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Bacteria (External Structures) (Appendages (Flagella -Appendage used…
Bacteria (External Structures)
Bacteria
-Unicellular organisms
-Oldest form of cellular life
-Also most widely dispersed & diverse form of life
-Have countless different functions
Appendages
Flagella
-Appendage used for motility
-Can perform full rotations
-Can be in single, double or multiple distributions over the entire cell
-Flagellated bacteria can detect & move in response to chemical signals (chemotaxis)
Axial Filaments
-Modified flagella which look like coiled threads; consist of a long, thin microfibril inserted into a hook
-AKA "Endoflagellum"
-Enclosed in the periplasmic space (between cell wall & cell membrane)
-Found in spirochetes & also used for motility (unusual, wriggly locomotion)
Parts
1. Filament
Long, thin helical structure made of proteins
2. Hook
Curved sheath
3. Basal Body
Stack of rings anchored to the cell wall
Arrangements
-4 Types of arrangements possible
2. Lophotrichous
Small branches from one end of the cell
3. Amphitrichous
Flagella found at both ends of the cell
1. Monoctrichous
Single flagellum found at one end of the cell
4. Peritrichous
Flagella found all over the cell surface
Chemotaxis
-Can be positive (movement towards the favourable stimulus)
-Can be negative (movement away from repellent compound)
-Run = Smooth linear swimming created by the flagella
-Tumble = Interruption of a run during which the flagellum reverses direction & the cell stops to change course
Pilli
-An elongate, rigid tubular structure made of pilin
-Only found in gram-negative bacteria
-Primarily involved in a mating process between cells known as "conjugation" which involves partial transfer of DNA from one cell to another
-Can also be used for adhesion purposes
Fimbriae
-Small, bristle-like fibres sprouting off the surface of the cell
-Responsible for adhesion/clinging to other cells
-Facilitate invasion for certain phagocytes
Cell Envelope
The complex of layers found external to the cell protoplasm; has 3 parts
2. Cell Wall
-Found immediately underneath the glycocalyx
-Determine the shape of a bacterium & provides structural support necessary to prevent bursting/collapsing
-
Peptidoglycan
is the unique macromolecule composed of a repeating framework of long glycan chains cross-linked by short peptide fragments that provides cell walls their strength & flexibility
-4 major divisions
Gram-Stain
-A staining technique developed by Hans Christian Gram to delineate two generally different forms of bacteria
2. Gram-Negative
-Has 2 major layers
-Made up of
lipopolysaccharide, lipoprotein (outer membrane)
&
peptidoglycan (inner membrane)
-Thinner (8-11 nm) but more complex than Gram-Positive
-Has an outer membrane & porin proteins
-Less penetrable to molecules
-Loses crystal violet & stains red from safranin counterstain
1. Gram-Positive
-Has 1 major layer
-Made up of peptidoglycan + 2 acidic polysaccharides (lipotechoic acid & techoic acid)
-Is 20-80 nm thick
-Has no outer membrane or porin proteins
-May have a periplasmic space
-More permeable to molecules
-Retains crystal violet & stains purple
3. No Cell Wall
-Some bacteria can have a cell wall & then lose it during part of their life cycle
-AKA "L-forms" or "L-phase variants"
-Arise naturally from a mutation in the wall-forming genes
-Created artificially by treatment w/ lysozyme/penicillin
-Protoplast = A Gram-Positive cell that completely loses its cell wall
-Spheroplast = A Gram-Negative cell that becomes severely weakened, but retains its outer membrane
4. Chemically Unique Cell Walls
-Ancient & primitive archaebacteria exhibit unusual & chemically distinct cell walls
-Some are composed entirely of either polysaccharides or pure protein
-Lack a true peptidoglycan structure
-Cell membrane likely serves dual functions of protection & transport
3. Cell Membrane
-AKA "Cytoplasmic Membrane"
-A very thin & flexible sheet molded completely around the cytoplasm
-Has a phospholipid bilayer w/ embedded proteins
Mesosomes
-Inward extensions of the cell membrane
-More prominent in Gram-Positive than Gram-Negative bacteria
-Functions are to increase internal surface area for membrane activities, participate in cell wall synthesis & cell division (by guiding duplicated chromatin bodies)
Functions
-Provides a site for functions such as energy reactions, nutrient processing & nutrient synthesis
-Regulates transport/passage of nutrients & discharge of wastes, enzymes & toxins into the extracellular environment
-Enzymes in the cell membrane synthesize structural macromolecules which are incorporated to the cell envelope & appendages
1. Glycocalyx
-A coating of macromolecules that protects the cell & helps it adhere to its environment
-2 Types
1. Capsule
-A highly organized, tightly attached arrangement of repeated polysaccharide/protein units
-Has a thicker, gummier consistency than a slime barrier
-Capsules protect bacteria from being engulfed by phagocytes
-Not absolutely necessary for survival
2. Slime Barrier
-A loose, soluble attached shield that protects the cell from loss of H2O & nutrients
-Slime barriers function to enable better adhesion, which in turn, results in easier colonization
-Not absolutely necessary for survival