Structure and Function of Prokaryotic Cells: Microbiology Study Notes

Overview of Prokaryotic Cells

Introduction

Prokaryotic cells are unicellular organisms that lack a membrane-bound nucleus and organelles. They are fundamental to microbiology, encompassing bacteria and archaea. Understanding their structure is essential for studying their physiology, classification, and roles in health and disease.

• Internal Components: Structures found within the cell, including the cytoplasmic matrix, nucleoid, ribosomes, and plasmids.

• Cell Envelope: Composed of the cytoplasmic membrane and cell wall, providing protection and shape.

• External Components: Surface structures such as flagella, pili, and capsules, which aid in motility, attachment, and protection.

Internal Components of Prokaryotic Cells

Cytoplasmic Matrix

The cytoplasmic matrix is the semi-fluid substance inside the cell membrane, containing water, enzymes, nutrients, wastes, and gases. It is the site of many metabolic reactions.

• Function: Supports cellular processes and houses the cell's internal structures.

Nucleoid

The nucleoid is the irregularly-shaped region within the cell where the genetic material (DNA) is located. Unlike eukaryotes, prokaryotes do not have a true nucleus.

• Chromosome: Typically a single, circular, double-stranded DNA molecule.

• Plasmids: Small, circular DNA molecules that replicate independently and often carry genes for antibiotic resistance or other specialized functions.

Ribosomes

Ribosomes are the molecular machines responsible for protein synthesis. Prokaryotic ribosomes are 70S, composed of 50S and 30S subunits.

• Function: Translate mRNA into proteins.

Inclusion Bodies

Inclusion bodies are granules of organic or inorganic material found in the cytoplasm. They serve as storage sites for nutrients and metabolic products.

• Types: Glycogen granules, polyphosphate granules, sulfur granules, gas vacuoles.

• Function: Storage of carbon, energy, phosphate, or other substances.

Cell Envelope

Cytoplasmic (Plasma) Membrane

The cytoplasmic membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins. It separates the cell interior from the environment.

• Structure: Phospholipid bilayer with proteins; in bacteria, ester linkages; in archaea, ether linkages.

• Function: Controls transport of substances, site of metabolic activities, energy generation.

Cell Wall

The cell wall provides structural support and shape to the cell. It protects against osmotic pressure and environmental stress.

• Peptidoglycan: A rigid polysaccharide layer found in most bacteria, composed of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by β-1,4 glycosidic bonds and cross-linked by peptides.

• Function: Maintains cell shape, prevents lysis, contributes to pathogenicity.

Gram-Positive vs. Gram-Negative Cell Walls

Bacteria are classified based on their cell wall structure, which affects their response to Gram staining.

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick, multi-layered	Thin, single-layered
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present (contains lipopolysaccharide)
Periplasmic Space	Small	Large
Gram Stain Result	Purple	Red/Pink

Gram Staining: A differential staining technique developed by Hans Christian Gram. Gram-positive bacteria retain the crystal violet stain, while Gram-negative bacteria do not and are counterstained with safranin.

External Components

Surface Structures

Prokaryotic cells possess various external structures that aid in protection, attachment, and motility.

• Capsules and Slime Layers: Polysaccharide layers outside the cell wall that protect against desiccation, phagocytosis, and aid in biofilm formation.

• Fimbriae and Pili: Short, hair-like appendages for attachment to surfaces and conjugation (transfer of genetic material).

• Flagella: Long, whip-like structures used for motility. Composed of filament, hook, and basal body.

Motility and Taxis

Motility allows bacteria to move toward or away from stimuli (taxis).

• Chemotaxis: Movement in response to chemical gradients.

• Phototaxis: Movement in response to light.

• Other types: Osmotaxis (ionic strength), hydrotaxis (water), aerotaxis (oxygen).

Major Bacterial Cell Shapes and Arrangements

Introduction

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

Shape	Description	Example
Coccus	Spherical or ovoid	Streptococcus
Bacillus	Rod-shaped	Escherichia coli
Spirillum	Rigid spiral	Spirillum volutans
Vibrio	Comma-shaped	Vibrio cholerae
Filamentous	Long, thread-like	Streptomyces
Stalk/Hypha	Budding and appendaged	Caulobacter

• Arrangements: Bacteria may form pairs (diplo-), chains (strepto-), clusters (staphylo-), or other groupings depending on the plane of division.

Size, Surface Area, and Volume

Introduction

The size and shape of prokaryotic cells affect their surface area-to-volume ratio, which influences nutrient uptake and metabolic efficiency.

• Small cells: Higher surface area-to-volume ratio, allowing efficient exchange of nutrients and waste.

• Large cells: Lower surface area-to-volume ratio, which can limit metabolic rates.

Formulas:

• Surface area of a sphere: $4\pi r^2$

• Volume of a sphere: $\frac{4}{3}\pi r^3$

• Surface area-to-volume ratio: $\frac{3}{r}$

Specialized Structures

Endospores

Endospores are highly resistant, dormant structures formed by some bacteria (e.g., Bacillus, Clostridium) under unfavorable conditions.

• Structure: Exosporium, spore coat, cortex, core (contains DNA, ribosomes, small acid-soluble proteins).

• Function: Resistance to heat, radiation, desiccation, chemicals; survival in harsh environments.

• Sporulation: Process of endospore formation triggered by nutrient limitation.

• Germination: Return to vegetative state when conditions improve.

Summary Table: Key Differences Between Bacteria and Archaea Cell Envelopes

Feature	Bacteria	Archaea
Membrane Lipids	Ester-linked phospholipids	Ether-linked phospholipids
Cell Wall Composition	Peptidoglycan	Pseudomurein, S-layer, or proteinaceous
Presence of Outer Membrane	Some (Gram-negative)	Rare

Additional info:

• Some details about cell envelope composition and specialized structures (e.g., S-layer, pseudomurein) were inferred from standard microbiology knowledge.

• Examples of bacterial genera and species were added for context.

• Formulas for surface area and volume were provided for completeness.