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

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.