PP4 membranes and cell walls

Introduction to Bacterial Cell Biology

  • Overview of E. Coli cells under a microscope

    • Image shown at 400x magnification

    • Cells appear broad or cylindrical in shape

Modern Microscopy Techniques

  • Development of microscopy techniques enhances understanding of bacterial cell biology

    • Electron microscopy provides detailed images of bacterial cells

    • Example: Scanning electron micrograph shows an elongated cell preparing to divide, forming a septum

Comparison of Bacterial and Eukaryotic Cells

  • Key structural differences between bacteria and eukaryotes

    • DNA Presence:

    • Bacteria: DNA located in a nucleoid, dispersed throughout the cytoplasm

    • Eukaryotes: DNA is condensed within a nucleus

    • Cell Walls:

    • Bacteria: Possess a cell wall

    • Animals and Humans: Do not have cell walls; plants and some fungi do

    • Cell Surface Appendages:

    • Bacteria: Have pili and flagella for movement

    • Future talks will elaborate on these appendages

Size Comparisons

  • Bacterial cell size comparison to eukaryotic structures

    • Example: Mitochondria approximately the same size as bacterial cells

    • Origin of mitochondria: ancient engulfment of a bacterial cell (endosymbiotic theory)

Functions of the Cell Membrane

  • Importance of the cell membrane

    • Permeability Barrier:

    • Retains ATP and amino acids, preventing leakage into the environment

    • Allows effective waste removal and selective integration of external molecules.

    • Protein Anchor:

    • Membrane is a site for embedding transport proteins

    • Essential for cellular interactions and movements

    • Energy Generation:

    • Proton pumping across the membrane creates a proton gradient used to generate ATP

  • Summary: All cells need membranes to maintain functions and interactions with their environment.

Movement Across Membranes

  • Rate of movement for various molecules across membranes

    • Water: 100% permeability

    • Glycerol: Moves across membranes at 1,000 times slower than water

    • Larger molecules (e.g., tryptophan, glucose): require transporters for uptake

    • Charged ions (e.g., sodium, potassium): need active transport due to slow movement

Structure of Cell Membranes

  • Lipid bilayer composition

    • Fatty Acid Tails: Hydrophobic

    • Hydrophilic Head Group: Composed of glycerol, phosphate, and organic groups

    • Lipid bilayers form with tails inward and heads outward

  • Fatty Acid Varieties:

    • Saturated fatty acids: straight tails, solid at room temperature

    • Unsaturated fatty acids: kinked tails due to double bonds, create fluid membranes

Biological Consequences of Fatty Acid Structure

  • Examples in cooking:

    • Saturated fats (coconut oil, animal fats): solidify at room temperature

    • Unsaturated fats (olive oil, vegetable oil): remain fluid

  • Relevance in biology:

    • Membrane fluidity adjusted based on environmental temperatures

Influence of Cholesterol and Hopanoids

  • Cholesterol:

    • Found in animal cells, reduces phospholipid mobility and membrane permeability

  • Hopanoids:

    • Similar function to cholesterol, found in some bacterial membranes

    • Serve as molecular fossils in geological samples dating back 1.5 billion years

Phospholipid Differences Across Life Domains

  • Bacteria and Eukaryotic Phospholipids:

    • Comprise fatty acid chains, ester linkages, and SN glycerol 3 phosphate

  • Archaea Phospholipids:

    • Include isoprene chains, ether linkages, and SN glycerol 1 phosphate

    • More stable linkages beneficial for extremophiles

Complexities in Archaea Phospholipid Structures

  • Modifications for thermophiles:

    • Fused Phospholipids: Forming monolayers, increasing rigidity

    • Aromatic Structures: Enhance stability and pack interactions

Tree of Life and Membrane Composition

  • Differences between archaea and bacteria membranes

    • Bacteria: fatty acid chains, ester linkages

    • Archaea: isoprene chains, ether linkages

  • The origin of life and the potential membrane structure of LUCA (Last Universal Common Ancestor) remains speculative

Overview of Peptidoglycan Structure

  • Two main classes of bacteria as classified by Gram stain

    • Gram-positive: retains purple stain due to thick peptidoglycan layer

    • Gram-negative: receives pink stain due to thin peptidoglycan layer

Peptidoglycan's Role and Structure

  • Peptidoglycan provides structural integrity to the cell

    • Composed of alternating sugars N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)

  • Cross-linking of peptide chains creates a stable cage-like structure

  • Unique amino acids (D amino acids and DAP) found in bacterial peptidoglycan
    aiding in structural rigidity

Transpeptidation Reaction in Peptidoglycan Synthesis

  • Transpeptidation involves linking peptide chains

    • Enzyme: Transpeptidase (penicillin binding protein)

    • Antibiotics like penicillin inhibit the transpeptidation process

Fluorescent Labeling of Peptidoglycan

  • Researchers use fluorescent amino acids to visualize new peptidoglycan synthesis

  • Example experimental setups for tracking new peptidoglycan insertion in filamentous bacteria

Other Cell Envelope Components

  • Teichoic Acids in Gram-Positive Bacteria:

    • Wall- and lipo-teichoic acids, anchor significant portions of mass to the cell wall

    • Play roles in nutrient trapping and defense against antimicrobial peptides

  • Lipopolysaccharides (LPS) in Gram-Negative Bacteria:

    • Major component of the outer membrane

    • Composed of lipid A, core polysaccharide, and O-specific polysaccharide

    • Variation in LPS structure among bacterial strains aids in immune evasion

Archaea Structures

  • Archaeal cell walls: Not peptidoglycan but pseudo-peptidoglycan with different sugar compositions

  • S-layers composed of organized protein arrays can be found in both archaea and some bacteria

  • Capsules and slime layers provide additional protection for bacteria against environmental threats

Conclusion

  • Summary of the discussed structures and functions in the biology of bacteria and archaea

  • Encouragement for further study and understanding of microbial cell biology concepts.