Cell Structure and Function Flashcards
MreB
- the skeleton that gives the bacterium its shape in bacilli
- as MreB polymerizes cell elongates
Peptidoglycan
- repeating units of B-1,4-N-acetylglucosamine (NAG) and N-acetyl-muramic acid (NAM).
- crosslinked through the NAM subunits by peptide bridges consisting of L and D amino acids and the unusual lysine derivative diaminopimelic acid (DAP)
Recognize structure of DAP
- 2 amino groups then -COOH on the end
Cross-linkage of gram positive cells
cross-linked to a much greater extend that gram negative cell walls, which give the gram negative cell wall more flexibility but less strength.
Cell wall components are made
- all cell wall components are made in the cytoplasm and must be transported outside (Gram +) or into the periplasm (Gram -) to be assembled.
Peptidoglycan synthesis in cytoplasm
- the carbohydrate subunits all start as NAG
- all the NAG is coupled to undecaprenyl-phosphate (UDP) (helps the sugar subunits cross the membrane)
- 1/2 is converted to UDP-NAM
- Amino acids are added sequentially to NAM to make UDP-NAM-aa (due to carboxylic acid)
- UDP-NAG and UDP-NAM-aa then cross the cytoplasmic membrane
recognize structure of UDP
- phosphate on the end
Peptidoglycan synthesis outside of the cytoplasm
- The sugar (NAG, NAM) subunits are polymerized via transglycosidation reactions
- The peptides are cross-linked via transpeptidases.
Further modifications in Gram positive cells
- addition of teichoic acid and lipotechoic acid.
techoic acid
- inserted vertically into the cell wall
- serves to strengthen the gram + cell wall by crosslinking the layers of peptidoglycan together via covalent bonds to NAM
liptechoic acid
- has a hydrophobic tail that inserts into the cell membrane
- attaches the cell wall to the lipid bilayer via a lipid anchor into the membrane
Cytoplasmic membrane
- permeability barrier. No solutes can pass through without going through specific transport proteins
- integral membrane proteins act as transporters to allow things in and out
- bacterial membranes are composed of a phospholipid bilayer - made up of hydrophobic lipid molecules (facing each other) with hydrophilic head groups (facing the water).
- the membrane is self-associated and not covalently bound together, so it is flexible yet resilient.
cytoplasmic gradient
- H+ on outside - gradient to bring substrates in
- relies heavily on this gradient to power processes and move the cell.
Outer membrane
- not as restrictive a barrier as cytoplasmic membrane
- many low molecular weight compounds are allowed free access through protein channels called porins
Lipopolysaccharide
- covered with carbohydrate
- O-specific - different for different strains - endotoxin
- innate immune recognizes this LPS
Nucleoid
- the region of the cell in which the chromosome is located that is not bound by a membrane.
- Most bacterial chromosomes are circular but must be greatly compacted in order to fit within the nucleoid space.
Locomotion
- used for finding nutrients, avoiding toxins, and colonizing favorable ecological niches
- bacteria use flagella to swim through aqueous solution
- to move on solid surface, bacteria can glide, swarm, and twitch.
Myxococcus
- not flagellum-related, slime-related, “caterpillar”-membrane cycling
- it is genetically determined, PMF driven, and MreB dependent.
- motors are localized to focal adhesions on one side of gliding cells.
Twitching motility
- mediated by pili or fimbrae
- used by bacteria to move over solid or semi-solid media
- twitching cells hyperpilated, and thought that the movement is mediated by the extension and retraction of pili.
Flagella structure
- contains a motor and a large extracellular filament
- filament and motor and connected via a transmission that crosses the periplasm and outer membrane and terminates in the hook.
- the hook makes it such that when the motor turns the filament turns in a sweeping motion that propels the bacterium like a boat propeller.
- the motor is powered by the PMF set up by the ETC
- the direction of the motor can change depending if a phosphate is added or not.
Spirochetes and spirilum
- use flagella to move but their flagella are not extracellular.
- they have a series of amphitrichous flagella that are contained with the periplasm and wrapped around the cell.
- when the flagella turn, the spirochete will twist and “corkscrew” through a viscous media.
Why move in this way?
- adopted based on their environment
- many are pathogens and live in viscous environments and must constantly evade host detection.
- corkscrew motion very efficient in moving the cell through very viscous environment such as mucus
- lacking a prominent external structure helps them to have fewer antigenic features.
Gliding
- Myxococcus move through gliding.
- do not use pili or flagella.
Chemotaxis
- bacteria move toward attractants (food) and away from toxins.
- in orer to do they have a sensors clustered in their membranes that are constantly sensing the presence of chemicals in their environment.
- not big enough tot sense a chemical gradient, so they must be on the move to detect it.
- sample the chemical, move, and sample again. If the concentration of an attract goes up, they keep going, if it goes down, they change direction.
Molecular basis for chemotaxis
- the cell has chemoreceptors in its membrane that detects chemo-attractants and toxins called methyl accepting chemotaxis proteins (MCPs)
- MCPs pass the signal (phosphate) to a sensor kinase protein called CheA with the help of a signal transducing domain.
CheA
transfers phosphate to CheY
CheY-P
binds the motor and causes it to turn counterclockwise, or run
When CheY not phosphorylated
it will not bind the motor, and the motor will turn clockwise. and tumble
Default mode for E. Coli
- tumble
- will only run when it senses a chemical attractant.
Adaptation
- make sure the bacterium continues up the gradient, it must “reset” the baseline of how much attractant it is encountering constantly.
- cell will only continue to “run” if it is encountering increasing amounts of attractant
CheR
- regulates how much of the substrate it takes to excite to the MCP.
- methyl-transferase that will methylate the MCP and cause them to be less sensitive to the chemical.
- # methyl groups determines how sensitive it is
CheB
- methylesterase
- resets the baseline
