Week 4: Cell membrane and passive transport Flashcards
what is the only site of active transport?
plasma membrane - solids reach plasma membrane via passive diffusion
nitrate vs glucose entering the cell
glucose = carbon and energy source
NO3- : nitrogen source (only of the most available)
- nitrate enters through coupled transport (think its an ion thats how they usually go through)
- glucose enters throough different type of active transport
what are the layers of a gram negative cell?
*from outer to inner
- capsule: outermost mucus layer - not all will have this
- outer membrane has LPS, an asymmetric lipid bilayer containing the O antigens
- creates a diffusion barrie to bile salts in Mcconkey agar
- outermembrane not a true barrier bc solutes enter through proin
- porous peptidoglycan layer
- plasma membrane: only site of active transport
describe the plasma membrane of bacteria, eukaryotes and archea
- bac and eukaryotes:
- ester linkages
- proteins that are able to move laterally within the plane of the membrane as well as between the two phospholipid bilayers
Archea
- fundamentally different membranes
- have ether lnkages and phoshplipids hae branched chains
- some form monolayers, can also form bilayers tho like bacteria and euk
describe bacterial cell wall
*cell wall is part with periplasm outside of the plasma membrane
- protects cell from harsh conditions
- major copmonent is peptidoglycan composed of long chains of NAG and NAM
- in gram -ve tetrapeptide chains extending from each NAM unit are directly cross linked
- in gram +ve tetrapeptide chains are linked by pentaglycine bridges
*once nutrient diffuses trhoguh cell wall alyers its at the periplasm of gram ve or external face of PM
gram pos vs neg cell wall
- gram positive cells
- many layers of peptidoglycan embedded with teichoic acids - thought to stabilize peptidoglycan and inc rigidity
- gram -ve
- much thinner peptidoglycan layer but more complex overall structure
- cell like matrix occupies periplasmic space between cell wall and plasma membrane
- has second lipid layer called outer membrane external to peptidoglycan (connected to peptidoglycan by murein lipoproteins)
- outer leaflet has lipopolysaccharides, act as an endotoxin in infections involving gram neg bacteri - composed of Lipid A, more polysaccharide and O antigen
-
how does cell wall portect cells from osmotic pressure
- osomotic pressure is due to differences in solute concentration on oposing sides of the semipermeable membrane - water can pass but solutes cannot
- cells lacking cell wall can become dehydrates causing crenation (shriveling in hypertonic env)
- cells with call wall undergo plasmolysis when in hyperotnic solution
^PM contracts and detaches from cell wall - dec interior volume
in isotonic cell wall reminas intact alowing cell to maintain some shape/integrity
- in hypotonic , water move into cell and cell wall counteracts osmotic pressure to prevent swelling and lysis
what are glycocalyses
- some prokaryotes have additional cell envelope liike glycocalyces and S layers
- Glycocalyx is a sugar coat: slime layer (EPS) and capsules
- capsular polysaccharide layer of organized layer outside the cell wall
- slime layer is lightly organized layer only loosely attached to cell wall and can be easily washed off
- glycocalyces allow cells to adhere to surfaces, aiding in biofilm formation
- capsular and EPS layers on biofilms hold water like sponge- preventing dessication, predation and hindering action of antibiotics and host immune responses
where is an S layer located?
S layer is omposed of a mix of structual proteins and glycoproteins
- in bacteria S layer is outside the cell wall
in some archea the S layer serves as a call wall
*hard to study, exact function not really understood
describe an oveview of transport across the plasma membrane
- PM is a selectivel permeable and fluid mosaic
- small uncharged mol like gases diffuse
- while water or glycerol (small unchanged polar mol) diffuse but less easily -> can move through aquamorine (NOT gram -ve porins)
*this passive movement is driven by brownian motion
- everything else needs avtive transport
where are the energy sources for active transport located?
- outside the PM in form of proton motice force or inside the cell as ATP
- bacteria and archea do not have energy generating organelles - membrane of organelles where is where energy generation happens is associated with the plasma mem of bacteria and archea
- ATP synthase is on PM of bacteria and archea -> allows for the entry of protons down conc gradient for generation of ATP
compare facilitated diffusion with simple diffusion
* both reply on a conentration gradient
- the facillitator is a transporter with specific binding sites for the substrate to move from high conc to low conc
- all transporters are substrate specific - bind, conformational change and then movement into or out of cell
- water can glycerol can passively diffuse but facilitated much faster
*membranes must be semi-fluid for transport and conformational change
- facilitated inc rate of diffusion until all sites occupied - will reach maximum this doesnt happen with simple
what is the driving force of coupled transport
- proton motive force
- used for:
- ATP found only inside cell and the atp dependent active transport and group translocation (only poss in certain bacteria)
what substrates use ABC transporters
ABC tranpsorters = ATP binding cassette transporters
- used by maltose and histidine
what expresses ATP transporters? what are they used for?
- most widepsread of membrane bound tranporters found in all domains of life
- can be recognized based on sequencing
- used for import and export (multidrug efflux)
*if oerexpresed in human cells can lead to being resistant to chemo drugs
how does ATP transporter work?
- has external substrate binding protein that is in periplasm or tethered to outer face of PM
1. transport initiated by substrate binding to substrate binding protein w/ high affinity
2. conformation all change allows it to bind to permease
3. interaction leads to release of substrate to the permease
4. tiggers another confomrational change of the permease that moves to the cytoplasmic phase of permease
5. interacts with ATP binding dimers, singals activation of enzyme to catalyze hydrolysis of ATP to ADP and Pi
6. another confomrational change in permease
7. hydrolysis prodiced energy to drive substrate into cell
describe the steps of group translocation
- used for import, substrate is altered by a high energy phosphoate group (energy source) from phosphoenol pyruvate
1. phosphate group transfered from PEP to enzyme 1, (dephosphorylated PEP is pyruvate)
2. Phosphorylated enzyme 1 transferred phosphoate to HPr (histidine protein)
3. HPr phosphorylates the E2A domain when there is substrate avilabale and bound to other side of permease
4. E2A transfers phosphate to 2B which transfers to the substrate
*each phosphorylation in thie reaction causes a conformational change
- once substrate is phosphorylated, E2C undergoes (the permase portion) undergoes conformational change and substrate is released to cytoplasm as the phosphorylated version - totally enw molecule
*glucose 6 phosphate now in cell and goes right into glycolytic pathway
ex of coupled transport - antipost
Na+, K+ antoport with H+
*coupled transport that sues energy from a concentration gradient (PMF)
