Chapter 3: Cells and Methods to Observe Them Study Notes

Chapter 3: Cells and Methods to Observe Them

A Glimpse of History

  • Hans Christian Joachim Gram (1853–1938)

    • Danish physician known for his work in microbiology

    • Worked at a morgue in Berlin and for Dr. Carl Friedlander

    • Developed staining methods for bacteria while attempting to identify pneumonia's cause

    • Some bacteria retained dye while others did not, leading to the categorization based on cell wall structure and chemistry

    • Established the basis for the modern Gram stain test

    • Identifies two major groups of bacteria:

    • Gram-positive

    • Gram-negative

Prokaryotic Versus Eukaryotic Cells

  • Two fundamental types of cells:

    • Prokaryotic Cells

    • All bacteria and archaea

    • Eukaryotic Cells

    • All animals, plants, protozoa, fungi, and algae

  • Similarities and Differences:

    • Differences have implications for human health:

    • Bacterial cell components target for antibacterial medications when treating diseases

    • Medications can selectively kill or inhibit bacteria without harming the patient

Prokaryotic Cell Characteristics

  • Size & Ratio:

    • Prokaryotic cells are generally much smaller than eukaryotic cells

    • High surface-area-to-volume ratio facilitates nutrient uptake and waste excretion, but also makes them vulnerable to threats (predators, parasites, competitors)

  • Structure Complexity:

    • Prokaryotes evolved unique features for increased survival

    • Eukaryotic cells more complex: larger, with membrane-bound compartments, including the nucleus

Structure of Prokaryotic Cells

  • Surface Layers:

    • Cell envelope consists of:

    • Cytoplasmic membrane

    • Cell wall

    • Capsule (if present)

    • Cytoplasm:

    • Houses nucleoid (location of chromosome)

    • May possess locomotor appendages (flagella, pili)

Prokaryotic Structures - Visual reference

  • Figure 3.1: Prokaryotic cell features labeled

    • Components such as pilus, ribosomes, cytoplasm, chromosome, nucleoid, and flagellum diagrammed

    • Scale: 0.5 μm

The Cytoplasmic Membrane

  • Definition:

    • Defines the cell's boundary; composed of a phospholipid bilayer embedded with proteins.

    • Hydrophobic tails face inward, hydrophilic heads face outward

    • Proteins serve various functions:

    • Selective gates

    • Environmental sensors

    • Enzymatic enzymes

    • Fluid Mosaic Model:

    • Proteins drift within the lipid bilayer

  • Bacteria vs. Archaea Cytoplasmic Structures:

    • Similar membrane structure but distinct compositions

    • Archaea lipid tails differ as they are not composed of fatty acids

Membrane Permeability

  • Selective Permeability:

    • Gases (O2, CO2, N2), small hydrophobic molecules, and water can pass freely

    • Aquaporins assist in facilitating water passage

    • Some molecules require active transport across the membrane via transport systems

Diffusion Mechanisms

  • Diffusion:

    • Movement from high to low concentration until equilibrium reached

    • Rate of diffusion increases with greater concentration differences

  • Osmosis:

    • Specific water diffusion across a selectively permeable membrane

    • Water moves from high water concentration (low solute concentration) to low water concentration (high solute concentration)

    • Water flows from hypotonic solutions to hypertonic ones; no net flow between isotonic solutions

    • Prokaryote Environments: Typically dilute (hypotonic) relative to cytoplasm, facilitating water entry with cell wall preventing bursting

Cytoplasmic Membrane in Energy Transformation

  • Electron Transport Chain (ETC):

    • Embedded in the cytoplasmic membrane, uses energy from electrons to transport protons out of the cell, creating an electrochemical gradient

    • Proton Motive Force:

    • Energy used for ATP synthesis and transport methods

Transport Mechanisms Across Cytoplasmic Membrane

  • Transport Systems:

    • Move nutrients across the cytoplasmic membrane via:

    • Transporters/Permeases/Carriers:

      • Specific for singular molecule types

      • Highly selective

    • Efflux Pumps:

    • Move waste products/toxic substances out of cells, enabling bacteria to resist certain antimicrobial treatments

  • Facilitated Diffusion:

    • Passive movement down the gradient, requires no energy

    • Less effective in low-nutrient environments

  • Active Transport:

    • Uses energy to move materials against their concentration gradient

    • Work may require ATP or is driven by proton motive force

    • Example: ABC transport system

  • Group Translocation:

    • Common in bacteria, alters compounds chemically during transport—phosphorylation is a typical modification

    • Often used for glucose uptake

Protein Secretion in Prokaryotic Cells

  • Active Movement:

    • Proteins move out of the cell, including exoenzymes and external structures

    • Polypeptides tagged for secretion using a signal sequence of amino acids

The Cell Wall of Prokaryotic Cells

  • Cell Wall Functionality:

    • Rigid structure protecting from bursting

    • Distinguishes major bacteria types:

    • Gram-positive

    • Gram-negative

Peptidoglycan Structure

  • Fundamental Component:

    • Layer of peptidoglycan in both Gram-positive and Gram-negative bacteria

    • Composed of:

    • Alternating N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) subunits

    • Tetrapeptide chains link glycan chains; direct links in Gram-negative cells and peptide interbridges in Gram-positive

Gram-positive and Gram-negative Cell Walls

  • Gram-positive:

    • Thick peptidoglycan layer with teichoic acids extending above it

    • Gel-like periplasmic material lies below the peptidoglycan

  • Gram-negative:

    • Thin peptidoglycan layer within a unique outer membrane

    • Outer membrane is mainly composed of lipopolysaccharide (LPS), which signals immune response upon invasion

    • Endotoxin: Can be deadly in large amounts, incorporating Lipid A (recognized by the immune system) and O antigen (used for species identification)

Mechanisms of Inhibition Using Antibiotics

  • Peptidoglycan Disruption:

    • Penicillin inhibits peptidoglycan synthesis by blocking cross-linking, effective mostly against Gram-positive bacteria

    • Lysozyme breaks glycan chain bonds, found naturally in body fluids, effective against Gram-positive bacteria

Bacteria Lacking a Cell Wall

  • Mycoplasma:

    • Bacteria species lacking a rigid cell wall, making them flexible and resistant to agents targeting cell wall synthesis

    • Survive by adopting sterols in their cytoplasmic membrane, strengthening it

Cell Walls of Archaea

  • Diverse Cell Wall Structures:

    • No peptidoglycan, some have pseudopeptidoglycan

    • Many exhibit self-assembling S-layers made of proteins or glycoproteins

Capsules and Slime Layers

  • Protective Layers:

    • Gel-like outer layers (Capsule vs. Slime Layer)

    • Glycocalyx: Most composed of polysaccharides, providing adhesion for biofilm formation (example: dental plaque), aiding in evasion of host immune responses

Motility: Flagella and Chemotaxis

  • Flagella Function:

    • Critical structure for motility, functioning like spinning propellers

    • Different arrangements (Peritrichous, Polar) assist with bacterial classification

  • Chemotaxis:

    • Ability of bacteria to sense chemical gradients and move toward nutrients or away from toxins

    • Movement described as a series of runs (straight line) and tumbles (direction changes)

Internal Components of Prokaryotic Cells

  • Nucleoid:

    • Gel-like region containing a single circular DNA molecule, tightly packed

  • Plasmids:

    • Smaller DNA molecules not essential for life, can be shared among bacteria, important for antibiotic resistance

  • Ribosomes:

    • Site for protein synthesis, prokaryotic ribosome size = 70S compared to eukaryotic's 80S which is crucial for antibiotic targeting

  • Cytoskeleton:

    • Interior structural framework involved in shape and division

  • Storage Granules:

    • Accumulations from excess nutrients (e.g., glycogen)

  • Protein-Based Compartments:

    • Gas vesicles for buoyancy and compartments for controlling metabolism (e.g., encapsulin nanocompartments)

Endospores

  • Dormant Cells:

    • Produced by Bacillus and Clostridium, resistant to harsh conditions

    • Can remain dormant for years and germinate back into vegetative cells

    • Triggered by nutrient limitation initiating sporulation, which involves protective layers that resist damage

    • Germination involves return to a vegetative state after favorable conditions resume

Eukaryotic Cell Structure and Functions

  • Diversity:

    • Eukaryotic cells exhibit variability in structures:

    • Animal Cells: Lack cell walls

    • Fungal Cells: Contain polysaccharide cell walls (e.g., chitin)

    • Plant Cells: Composed of cellulose walls

  • Organelles:

    • More complex than prokaryotic cells, containing membrane-enclosed compartments (e.g., nucleus, mitochondria)

  • Cytoplasmic Membrane:

    • Similar to prokaryotic but may include cholesterol, glycoproteins, and provide cell communication

Transfer of Molecules Across Cytoplasmic Membrane

  • Mechanisms:

    • Aquaporins for water, channels for small molecules, carriers for active transport

Vesicle Trafficking and Protein Secretion

  • **Endocytosis and Exocytosis:

    • Cell processes to uptake material or release it via vesicles

  • Protein Secretion Mechanism:

    • Involves signal sequences that tag proteins for secretion, typically using endoplasmic reticulum and Golgi apparatus

Eukaryotic Structures: Membrane-Bound Organelles

  • Nucleus:

    • Contains genetic material, double membrane, nucleolus for rRNA synthesis

  • Endoplasmic Reticulum:

    • Rough (with ribosomes) synthesizes proteins; Smooth synthesizes lipids and stores calcium

  • Golgi Apparatus:

    • Modifies and sorts proteins from the ER for delivery

  • Lysosomes:

    • Contain enzymes for degrading cellular waste, vital for autophagy processes

  • Mitochondria and Chloroplasts:

    • Energy-producing organelles, each containing their own DNA and ribosomes (70S), descended from endosymbiotic ancestors

Principles of Microscopy

  • Light Microscopes:

    • Magnify up to 1,000×; principles of magnification and resolution

    • Contrast is essential for visualization, various types of light techniques (such as dark-field and phase-contrast) improve observation clarity

  • Electron Microscopes:

    • Transmission and scanning types allow for magnifications exceeding 100,000×, observing internal structures and surfaces with detailed precision

    • Sample preparation unique to avoid air interference required for electron beams

Preparing and Staining Specimens

  • Staining Techniques:

    • Simple Staining: Uses one dye to enhance visibility of structures

    • Differential Staining: (e.g., Gram staining) distinguishes between cell types based on cell wall properties

    • Specialized Stains: (e.g., acid-fast for certain bacteria or endospore staining) to visualize specific components

Key Takeaways

  • Understanding cellular structures and functions helps design effective antimicrobial therapies, emphasizes the critical balance between human health and microbial existence, and improves our ability to leverage microscopy techniques for cellular study.