Introduction to Prokaryotic Cells: Structure, Function, and Diversity

Prokaryotic Cell Basics

Domains of Prokaryotes

Prokaryotes are classified into two major domains: Bacteria and Archaea. These domains share fundamental similarities but also exhibit key differences.

• Bacteria and Archaea both reproduce asexually by binary fission.

• Difference: Bacterial cell walls contain peptidoglycan, whereas archaeal cell walls do not. Archaea have unique cell wall and membrane lipids.

• Similarity: Both lack a membrane-bound nucleus and organelles.

Shapes and Arrangements of Prokaryotes

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

• Shapes:

  • Cocci: Spherical

  • Bacilli: Rod-shaped

  • Spirals: Spiral-shaped

  • Vibrios: Curved rods

• Arrangements: Cells may form pairs (diplo-), chains (strepto-), clusters (staphylo-), or other groupings depending on their division patterns and whether they remain attached after division.

Binary Fission

Binary fission is the primary method of asexual reproduction in prokaryotes.

• The cell replicates its DNA.

• The cell enlarges and the DNA copies move to opposite ends.

• The plasma membrane and cell wall grow inward, dividing the cell into two genetically identical daughter cells.

Equation for binary fission population growth:

$N = N_0 \times 2^n$ Where $N$ = final cell number, $N_0$ = initial cell number, $n$ = number of generations.

Extracellular Structures

Prokaryotic Plasma Membranes

The plasma membrane is a thin, flexible barrier composed of a phospholipid bilayer with embedded proteins.

• Acts as a selective barrier, regulating entry and exit of substances.

• Site of energy production (e.g., electron transport chain) in prokaryotes, which lack mitochondria.

• Involved in nutrient transport, waste removal, and metabolic reactions.

Comparison of Bacterial and Archaeal Membranes and Cell Walls

• Bacterial cell walls contain peptidoglycan, a polymer unique to bacteria.

• Archaeal cell walls lack peptidoglycan and may contain pseudopeptidoglycan or other unique polymers.

• Archaeal membranes have unique ether-linked lipids, while bacterial membranes have ester-linked lipids.

Gram-Positive, Gram-Negative, and Acid-Fast Cell Walls

Prokaryotic cell walls can be classified based on their structure and staining properties:

Type	Peptidoglycan Layer	Outer Membrane	Stain Color	Special Features
Gram-Positive	Thick	Absent	Purple	Teichoic acids present
Gram-Negative	Thin	Present	Pink	Contains lipopolysaccharide (LPS)
Acid-Fast	Variable	Absent	Resistant to many stains	Waxy mycolic acids in cell wall

Transport Mechanisms

Cells use various mechanisms to move substances across their membranes:

• Passive Transport (no energy required):

  • Simple diffusion: Movement of small, nonpolar molecules down their concentration gradient.

  • Osmosis: Diffusion of water across a selectively permeable membrane.

  • Facilitated diffusion: Movement of substances via specific transport proteins.

• Active Transport (requires ATP):

  • Moves substances against their concentration gradient using transport proteins or pumps.

Roles of Flagella, Fimbriae, Pili, and Glycocalyx

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

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

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

• Glycocalyx: Sticky outer layer (capsule or slime layer) for protection, adhesion, and biofilm formation.

Intracellular Structures

Nucleoid

The nucleoid is the region in a prokaryotic cell where the genetic material (DNA) is located. It is not surrounded by a membrane and contains DNA, RNA, and proteins.

Prokaryotic Cytoskeleton

The cytoskeleton in prokaryotes consists of long protein filaments that provide structural support, help organize cell division, guide cell wall synthesis, and maintain cytoplasmic organization.

Prokaryotic Ribosomes and the Endosymbiotic Theory

• Ribosomes are the sites of protein synthesis, translating mRNA into polypeptides.

• Prokaryotic ribosomes (70S) are smaller than eukaryotic ribosomes (80S).

• The similarity between prokaryotic ribosomes and those found in mitochondria and chloroplasts supports the endosymbiotic theory, which proposes that these organelles originated from ancient prokaryotic cells.

Endospores

Endospores are highly resistant, dormant structures formed by certain bacteria (e.g., Bacillus and Clostridium species) to survive harsh conditions.

• Endospores can withstand extreme heat, desiccation, chemicals, and radiation.

• They pose significant challenges in healthcare settings because they are difficult to destroy and can lead to persistent infections.

Example: Clostridium difficile endospores can survive routine cleaning and cause hospital-acquired infections.