Introduction to Prokaryotic Cells: Structure, Function, and Diversity

Introduction to Prokaryotic Cells

Overview of Cell Types

Cells are the fundamental units of life, classified into two major types: prokaryotic and eukaryotic. Prokaryotic cells are simpler, lacking a membrane-bound nucleus and organelles, while eukaryotic cells possess these structures.

• Prokaryotic cells: Found in Bacteria and Archaea domains.

• Eukaryotic cells: Make up the domain Eukarya.

Origin and Domains

Life on Earth likely began as prokaryotic cells approximately 3.8 billion years ago. Prokaryotes are divided into two domains: Bacteria and Archaea. Eukaryotes form the domain Eukarya.

• Bacteria: Typical prokaryotes found in diverse environments.

• Archaea: Often inhabit extreme environments; structurally distinct from bacteria.

Prokaryotic Cell Structure

General Features

Prokaryotes are unicellular organisms that lack a membrane-bound nucleus and organelles. Their cellular components are distributed within the cytoplasm.

• Ribosomes: 70S ribosomes (smaller than eukaryotic 80S ribosomes); essential for protein synthesis.

• Inclusion bodies: Storage sites for nutrients and other substances; may be membrane-bound or granules.

• Chromosomes: Usually a single, circular DNA molecule located in the nucleoid region.

• Cytoplasm: Watery matrix where most biochemical reactions occur.

• Plasma membrane: Phospholipid bilayer acting as a selective barrier.

• Cell wall: Provides rigidity and protection; animal cells lack cell walls.

• Capsules: Carbohydrate-based outer layer; increases pathogenicity.

• Fimbriae: Short, bristle-like protein structures for adhesion.

• Flagella: Tail-like structures for motility.

Prokaryotic Diversity: Sizes, Shapes, and Arrangements

Cell Size

• Mycoplasma: Among the smallest prokaryotes; lack cell wall, triple plasma membrane, pleomorphic, cause atypical pneumonia.

• Thiomargarita namibiensis: Among the largest; up to 0.75 mm in diameter.

• Typical prokaryote size: 0.2–750 μm diameter; most are 0.5–2.0 μm.

• Size is limited by surface area-to-volume ratio due to nutrient diffusion.

Cell Shapes

• Bacilli: Rod-shaped.

• Cocci: Spherical.

• Vibrio: Comma-shaped (e.g., Vibrio cholerae causes cholera).

• Stella: Star-shaped.

• Coccobacilli: Ovoid (e.g., Haemophilus influenzae, Gardnerella vaginalis, Chlamydia trachomatis).

• Spirochetes: Spiral-shaped, corkscrew motion (e.g., Treponema pallidum).

Cell Arrangements

• Diplococci: Paired cocci.

• Streptococci: Chains of cocci.

• Staphylococci: Grapelike clusters.

• Diplobacilli: Paired bacilli.

• Streptobacilli: Chains of bacilli.

• Palisade: Clusters of bacilli.

Prokaryotic Cell Division

Binary Fission

Prokaryotic cells reproduce primarily by binary fission, a process resulting in two identical daughter cells.

• DNA is copied (DNA replication).

• Cell grows; copied chromosome moves to opposite ends.

• Septum (partition) forms at the midpoint.

• Septum walls off the two daughter cells.

Cell Barriers: Plasma Membrane and Cell Wall

Plasma Membrane

• Thin, flexible phospholipid bilayer.

• Acts as a selective barrier (selective permeability).

• Proteins in the membrane serve as transporters, anchors, receptors, and enzymes.

Membrane Fluidity

• Lipid bilayer is a fluid-mosaic; lipids and proteins move freely.

• Fluidity is essential for function; affected by temperature and fatty acid content.

• Unsaturated fatty acids increase fluidity; saturated fatty acids decrease fluidity.

Archaea Plasma Membranes

• Bacteria: linear fatty acids.

• Archaea: long-branched fatty acids; some form lipid monolayers for extreme environments.

Cell Wall Structure and Function

• Provides rigidity and protection.

• Bacteria: peptidoglycan is the main component.

• Archaea: pseudopeptidoglycan is the main component.

Table: Comparison of Gram-Positive and Gram-Negative Bacteria

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick (20–80 nm)	Thin (2–8 nm)
Outer Membrane	Absent	Present
Teichoic Acid	Present	Absent
Lipopolysaccharide	Absent	Present
Porins	Absent	Present
Clinical Implications	More sensitive to agents targeting peptidoglycan	More resistant to chemicals, drugs, detergents

Additional info:

• Gram staining is a key diagnostic tool for classifying bacteria based on cell wall structure.

• Monomorphic bacteria maintain a single shape, while pleomorphic bacteria can vary in form due to genetic or environmental factors.

• Surface area-to-volume ratio is a critical factor limiting cell size in prokaryotes.