Prokaryotic Cell Structure and Function: Study Notes for Microbiology

Prokaryotic Cell Basics

Prokaryotic Domains: Bacteria and Archaea

Prokaryotes are divided into two major domains: Bacteria and Archaea. These domains differ in several fundamental ways, but also share key similarities.

• Differences: Bacteria have peptidoglycan in their cell walls, while Archaea do not. Archaea often inhabit extreme environments and have unique membrane lipids.

• Similarities: Both lack a nucleus and membrane-bound organelles, and both have circular DNA.

• Example: Escherichia coli is a common bacterium; Halobacterium is an archaeon found in salty environments.

Shapes and Arrangements of Prokaryotes

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

• Shapes: Main shapes include coccus (spherical), bacillus (rod-shaped), spirillum (spiral), vibrio (comma-shaped), and spirochete (flexible spiral).

• Arrangements: Cells may occur singly, in pairs (diplo-), chains (strepto-), clusters (staphylo-), or other groupings.

• Example: Streptococcus forms chains of cocci; Staphylococcus forms clusters.

Binary Fission: Prokaryotic Cell Division

Prokaryotes reproduce asexually by binary fission, a process that results in two genetically identical daughter cells.

• Process: The cell grows, replicates its DNA, and divides by constriction of the plasma membrane and cell wall.

• Steps: DNA replication → cell elongation → septum formation → cell separation.

• Example: Rapid binary fission allows bacteria to multiply quickly in favorable conditions.

Extracellular Structures

Prokaryotic Plasma Membrane: Structure and Function

The plasma membrane of prokaryotes is a selectively permeable barrier that separates the cell interior from the environment.

• Structure: Composed of a phospholipid bilayer with embedded proteins.

• Function: Regulates transport, supports energy generation, and anchors extracellular structures.

• Example: The plasma membrane allows nutrient uptake and waste removal.

Cell Wall Composition: Bacteria vs. Archaea

Cell walls provide structural support and protection. Their composition varies between bacteria and archaea.

• Bacterial Cell Walls: Contain peptidoglycan, a polymer of sugars and amino acids.

• Archaeal Cell Walls: Lack peptidoglycan; may contain pseudopeptidoglycan or other polymers.

• Gram Staining: Bacteria are classified as Gram-positive (thick peptidoglycan) or Gram-negative (thin peptidoglycan, outer membrane).

• Example: Bacillus subtilis is Gram-positive; Escherichia coli is Gram-negative.

Passive and Active Transport Mechanisms

Transport across the plasma membrane occurs via passive and active mechanisms.

• Passive Transport: Includes diffusion, facilitated diffusion, and osmosis; does not require energy.

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

• Example: Uptake of glucose by facilitated diffusion; sodium ion transport by active pumps.

Extracellular Structures: Flagella, Fimbriae, Pili, Glycocalyx

Prokaryotes possess various extracellular structures that aid in movement, attachment, and protection.

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

• Fimbriae: Short, hair-like projections for attachment to surfaces.

• Pili: Longer than fimbriae; involved in DNA transfer (conjugation).

• Glycocalyx: Capsule or slime layer; protects against desiccation and immune attack.

• Example: Neisseria gonorrhoeae uses pili for gene transfer; Streptococcus pneumoniae has a protective capsule.

Intracellular Structures

Nucleoid: Definition and Function

The nucleoid is the region in a prokaryotic cell where the circular DNA chromosome is located.

• Structure: Not membrane-bound; DNA is compacted by proteins.

• Function: Contains genetic information for cell function and reproduction.

• Example: The nucleoid is visible under electron microscopy as a dense region.

Prokaryotic Cytoskeleton

Although lacking the complex cytoskeleton of eukaryotes, prokaryotes possess protein fibers that help maintain cell shape and organize cellular components.

• Proteins: Actin-like and tubulin-like proteins form filaments.

• Function: Cell division, shape maintenance, and intracellular organization.

• Example: MreB protein helps maintain rod shape in bacteria.

Prokaryotic Ribosomes and the Endosymbiotic Theory

Prokaryotic ribosomes are smaller (70S) than eukaryotic ribosomes (80S) and are essential for protein synthesis.

• Structure: Composed of rRNA and proteins; two subunits (30S and 50S).

• Function: Translate mRNA into proteins.

• Endosymbiotic Theory: The similarity of prokaryotic ribosomes to those in mitochondria and chloroplasts supports the theory that these organelles originated from prokaryotic cells.

• Example: Antibiotics like tetracycline target bacterial ribosomes.

Endospores: Structure, Function, and Healthcare Challenges

Endospores are highly resistant, dormant structures formed by some bacteria to survive harsh conditions.

• Structure: Thick protective coat, dehydrated core, and DNA.

• Function: Enable survival during extreme heat, desiccation, chemicals, and radiation.

• Healthcare Challenge: Endospores resist disinfection and cause persistent infections (e.g., Bacillus anthracis, Clostridium difficile).

Summary Table: Comparison of Bacterial and Archaeal Cell Walls

Feature	Bacteria	Archaea
Main Polymer	Peptidoglycan	Pseudopeptidoglycan or other polymers
Gram Stain	Positive/Negative	Not applicable
Membrane Lipids	Ester-linked	Ether-linked
Example Organism	Escherichia coli	Halobacterium

Key Equations

• Binary Fission Growth:

$N = N_0 imes 2^n$

Where $N$ is the final number of cells, $N_0$ is the initial number of cells, and $n$ is the number of generations.

• Osmosis (Passive Transport):

$J = -P (C_1 - C_2)$

Where $J$ is the flux, $P$ is the permeability coefficient, and $C_1$ and $C_2$ are concentrations on either side of the membrane.

Additional info: Academic context and examples have been added to expand on brief points and ensure completeness.