Cell Structure and Function

Overview of Cellular Life

Cells are fundamental living entities, surrounded by a membrane, capable of growth, reproduction, response to stimuli, and metabolism. Microbiology distinguishes between prokaryotic and eukaryotic cells based on structural complexity and the presence of membrane-bound organelles.

• Prokaryotic cells: Lack membrane-bound organelles; simpler structure (e.g., E. coli).

• Eukaryotic cells: Contain membrane-bound organelles; more complex (e.g., human cells).

External Structures of Prokaryotic Cells

Glycocalyces

The glycocalyx is a sticky layer outside the cell envelope, composed of polysaccharides and/or polypeptides. It prevents desiccation and helps pathogens evade detection by the host immune system.

• All bacteria possess a glycocalyx.

• Two types: Capsule (firmly attached, organized) and Slime layer (loosely attached, sticky, water-soluble).

• Capsules can mimic host chemicals, aiding in immune evasion.

• Slime layers facilitate biofilm formation and adherence to surfaces.

Eukaryotic Glycocalyces

Eukaryotic glycocalyces are present only in cells lacking cell walls (animal and protozoan cells). They are less organized than prokaryotic capsules and serve functions such as anchoring cells, strengthening surfaces, protection against dehydration, and cell-to-cell communication.

• Absent in eukaryotes with cell walls (plants, fungi).

• Never as structurally organized as prokaryotic capsules.

Bacterial Cell Walls

Peptidoglycan Structure

Bacterial cell walls are primarily composed of peptidoglycan, a complex polysaccharide. Peptidoglycan consists of alternating sugars, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), linked by tetrapeptide crossbridges and connecting chains of amino acids.

• Provides structural support and shape.

• Peptides and sugars form a mesh-like structure.

Gram Stain and Cell Wall Types

The Gram stain procedure distinguishes two basic types of bacterial cell walls based on their reaction to crystal violet and safranin stains, reflecting differences in cell wall structure.

• Gram-positive bacteria: Retain crystal violet, appear purple.

• Gram-negative bacteria: Do not retain crystal violet, appear pink/red.

Gram-Positive Bacterial Cell Walls

Gram-positive cell walls contain a thick layer of peptidoglycan and teichoic acids, which anchor the peptidoglycan to the cytoplasmic membrane. Some teichoic acids are covalently linked to lipids (lipoteichoic acids). Acid-fast bacteria, such as Mycobacterium tuberculosis, have cell walls with mycolic acids, making them resistant to desiccation and antibiotics.

• Thick peptidoglycan layer.

• Teichoic and lipoteichoic acids present.

• Acid-fast cell walls contain mycolic acids (waxy lipid).

Gram-Negative Bacterial Cell Envelopes

Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides (LPS). LPS includes Lipid A, which can be toxic to humans. The periplasmic space lies between the cytoplasmic membrane and the outer membrane.

• Thin peptidoglycan layer.

• Outer membrane with LPS and Lipid A.

• Appear pink/red after Gram staining.

Motility and External Appendages

Flagella

Flagella are whip-like structures that enable motility in prokaryotes. They are composed of flagellin proteins and consist of a filament, hook, and basal body. The basal body anchors the flagellum in the cell envelope, and its rotation drives movement.

• Movement in response to stimuli (chemotaxis).

• Swimming motility; not all bacteria have flagella.

• Structure differs between Gram-positive and Gram-negative bacteria.

• Rotation direction determines movement: clockwise (tumble), counterclockwise (run).

Fimbriae

Fimbriae are short, sticky, bristle-like fibers made of protein with adhesive tips. They facilitate adherence to surfaces, biofilm formation, and communication within biofilms. Some fimbriae carry enzymes that detoxify toxic metal ions.

• Important for biofilm formation.

• May be hundreds per cell; usually shorter than flagella.

• Can conduct electrical signals in biofilms.

Pili

Pili are hollow tubes made of pilin protein, longer than fimbriae but shorter than flagella. They mediate the transfer of DNA between cells (conjugation), also known as the conjugation pilus or F pilus. Pili can also aid in attachment, motility, microbial warfare, and electrical conduction.

• Usually one or a few per cell.

• Important for spreading antibiotic resistance.

Eukaryotic Flagella

Eukaryotic flagella are located inside the cell, surrounded by the cytoplasmic membrane, and composed of tubulin. Unlike prokaryotic flagella, they do not rotate but undulate rhythmically. Eukaryotic flagella move in response to stimuli but not by runs and tumbles.

• Structure: "9 + 2" array of microtubules.

• Movement: wave-like motion.

Summary Table: Comparison of Prokaryotic and Eukaryotic Cell Structures

Key Terms and Definitions

• Glycocalyx: Extracellular layer of polysaccharides/polypeptides; capsule (organized) or slime layer (loose).

• Peptidoglycan: Structural polysaccharide in bacterial cell walls, composed of NAG and NAM sugars linked by peptides.

• Teichoic acids: Polymers in Gram-positive cell walls, anchoring peptidoglycan.

• Lipopolysaccharide (LPS): Molecule in Gram-negative outer membrane; contains toxic Lipid A.

• Flagella: Motility appendage; rotates in prokaryotes, undulates in eukaryotes.

• Fimbriae: Short, adhesive appendages for attachment and biofilm formation.

• Pili: Tubular appendages for DNA transfer (conjugation).

Relevant Equations

Peptidoglycan cross-linking:

Gram stain reaction: