prokaryotes and eukaryotes .

Membrane Structures in Cell Biology

  • Prokaryotes

    • Basic components of a cell include:

    • A. Plasma membrane

      • Function: Separates the internal contents of the cell from the exterior environment.

      • Present in both prokaryotic and eukaryotic cells.

    • B. DNA

      • Exist freely floating inside the plasma membrane in prokaryotes.

    • C. Ribosomes

      • Responsible for synthesizing proteins, associated specifically with the rough endoplasmic reticulum (RER).

    • D. Plasma membrane Structure

      • Made up of a bilayer of phospholipids consisting of:

        1. Transport proteins

        2. Receptor glycoproteins

        3. Recognition antigens

      • The surface exhibits both hydrophilic (water-attracting) and hydrophobic (water-repelling) characteristics, allowing amphipathic interactions.

Lipid Composition of Membranes

  • Saturated vs Unsaturated Fats

    • Saturated fats:

      • Characteristics: Solid at room temperature.

      • Structure: Lack double bonds in their hydrophobic tails.

    • Unsaturated fats:

      • Characteristics: Fluid at room temperature.

      • Structure: Contain double bonds causing kinks in the tail.

    • Cholesterol Presence:

      • Cholesterol can alter membrane composition and reinforce membrane stability (membrane lysis and protection).

Membrane Transport Mechanisms

  • Membrane Transport Functions

    • A. Regulates moving materials in and out of the cell

      • Nature: Selectively permeable.

    • B. Passive Transport

      1. Simple Diffusion

        • Definition: Materials cross membranes along the concentration gradient (high concentration to low concentration).

        • Example: Oxygen diffusion (from 4 molecules to 0 molecules, illustrating concentration drop).

      2. Facilitated Diffusion

        • Definition: Passive transport occurring through channels; materials move based on concentration gradients.

        • Example: Sugar molecules transport from areas of high to low concentration.

      3. Osmosis

        • Definition: Movement of water from high-water concentration to low-water concentration; involves saturation to water (H₂O).

        • Concept: If water is bound to solutes, free water content may decrease, impacting overall movement.

        • Example: In a high salt environment, water tends to move toward lower concentrations.

    • C. Active Transport

      • Characteristics: Requires energy to move molecules against their concentration gradient.

      1. Example: Active Transport of Potassium ions:

        • Transitioning from high concentration (inside the cell) to low concentration (outside).

      2. Group Translocation:

        • Process: Activates transport via high-energy components like phosphoenolpyruvate (PEP) adding phosphate to glucose, preventing it from leaving the cell.

        • Examples: High glucose conditions to low glucose conditions.

Prokaryotic Cells Overview

  • A. Characteristics of Prokaryotic Cells

    • Definition: Cells lacking membrane-bound organelles and specialization; generally do not form multicellular organisms.

    • Forms: Exhibit various shapes and arrangements based on division planes.

      1. Diplococcus: Division along one plane.

      2. Streptococcus: Chains formed by division along one plane.

      3. Tetrad: Groups of four bacteria due to division along two perpendicular planes; variable arrangements.

  • B. External Layers of Prokaryotic Cells

    • Glycocalyx:

      • Definition: Surface covering made of polysaccharides, polypeptides, or both.

      • Capsule: Well-organized glycocalyx firmly attached to the cell wall.

      • Slime Layer: Disorganized glycocalyx structure.

    • Functions of Glycocalyx:

      1. Nutritional source in times of distress.

      2. Assist with virulence by protecting against phagocytosis.

      3. Protect from dehydration.

      4. Inhibit the movement of materials out of cells.

      5. Produce biofilm.

Prokaryotic Cell Structure and Functionality

  • C. Cell Shape Maintenance

    • Slayer: Crystalline layer aiding in cell shape and adhesion.

      • Pathogens utilizing it can remain in hosts longer; increasing disease likelihood (e.g., Clostridium difficile, a nosocomially acquired pathogen causing antibiotic-associated diarrhea).

  • D. Prokaryotic Appendages

    • Flagella:

      • Composition: Chains of proteins attached to a protein hook and anchored to the cell membrane by the basal body.

      • Types of arrangements:

        1. Monotrichous: One flagella

        2. Peritrichous: Flagella covering the entire surface

        3. Lophotrichous: Clustered flagella at one end

        4. Amphitrichous: Flagella at both ends

      • Importance: Contributes to virulence and capacity to cause harm.

    • Fimbrae:

      • Found in many Gram-negative bacteria; function: enable adhesion to surfaces.

    • Pili:

      • Longer than fimbrae; assists in transferring DNA during conjugation (this can contribute to antibiotic resistance).

Prokaryotic Cell Wall and Its Components

  • A. Peptidoglycan Structure:

    • Composition: Long chains of sugar connected by protein links.

  • B. Classifications of Gram Staining:

    • Represents differences in cell wall compositions:

      1. Gram-positive (G+): High peptidoglycan content; may retain purple dye.

      2. Gram-negative (G-): Contains lipopolysaccharides (endotoxins) and low peptidoglycan content; does not retain purple dye leading to contrast with a pink stain after decolorization.

    • C. Gram Stain Mechanism:

      • Results: G+ appears purple (intensely colored), G- becomes colorless upon decolorization (subsequent pink after secondary staining).

  • D. Atypical Cell Walls

    1. Mycoplasmas: Lack cell walls; possess sterols in membranes.

    2. Archaea: May appear Gram-positive because they lack peptidoglycan.

    3. Acid-fast Bacteria: Contain mycolic acid; stain Gram-positive due to this composition.

  • E. Damage to the Cell Wall:

    1. Lysosome: Effective against Gram-positive bacteria; breaks bonds between disaccharides in peptidoglycan.

    2. Antibiotics: Target peptidoglycan structure leading to effective bacterial cell mortality.

Prokaryotic Internal Structure

  • A. Cytoplasm Composition:

    • Contains:

      1. Water

      2. Proteins

      3. DNA

      4. Ribosomes

  • B. Nucleoid Region:

    • Definition: A single continuous double-stranded DNA chromosome attached to the plasma membrane; may also contain extra plasmids that, while not crucial for survival, are helpful (e.g., antibiotic resistance).

  • C. Ribosomes:

    • Function: Protein production; smaller than eukaryotic ribosomes and targeted by specific antibiotics for selective toxicity.

  • D. Inclusions:

    • Various inclusion types include:

      1. Metachromatic granules (phosphate)

      2. Polysaccharide granules

      3. Lipid inclusions

      4. Sulfur granules

      5. Carboxysomes

      6. Gas vacuoles

      7. Magnetosomes

      8. Microcompartments

    • E. Endospores: Not found in all microorganisms; form under restricted conditions (e.g., Clostridium, Bacillus species) for survival.

    • F. Sporulation Process:

      • Steps: DNA is separated amid plasma membranes that secrete and surround the forespore leading to spore coat formation then spore releases.

      • Importance: Example of Bacillus anthracis, known for causing anthrax, and utilized as a bioweapon in 2001 (cases of transmission via US mail).

Eukaryotic Cell Structures

  • A. Eukaryotic Organisms

    • Characteristics: Contain organelles which are membrane-bound compartments.

    • Size: Eukaryotic cells are larger than bacteria; generally able to accommodate about 1000 bacteria.

    • Specialization: Capable of forming tissues in multicellular animals and plants.

  • B. Plasma Membrane:

    • Features: Presence of cholesterol, critical for maintaining membrane fluidity and functionality.

  • C. Movement Mechanisms in Eukaryotes:

    1. Endocytosis: The process to bring material into the cell.

    2. Exocytosis: The process by which materials are expelled from a cell.

    3. Phagocytosis: A form of endocytosis using pseudopodia to engulf large particles.

  • D. Eukaryotic Cytoplasm and Skeletal System:

    • Contains:

      • Cytoskeleton providing shape and support; involved in intracellular transport.

      • Ribosomes: where protein synthesis occurs, constituted of two subunits from protein and ribosomal RNA.

Eukaryotic Organelles in Protein Production

  • 1. Nucleus:

    • Function: Houses DNA; involved in ribosome production.

  • 2. Rough Endoplasmic Reticulum (RER):

    • Structure: A series of membrane-bound channels with ribosomes present.

    • Function: Receives material from the nucleus and synthesizes proteins based on that material.

  • 3. Golgi Apparatus:

    • Characteristics: A series of membrane-bound discs.

    • Function:

      • Accepts material from the ER,

      • Modifies materials,

      • Sorts and packages for transport via vesicles (transitional vesicles for material between locations and secretory vesicles for delivery).

  • 4. Other Organelles:

    • A. Smooth Endoplasmic Reticulum:

      • Function: Synthesizes lipids and detoxifies some harmful chemicals.

    • B. Lysosomes:

      • Contains digestive enzymes; involved in digesting large macromolecules, organelles, and pathogens.

    • C. Mitochondria:

      • Roles: Site of cellular respiration; extracts energy from food and stores it in ATP bonds.

      • Features:

        1. Double membrane-bound sync.

        2. Supports the endosymbiotic hypothesis (contains DNA, can make proteins, and reproduce via binary fission).

  • 5. Plastids:

    • Example: Chloroplast

      1. Function: Site of photosynthesis in plants and algae.

      2. Structure: Double-membraned organelle.

External Features of Eukaryotic Cells

  • A. Eukaryotic Cell Wall (if present):

    • Lacks peptidoglycan. The material composition varies by organism.

  • B. Glycocalyx:

    • A carbohydrate layer that covers the outer surface of cells, sometimes underneath.

  • C. Flagella and Cilia:

    • Projections made of microtubules that contain cytoplasm, enclosed by the cell membrane; used for movement and sensory functions in cells.