Cell Biology and Microscopy Quiz

Prokaryotic Cell Structure - BIO-440

• The course covers prokaryotic cell structure, focusing on various components and their functions.
• Topics include: introduction to microscopy and cell structure, the cell envelope (cytoplasmic membrane, cell wall, outer membrane, S-layers), cell surface and inclusions, cell locomotion, and chemotaxis.
• Early microscopy: Robert Hooke used a microscope in 1664 and Antoni van Leeuwenhoek created drawings of bacteria in 1684.
• Light microscopy: uses visible light and lenses to magnify specimens. Contrast is crucial for visualization using methods like bright-field, phase-contrast, differential interference contrast, dark-field, and fluorescence. Samples may require staining for contrast. Techniques of Gram staining and fluorescent stains are mentioned.
• Electron Microscopy: uses electrons instead of light. High magnification and resolution, allowing viewing of structures at the molecular level. Transmission electron microscopy (TEM) sends electrons through a thin sample to study internal structures, while scanning electron microscopy (SEM) scans the surface to create a 3D image.

Cell Envelope

• The cell envelope is a layered structure surrounding the cytoplasm, governing interactions with the external environment.
• Components include: cytoplasmic membrane, cell wall, outer membrane, and S-layers.

Cytoplasmic Membrane

• Surrounds the cytoplasm.
• Composed of a phospholipid bilayer.
• Contains hydrophilic head groups and hydrophobic tails (present in bacteria and eukarya).
• Integral proteins (embedded in membrane) and peripheral proteins (associated with membrane surface) are present. Essential for interactions with environments.
• Functions: permeability barrier—controls entry/exit of substances, protein anchor—site for proteins involved in transport, bioenergetics, and chemotaxis, and energy conservation—site of generation and dissipation of proton motive force.
• Bacteria and Archaea have variations in membrane structure, impacting survival in extreme conditions. Archaea use ether linkages in their membranes

Cell Wall

• Layer outside the cytoplasmic membrane.
• Provides protection against osmotic lysis (bursting due to water pressure).
• Maintains cell shape and rigidity.
• Gram-positive cell walls are thick, while gram-negative cell walls are thinner. Structure differs depending on whether bacteria is gram positive or gram negative.
• Peptidoglycan is the major component of bacterial cell walls (polysaccharide backbone).
• Archaea have a different type of cell wall without peptidoglycan called pseudomurein.
• Mycobacteria do not have a cell wall.

Outer Membrane

• Second lipid bilayer exterior to the cell wall (only in Gram-negative bacteria).
• Structure and functions distinct from cytoplasmic membrane.
• Contains lipopolysaccharide (LPS).
• Porins are channels that create permeability. Outer membrane is more permeable than cytoplasmic membrane.
• Functions for surface recognition, virulence factor, mechanical strength.

LPS (Lipopolysaccharide)

• Often referred to as endotoxin due to toxic effects.
• Highly variable among species.
• Components include: O-specific polysaccharide, core polysaccharide, and lipid A.

Porins

• Unique to outer membrane.
• Act as channels for the passage of solutes.

Periplasm

• Region between the outer and cytoplasmic membranes.
• Contains hydrolytic enzymes, binding proteins (for transport), and chemoreceptors.

S-layers

• Found in many bacteria and nearly all archaea.
• Always the outermost layer of the cell envelope.
• May play the role of the cell wall, contributing to structural strength, protection against osmotic lysis, and maintaining cell shape.
• May form a periplasmic-like space in Archaea.
• Different configurations of the cell envelope structures are shown across various species.

Cell surface and inclusions

• Includes: capsules and slime layers, fimbriae and pili, carbon storage polymers, polyphosphate, sulfur, and carbonate minerals, and gas vesicles.

Capsules and slime layers

• Sticky polysaccharide layers exterior to the cell envelope.
• Involved in attachment and protection.

Fimbriae and pili

• Filamentous protein structures extending from the cell surface.
• Functions in attachment, motility, cell recognition, and genetic exchange.

Carbon storage polymers

• Poly-β-hydroxyalkanoates (PHAs) and glycogen are common intracellular carbon storage inclusions.

Polyphosphate, sulfur, and carbonate minerals

• Inorganic materials that microbes accumulate as storage.

Gas vesicles

• Confer buoyancy to some bacteria and archaea allowing the positioning in the water column where it's best for them metabolically.

Cell Locomotion

• Flagella (bacteria) and archaella (archaea): tiny rotating machines that push or pull the cell through liquids.
• Polar flagella: attached to one end or both ends of the cell.
• Peritrichous flagella: groups or tufts of flagella around the cell, causing rotation.

Flagella structure and functions

• Flagellar motor: composed of several proteins anchored in the cytoplasmic membrane, and cell wall.
• Components include the filament, hook, rings.
• Proton motive force drives the rotation of the flagellum for movement.

Surface motility

• Differs from flagella-driven motility
• Additional ways bacteria move
• Gliding: smooth motion along the long axis of a cell without external structures like pili/attachment organelles
• Twitching: driven by the extension/retraction of type IV pili
• Additional configurations seen in different prokaryotic organisms

Chemotaxis

• Directed movement toward (positive) or away from (negative) a chemical gradient.
• A method for measuring chemotaxis utilizes a chemotaxis assay.

Other taxis (Phototaxis and Magnetotaxis)

• Phototaxis: movement in response to light.
• Magnetotaxis: movement in response to magnetic fields. Different configurations observed in different species.