Prokaryotic Cell Basics

Domains of Prokaryotes

Prokaryotes are classified into two major domains: Bacteria and Archaea. Both domains consist of unicellular organisms that lack a membrane-bound nucleus.

• Bacteria: Most common prokaryotes, found in diverse environments.

• Archaea: Often found in extreme environments; differ from bacteria in cell wall composition and membrane lipids.

• Unicellular: Both domains consist of single-celled organisms.

Key Difference: Bacteria have peptidoglycan in their cell walls, while archaea do not.

Shapes and Arrangements of Prokaryotes

Prokaryotic cells exhibit a variety of shapes and arrangements, which are important for identification and classification.

• Coccus: Spherical shape

• Bacillus: Rod-shaped

• Spirillum: Spiral-shaped

• Arrangements: Chains (strepto-), clusters (staphylo-), pairs (diplo-)

• Example: Thiomargarita namibiensis is the largest known bacterium.

Binary Fission

Binary fission is the primary method of reproduction in prokaryotes. It is an asexual process in which a single cell divides to produce two identical daughter cells.

• DNA replication occurs first.

• The cell elongates and divides by forming a septum.

• Results in two genetically identical cells.

Extracellular Structures

Prokaryotic Plasma Membranes

The plasma membrane acts as a barrier and is composed of a phospholipid bilayer. It regulates the movement of substances in and out of the cell.

• Function: Selective permeability, energy generation, and cell signaling.

• Structure: Phospholipid bilayer with embedded proteins.

Cell Wall and Plasma Membrane as Barriers

• Cell Wall: Provides structural support and protection.

• Plasma Membrane: Controls transport and communication.

Comparison of Bacterial and Archaeal Plasma Membranes and Cell Walls

• Bacterial Cell Walls: Contain peptidoglycan; ester bonds link fatty acids to glycerol.

• Archaeal Cell Walls: Lack peptidoglycan; ether bonds link glycerol to isoprenoids.

Gram Staining and Cell Wall Types

Gram staining differentiates bacteria based on cell wall structure.

• Gram-positive: Thick peptidoglycan layer, retains crystal violet stain.

• Gram-negative: Thin peptidoglycan layer, outer membrane present, does not retain crystal violet.

• Acid-fast: Cell walls contain mycolic acids, resist decolorization.

Passive and Active Transport Mechanisms

Cells use various mechanisms to transport substances across membranes.

• Passive Transport: Movement without energy input (e.g., diffusion, osmosis).

• Active Transport: Requires energy (ATP) to move substances against their concentration gradient.

Surface Structures: Flagella, Fimbriae, Pili, Glycocalyx

• Flagella: Long, whip-like structures for motility.

• Fimbriae: Short, bristle-like structures for attachment.

• Pili: Longer, hair-like structures involved in attachment and conjugation.

• Glycocalyx: Protective outer layer; can be a capsule or slime layer.

Intracellular Structures

Nucleoid

The nucleoid is the region within a prokaryotic cell where the genetic material (DNA) is located. It is not surrounded by a membrane.

• Contains the cell's chromosome.

• Genetic material is typically a single, circular DNA molecule.

Prokaryotic Cytoskeleton

The cytoskeleton in prokaryotes is composed of protein filaments that provide structural support and shape to the cell.

• Helps maintain cell shape.

• Involved in cell division and chromosome segregation.

Prokaryotic Ribosomes

Ribosomes are the sites of protein synthesis. Prokaryotic ribosomes are smaller (70S) than eukaryotic ribosomes (80S).

• Composed of rRNA and proteins.

• Structural differences are targeted by certain antibiotics.

• Support the endosymbiotic theory.

Endospores

Endospores are metabolically inactive structures formed by some bacteria to survive harsh conditions.

• Highly resistant to heat, drying, radiation, and chemicals.

• Allow bacteria to remain dormant for extended periods.

• Present challenges in healthcare due to resistance to sterilization.

Endospore Formation Equation

Endospore formation is a complex process involving multiple steps, but can be summarized as:

Example: Bacillus and Clostridium species are well-known for endospore formation.

Additional info: The endosymbiotic theory suggests that mitochondria and chloroplasts originated from prokaryotic cells through symbiosis.