cyanobacteria

    • Gram-positive bacteria

  • Domain Archaea:

    • Shares characteristics with both bacteria and eukaryotes and can thrive in extreme environments.

    • Includes five major groupings:

    • Euryarchaeotes

    • Thaumarchaeota

    • Algarchaeota

    • Crenarchaeota

    • Korarchaeota

Prokaryotes: Importance

  • Chemical Recycling: Prokaryotes play a vital role in decomposing organic matter, recycling essential chemicals (e.g., carbon CC, nitrogen NN, oxygen OO, sulfur SS, phosphorus PP , potassium KK, etc.) back into ecosystems.

  • Ecological Interactions:

    • Prokaryotes engage in various interactions within their ecological communities:

    • Mutualism: Both species benefit.

    • Commensalism: One species benefits, the other is unaffected.

    • Parasitism: One species benefits at the expense of the other.

  • Human Interactions:

    • Prokaryotes can have both beneficial effects (e.g., aiding in nutrient absorption) and pathogenic impacts (e.g., producing harmful toxins).