Prokaryotic Cell Structure and Classification

Domains and Characteristics

Prokaryotic cells are fundamental to microbiology and are classified into two domains: Archaea and Bacteria. These cells are distinct from eukaryotic cells in several key ways.

• Unicellular: Prokaryotes are single-celled organisms.

• Lack a membrane-bound nucleus: Their genetic material is not enclosed within a nuclear membrane.

• Lack membrane-bound organelles: Organelles such as mitochondria and endoplasmic reticulum are absent.

• Plasma membrane: Composed of a permeable lipid bilayer, often with a cell wall.

• Nucleoid: Region where the prokaryotic chromosome (double-stranded DNA) is located.

Prokaryotic Cell Morphology

Shapes and Arrangements

Prokaryotes exhibit a variety of shapes, sizes, and arrangements, which are important for identification and understanding pathogenicity.

• Monomorphic bacteria: Have a single, consistent shape.

• Pleomorphic bacteria: Can take on different forms, enhancing survival and transmission.

Common shapes include:

• Cocci: Spherical cells (e.g., Staphylococcus).

• Bacilli: Rod-shaped cells.

• Vibrio: Comma-shaped cells.

• Stella: Star-shaped cells.

• Coccobacilli: Ovoid cells.

• Spirochetes: Spiral-shaped cells with corkscrew motion.

Cell Arrangements

Cell division patterns lead to characteristic arrangements, aiding in identification.

• Diplococci: Paired cocci.

• Streptococci: Chains of cocci.

• Staphylococci: Grapelike clusters of cocci.

• Diplobacilli: Paired bacilli.

• Streptobacilli: Chains of bacilli.

• Palisade: Clusters of bacilli.

Other shapes: Vibrio, coccobacillus, spiral, spirochete, stella, filamentous, pleomorphic.

Prokaryotic Cell Division

Binary Fission

Prokaryotic cells primarily reproduce by binary fission, a process that ensures genetic continuity.

• Cell grows

• Chromosome is copied and drawn to opposite ends

• Septum (partition) begins to form at the midpoint

• Septum eventually walls off the resulting two daughter cells

Steps in binary fission:

1. DNA replication

2. Cell elongates

3. Septum starts to form

4. Cell is partitioned

5. Daughter cells separate

Prokaryotic chromosome: Typically consists of double-stranded DNA.

Prokaryotic Cell Membrane Structure and Function

Selective Permeability

The plasma membrane is selectively permeable, allowing water and small non-charged substances to diffuse freely, while ions and larger molecules require protein transporters.

Membrane Fluidity

The lipid bilayer is described as a fluid-mosaic model, where membrane lipids and proteins move around, ensuring proper cell function.

• Proteins relocate to areas of the membrane where needed.

• Membrane fluidity is essential for physiological functions.

• Fluidity is affected by temperature, fatty acid content, and saturation.

• Warmer temperatures increase fluidity; colder temperatures reduce it.

• Unsaturated fatty acids prevent tight packing, improving fluidity in cold.

• Saturated fatty acids pack tightly, making the bilayer more rigid in cold.

Prokaryotic Cell Wall Structure

Peptidoglycan and Gram Staining

Prokaryotic cell walls are primarily composed of peptidoglycan, with NAG (N-acetylglucosamine) and NAM (N-acetylmuramic acid) as building blocks.

• Gram-negative cell wall: Has a double-layer membrane and contains lipopolysaccharide (LPS), including lipid A (endotoxin).

• Gram-positive cell wall: Has a thick peptidoglycan layer and only one plasma membrane.

Importance of Cell Wall Structure

• Gram-negative bacteria release lipid A (endotoxin) upon cell wall breakdown, which can trigger immune responses.

• Cell wall structure is crucial for bacterial identification and antibiotic targeting.

Summary Table: Key Prokaryotic Features

Key Terms and Definitions

• Prokaryote: A unicellular organism lacking a nucleus and membrane-bound organelles.

• Binary fission: Asexual reproduction method in prokaryotes involving cell growth, DNA replication, and division into two daughter cells.

• Peptidoglycan: Polymer forming the cell wall in most bacteria, composed of NAG and NAM subunits.

• Gram stain: Differential staining technique to classify bacteria based on cell wall structure.

• Lipid A: Component of LPS in Gram-negative bacteria, acts as an endotoxin.

Relevant Equations

• Binary Fission Growth Equation:

Where: N = final number of cells N_0 = initial number of cells n = number of generations

Additional info: Academic context was added to clarify the importance of cell wall structure, binary fission steps, and the fluid-mosaic model, as well as to provide definitions and a summary table for exam preparation.