Prokaryotic Cell Structure and Function: Chapter 3 Study Notes

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.

Structure	Main Function	Location
Flagella	Motility	External
Fimbriae	Attachment	External
Pili	Attachment, DNA transfer	External
Glycocalyx	Protection, evasion of immune system	External

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:

$\text{Vegetative Cell} \rightarrow \text{Sporulation} \rightarrow \text{Endospore}$

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.