Bacterial Cell Structure and Function Quiz

Lecture 3: Cell Structure and Function

• Bacterial cells have a complex cell envelope, a compact genome, and tightly coordinated functions.
• Archaea, like bacteria, are prokaryotes but have unique membrane and envelope structures.
• Eukaryotic cells have a nucleus and extensive membranous organelles.

Chapter Overview

• 3.1 The Bacterial Cell: An Overview: Provides general information about bacterial cells.
• 3.2 Membrane Molecules and Transport: Details how molecules move in and out of bacterial cells.
• 3.3 Cell Envelope: Describes the layers surrounding the bacterial cell.
• 3.4 Bacterial Cytoskeleton and Cell Division: Discusses the internal framework and how bacteria reproduce.
• 3.5 Cell Asymmetry: Explains the structural differences between various parts of a microbial cell.
• 3.6 Specialized Structures: Outlines specialized components like storage granules, magnetosomes, and pili.

The Bacterial Cell

• Most bacterial cells share fundamental features, including a complex cell envelope, compact genome, and tightly coordinated functions.
• Archaea are prokaryotes with unique membranes and envelopes.
• Eukaryotic cells have a nucleus and extensive membranous organelles.
• Early 20th-century models viewed the cell as a "soup" of ribosomes and enzymes. Modern research shows an ordered, yet flexible cell structure.

Model of a Bacterial Cell

• Cytoplasm: Viscous, gel-like substance.
• Cell Membrane: Encloses the cytoplasm.
• Cell Wall: Covers the cell membrane.
• Nucleoid: Non-membrane-bound area of the cytoplasm containing the chromosome.
• Flagellum: External helical filament propelling the cell.

Studying Cell Components

• Cell study requires isolating and analyzing cell parts.
• Cell Fractionation: Cells are broken down allowing intracellular parts to remain intact. Methods include using mild detergents, enzymes, sonication, and mechanical disruption.
• Subcellular components are separated using an ultracentrifuge.
• Parts are then subjected to structural and biochemical analysis.

Studying Cell Parts

• Employing genetic analysis (complementing cell fractionation).
• Uses different genotypes (mutant strains selected for specific loss/alteration of gene function).
• Uses Reporter genes to express protein function of interest (e.g., GFP).
• The mutant cell phenotype reveals information about the function of the altered part.

Biochemical Composition of Bacteria

• All cells contain common chemical components (water, essential ions, small organic molecules, and macromolecules).
• Cell composition varies with species, growth phase, and environmental conditions.
• A table provides the composition of Escherichia coli.

Cell Membrane

• The cell membrane defines the cell's existence.
• It's a two-dimensional fluid comprised of lipids and proteins.
• Lipids are arranged in a bilayer.
• It contains the cytoplasm, mediates transport, and carries proteins serving various physiological roles.

Membrane Lipids

• Phospholipids consist of glycerol, fatty acids, and a phosphoryl head group.
• Some phospholipids have side chains.
• Phospholipids are amphipathic (polar/charged hydrophilic heads and hydrophobic fatty acid tails).
• The two layers of phospholipids are called leaflets.

Phospholipid Diversity

• Phospholipids vary, differing in phosphoryl head groups and fatty acid side chains.
• Some examples include phosphatidyl-ethanolamine, phosphatidyl-serine, phosphatidyl-choline, sphingomyelin, and sphingosine.

Phospholipid Diversity (Cardiolipin/Glycerol)

• Cardiolipin (diphosphatidylglycerol) is a double phospholipid.
• It increases in bacteria grown to starvation, localizes to cell poles, and stabilizes membrane curves.

Phospholipid Diversity (Saturated/Unsaturated, Cyclization)

• Unsaturated fatty acids increase membrane fluidity at low temperatures
• Saturated fatty acids decrease fluidity at high temperatures.
• Cyclization of fatty acids can decrease fluidity by forming rigid rings.

Hopanoids

• Membranes include planar molecules that fill gaps between hydrocarbon chains.
• Bacterial reinforcing agents are hopanoids/hopanes.
• Hopanoids are pentacyclic lipids modifying membrane fluidity.
• They comprise a small percentage of total lipids (not present in archaea).
• Sterols (like cholesterol) are analogous components in eukaryotic membranes.

Membrane Lipids of Archaea

• Archaea exhibit variations in phospholipid side-chain structures.
• Some have ether linkages between glycerol and fatty acids.
• Hydrocarbon chains may be branched terpenoids (containing isoprene rings).

Membrane Proteins

• Membrane proteins perform various roles.
• They provide structural support, detect environmental signals, secrete virulence factors, transport ions, and store energy.
• Proteins possess hydrophilic and hydrophobic regions locking them into the membrane.

Transport Across the Cell Membrane

• The cell membrane acts as a semi-permeable barrier.
• Small uncharged molecules (O2 and CO2) diffuse across it.
• Water diffuses across through a process called osmosis.
• Solutes move along concentration gradients (high to low).

Transport Across the Cell Membrane (Weak Acids/Bases)

• Weak acids/bases partially exist in uncharged forms in membranes.
• Uncharged forms can move across the membrane and affect cell pH.

Transport Across the Cell Membrane (Passive/Active Transport)

• Large polar molecules/charged molecules require protein transporters.
• Passive transport: molecules move along concentration gradients (facilitated diffusion).
• Active transport: molecules move against concentration gradients, requiring energy (ATP hydrolysis).
• Coupled transport uses energy from one substance's gradient to transport another against its gradient (symporters, antiporters).

Passive Transport (Facilitated Diffusion)

• Facilitated diffusion uses concentration gradients to transport molecules (high-to-low).
• Moves large or too polar molecules for direct diffusion.
• Example: aquaporins transport water and small polar molecules.

Active Transport: ATP Hydrolysis

• ATP stored energy drives active transport.
• ATP-binding cassette (ABC) transporters: the largest active transport family in prokaryotes.
• These involve integral membrane permeases, ATP-binding/hydrolysis (ATPase), and substrate-binding subunits.

Active Transport: Coupled Transport

• Coupled transport uses energy released from one substance travelling down its gradient to move a second substance against its gradient.
• Symporters move two substances in the same direction.
• Antiporters move two substances in opposite directions.

Active Transport: Group Translocation

• Phosphoenolpyruvate (PEP) group translocation or Phosphate transfer energizes the import of substances like glucose into bacterial cells.

The Cell Wall

• The cell wall provides shape, rigidity, and resistance to turgor pressure. It's a single molecule composed of peptidoglycan.
• The peptidoglycan sacculus (cell wall) is a single, interlinked molecule. In Escherichia coli.

Peptidoglycan Structure

• Bacterial cell walls are composed of peptidoglycan.
• Peptidoglycan's two main components are N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).
• Peptides cross-link these sugars (peptidoglycan) and transpeptidases (PBPs) cross-link wall peptides.
• Transglycosylases extend the polysaccharide chains of the peptidoglycan molecule.

Peptidoglycan Structure (Antibiotics)

• Enzymes in peptidoglycan synthesis are excellent antibiotic targets.
• Penicillin inhibits transpeptidases.
• Vancomycin inhibits cross-bridge formation.

Peptidoglycan Structure (Bacteria Synthesis)

• Cell walls are synthesized differently in various bacterial species.

Pseudomurein of Archaea

• Pseudomurein is found in archaea and is structurally distinct from bacterial peptidoglycan.
• It is insensitive to lysozyme due to NAT instead of NAM, and NAG-β(1,3)-NAT linkages.
• It has unique side-chain peptide cross-links.

Gram-Positive and Gram-Negative Bacteria

• Most bacteria have additional envelope layers providing structural support and protection.
• Gram-positive bacteria have thick cell walls (e.g., Staphylococcus aureus).
• Gram-negative bacteria have thin cell walls (e.g., Yersinia pestis).

Gram-Positive Cell Envelope (Teichoic Acids/S-layer)

• Gram-positive envelopes have multiple peptidoglycan layers, and teichoic acids (WTA/LTA) are present.
• S-layer is an additional protective protein layer found in free-living bacteria, and archaea.

Mycobacterial Cell Envelope

• Mycobacteria (like M. tuberculosis and M. leprae) have complex envelopes containing unique lipids (mycolic acids) and unusual sugar derivatives (arabinogalactans).

Gram-Negative Cell Envelope (LPS/Braun Lipoprotein)

• The outer membrane of gram-negative bacteria is crucial for defense and toxin production.
• Braun lipoprotein is covalently linked to peptidoglycan (PG).
• Lipopolysaccharide (LPS) is a component of the outer membrane and can act as an endotoxin, causing septic shock.

Eukaryotic Microbes (Osmotic Shock)

• Eukaryotic microbes have their own structures to prevent osmotic shock (water influx). Examples include algae (cellulose walls), fungi (chitin walls), diatoms (silica exoskeletons), and paramecia (contractile vacuoles).

Bacterial Cytoskeleton

• Shape-determining proteins (like FtsZ, MreB, and CreS) in bacteria affect cell division and elongation.

Bacterial Cell Division (Binary Fission)

• Prokaryotes divide by binary fission, where a single mother cell splits to form two daughter cells.
• This process requires coordinated growth and expansion of all cell parts.

Bacterial Cell Division: Chromosome Replication

• Replication of the circular chromosome precedes binary fission.
• Replication starts at the ori site and proceeds outward in both directions (bidirectional).

Bacterial Cell Division: Divisome

• FtsZ initiates divisome formation.
• Divisomes' purpose is to coordinate peptidoglycan and lipid membrane synthesis, and coordinate chromosome segregation.

Bacterial Cell Division: Septation

• The division septum is formed at the Z-ring, where the inward growth constricts and separates daughter cells to complete cell division.

Bacterial Cell Division: Septal Planes

• The spatial orientation of septation influences coccus shape/arrangement (parallel planes, random planes, perpendicular planes).

Cell Asymmetry and Aging

• Bacterial cells' poles differ in origin and age (polar aging).
• Asymmetrical cell division (e.g., sporulation in Bacillus) creates differences in daughter cells.
• Some bacteria only grow at a specific pole,
• Differences in poles can affect susceptibility to antibiotics.

Cell Asymmetry (Daughter Cell Types)

• Some bacteria generate two types of daughter cells (stationary or mobile).

Specialized Structures: Membrane Vesicles

• Some microbes export cytoplasm via membrane vesicles.
• These vesicles carry proteins, nucleic acids, toxins, and immunogenic molecules.

Specialized Structures: Thylakoids, Carboxysomes, Gas Vesicles

• Thylakoids: extensively folded membranes for photon absorption and photosynthesis.
• Carboxysomes: polyhedral bodies packed with Rubisco (CO2 fixation enzyme).
• Gas vesicles: increase buoyancy.

Specialized Structures: Storage Granules and Magnetosomes

• Storage granules: store things like glycogen (energy) and sulfur (oxidation).
• Magnetosomes: membrane-embedded magnetite crystals that orient magnetotactic bacteria.

Specialized Structures: Pili and Stalks

• Pili/fimbriae: straight filaments of protein for attachment, motility, or conjugation.
• Sex pili: specialized pili for DNA transfer.
• Stalks: membrane-embedded cytoplasmic extensions with adhesion factors (holdfasts).
• Nanotubes: intercellular connections for material transfer.

Specialized Structures: Cryo-ET

• Cryo-electron tomography (Cryo-ET) revealed novel structures.
• Example features found include pearling tubes and nanopod extensions.

Specialized Structures: Rotary Flagella

• Motile prokaryotes have flagella for movement.
• Flagella types include peritrichous, lophotrichous, amphitrichous, and monotrichous, differing in arrangement and number.
• Flagella rotate via a motor powered by the proton motive force, affecting movement and chemotaxis.

Chemotaxis

• Chemotaxis is the bacterial movement in response to chemical gradients.
• Attractants cause counterclockwise (CCW) rotation and flagella bundling ("run").
• Repellents cause clockwise (CW) rotation and flagella unbundling ("tumble").

Chapter Summary

• Prokaryotes share fundamental traits but are diverse.
• Studying cells uses various methods including fractionation, structural, and genetic analysis.
• Bacterial cell membranes have phospholipid bilayers and ether linkages while archaea have ether linkages.
• Gram-negative cell envelopes are more complex than Gram-positive. Bacterial cell divisoin and septation are highly coordinated.
• Prokaryotic cell division proceeds via binary fission involving the divisome complex. Different specialized structures aid in functions ranging from movement to nutrient acquisition and sensing external environments.