Prokaryotic Cell Structure

Major Characteristics of Prokaryotic Cells

Prokaryotic cells are fundamental units of life in bacteria and archaea. They are structurally simpler than eukaryotic cells and lack membrane-bound organelles.

• No nucleus: Genetic material is located in a nucleoid region.

• Single circular chromosome: Most prokaryotes possess a single, circular DNA molecule.

• Cell wall: Provides structural support and shape; composition varies between Gram-positive and Gram-negative bacteria.

• Ribosomes: 70S ribosomes are responsible for protein synthesis.

• Inclusions: Storage granules for nutrients and other substances.

• Endospores: Some bacteria form highly resistant, dormant structures for survival under adverse conditions.

Cytoplasmic (Plasma) Membrane

Structure and Composition

The cytoplasmic membrane encloses the cytoplasm and regulates the movement of substances in and out of the cell.

• Phospholipid bilayer: Composed of two layers of phospholipids with embedded proteins.

• Hydrophilic exterior: The polar heads of phospholipids face outward toward the aqueous environment.

• Hydrophobic interior: Nonpolar fatty acid tails face inward, away from water.

• Proteins: Integral and peripheral proteins serve as transporters, enzymes, and receptors.

Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which emphasizes the dynamic arrangement of lipids and proteins.

• Integral proteins: Span the membrane and are involved in transport and signaling.

• Peripheral proteins: Attached to the membrane surface, often involved in cell signaling or maintaining cell shape.

Transport Across the Membrane

Passive Transport

Passive transport does not require energy and moves substances down their concentration gradient.

• Simple diffusion: Movement of small, uncharged molecules directly through the lipid bilayer.

• Facilitated diffusion: Movement of larger or charged molecules via specific membrane proteins.

• Osmosis: Movement of water across the membrane.

Comparison Table: Simple vs. Facilitated Diffusion

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradient.

• Specific transporters: Each transporter is specific for a particular substance or group of substances.

• Energy expenditure: ATP or other high-energy compounds are used to drive transport.

Equation: Active Transport Energy Requirement

Effects of Osmotic Environments

Bacterial cells respond differently to isotonic, hypotonic, and hypertonic solutions:

• Isotonic: No net movement of water; cell remains unchanged.

• Hypotonic: Water enters the cell; cell may swell and undergo osmotic lysis if the cell wall is damaged.

• Hypertonic: Water leaves the cell; cell shrinks (plasmolysis).

Bacterial Cell Wall Structure

Gram-Positive vs. Gram-Negative Cell Walls

Bacterial cell walls differ in structure and composition, which affects staining and antibiotic susceptibility.

• Gram-positive: Thick peptidoglycan layer, teichoic acids, no outer membrane.

• Gram-negative: Thin peptidoglycan layer, outer membrane containing lipopolysaccharide (LPS), periplasmic space.

Comparison Table: Gram-Positive vs. Gram-Negative Cell Walls

Bacterial Ribosomes

Structure and Function

Bacterial ribosomes are the sites of protein synthesis (translation).

• 70S ribosome: Composed of a small (30S) and large (50S) subunit.

• Made of protein and rRNA: Both subunits contain ribosomal RNA and proteins.

• Function: Translate mRNA into polypeptides.

Bacterial Inclusions

Types and Functions

Inclusions are reserve deposits found in prokaryotic cells, storing nutrients and reducing osmotic pressure.

• Metachromatic granules (volutin): Store inorganic phosphate for ATP synthesis.

• Polysaccharide granules: Store glycogen and starch.

• Carboxysomes: Contain enzymes for carbon fixation (photosynthesis).

• Gas vacuoles: Provide buoyancy for aquatic bacteria.

• Magnetosomes: Contain iron oxide, allowing bacteria to orient along magnetic fields (magnetotaxis).

Bacterial Endospores

Structure and Function

Endospores are highly resistant, dormant structures formed by certain Gram-positive bacteria (e.g., Bacillus and Clostridium genera).

• Survival: Endospores enable bacteria to withstand extreme conditions (heat, chemicals, desiccation).

• Layers: Multiple protective layers surround the core containing DNA and essential cell components.

• Germination: Under favorable conditions, endospores return to vegetative cells.

Sporulation and Germination

Sporulation is the process of endospore formation, while germination is the return to active growth.

• Sporulation: DNA is replicated, and the cell divides asymmetrically; the forespore is engulfed and surrounded by protective layers.

• Germination: Triggered by environmental signals; endospore absorbs water, breaks down protective layers, and resumes metabolism.

Endospore Staining Procedure

Steps and Expected Results

Endospore staining differentiates endospores from vegetative cells using specific dyes.

• Primary stain: Malachite green is applied to penetrate endospores.

• Counterstain: Safranin stains vegetative cells red.

• Result: Endospores appear green; vegetative cells appear red.

Table: Endospore Staining Results

Additional info:

• These notes are based on lecture slides and textbook references for a college-level microbiology course.

• Images referenced in the slides depict Gram-positive and Gram-negative cell wall structures, the fluid mosaic model of membranes, and endospore formation.