S2W2 - Membrane Proteins and Transport Across Membranes Flashcards
Permeability of an artificial bilayer as opposed to a cell membrane
Artificial bilayer (protein-free liposome): impermeable to most water-soluble molecules
Cell membrane: transports proteins to transfer specific molecules via facilitated transport
Movement across the lipid bilayer - small non polar molecules and small, uncharged polar molecules
Permeable - movement via simple diffusion through the lipid bilayer
1. high concentration to low concentration down the concentration gradient
2. more hydrophobic/nonpolar molecules have faster diffusion across the lipid bilayer
Small non-polar molecules
oxygen, carbon dioxide, nitrogen, steroid hormones
Small, uncharged polar molecules
water, ethanol - can diffuse across lipid bilayer
glycerol - cannot move across very well
Movement across the lipid bilayer - larger uncharged polar molecules and ions
Impermeable - require membrane proteins for transport
larger uncharged polar molecules
amino acids, nucleosides
glucose - a little can get across
Ions
H+, Na+, K+, Ca2+, Cl-, Mg2+, HCO3-
transmembrane transport proteins
- create a protein-lined hydrophilic path across cell membrane
- transport polar and charged molecules (amino acids, ions, sugars, nucleotides, various cell metabolites)
- each transport protein is selective and transports a specific class of molecules
- different cell membranes have a different complement of transport proteins
two main types of membrane transport proteins
- channels
- transporters
channel proteins vs transporter proteins in terms of selectivity and transport
channel proteins:
- selectivity: size and charge of electric solute
- transport: transient interactions as solute passes through. no conformational changes for transport through an open channel
transporter proteins:
- selectivity: solute fits into binding site
- transport: specific binding of solute. series of conformational changes for transport
passive transport
driven by the concentration gradient, no energy required
active transport
against concentration gradient, requiring energy
is glucose charged?
no
electrochemical gradient (electrical gradient) =
concentration gradient + membrane potential
what is the effect of the electrochemical gradient when the voltage and the concentration gradients work in the same direction?
- positive ions attracted to negative ions on the opposite side of the membrane (due to resting potential)
- this is additive to the effect of the concentration gradient
- greater net driving force
what is the effect of the electrochemical gradient when the voltage and the concentration gradients work in opposite directions?
- positive ions attracted to negative ions on the same side of the membrane (due to resting potential)
- smaller net driving force
describe voltage across a membrane
differences in charges of ions
resting membrane potential
stable electrical charge difference across a cell membrane when the cell is at rest and not actively sending signals
describe how channel proteins work
- hydrophilic pore across membrane
- most channel proteins are selective (eg ion channels transport a specific ion, determined by ion size and electric charge
- passive transport of solute
- transient interactions with channel wall as solute passes through (selectivity)
which are faster - channels or transporters?
channels
two broad categories of ion channels
- non-gated ion channels
- gated ion channels
non-gated ion channels
always open
gated ion channels
- some type of signal required for channel opening
- even when open, they are not open ALL the time, they just are open for more of the time
- specific ions are transported.
K+ leak channels
- non gated channel
- K+ moves out of cell
- major role in generating resting membrane potential in plasma membrane of animal cells
in what organisms are ion channels found?
animals, plants, microorganisms
types of gated ion channels
- mechanically-gated
- ligand-gated (extracellular ligand)
- ligand-gated (intracellular ligand)
- voltage-gated
mechanically-gated ion channels
signal - mechanical stress
eg plasma membrane may get stretched and that causes it to open up
ligand-gated (extracellular ligand)
signal - ligand from outside of the cell
eg neurotransmitter
ligand-gated (intracellular ligand)
signal - ligand from inside the cell
eg ion, nucleotide
voltage-gated
signal - change in voltage across membrane by membrane depolarisation
what is the difference between the signal to open and the transported material?
the signal causes the channel to open, it is not necessarily what is transported through the channel
main types of transporter proteins
- passive transport by transporter proteins
- uniport - active transport by transporter proteins
- gradient-driven pumps (symport, antiport)
- ATP-driven pumps (P-type pump, V-type proton pump, ABC transporter)
how do transporter proteins work?
bind a specific solute
- goes through a conformational change to transport solute across the membrane
compare transporter proteins to channel proteins in terms of rate of transport by drawing graph
different kinetics - rate of transport in channels gets faster and faster as concentration difference of transported molecule increases. for transporter, because it has to undergo conformational changes, it starts off at a fast rate but eventually hits Vmax as all binding sites get saturated
uniport
one solute
- passive transport down its electrochemical gradient
- direction of transport is reversible - dependent on concentration gradient
glucose transporter
GLUT uniporter
- transports D-glucose down the concentration gradient
- can work in either direction (glucose in or out of the cell)
gradient-driven pump
- 1st solute down its gradient, providing energy
- 2nd solute against its gradient using this energy
ATP-driven pump (ATPases)
ATP hydrolysis provides energy to move the solute against its gradient
Light-driven pump (bacteria)
uses light energy to move solute against its gradient
two types of gradient-driven pumps
symport and antiport
symport
two solutes moved in the same direction
antiport
two solutes moved in opposite direction
Na+ glucose symporter
- sodium going down its electrochemical gradient, providing energy
- glucose is going against its concentration gradient
- random oscillations between conformations which are reversible
occluded state
transporter closed - either occupied or empty
when do conformational changes in the Na+ glucose symporter occur?
both sites occupied: cooperative binding of Na+ and glucose
both sites empty: both Na+ and glucose dissociate
describe the process behind the Na+ glucose symporter
- The symporter has two binding sites: one for Na+ and one for glucose.
- In its outward-open state, facing the extracellular space where Na+ concentration is high, Na+ readily binds to its site on the symporter.
- Once Na+ is bound, it increases the affinity of the transporter for glucose. When a glucose molecule binds to its site, this triggers a conformational change in the protein.
- The transporter then transitions to an occluded state where both binding sites are inaccessible from either side of the membrane. This occluded state can only be reached when both Na+ and glucose are bound or when neither is bound (occluded-empty).
- From this occluded state, another conformational change occurs that opens up towards the cytosol (inward-open state), where Na+ concentration is low.
- Due to this low intracellular concentration, Na+ dissociates from its binding site and enters into cytosol.
- The release of Na+ reduces affinity for glucose at its binding site; thus, glucose also dissociates and enters into cytosol.
how does Na+-H+ exchanger work
antiport: Na+ down its electrochemical gradient provides energy to move H+ against its electrochemical gradient
- sodium goes into the cytosol
- H+ goes out of the cell
what is the function of the Na+-H+ exchanger?
- cytosolic pH needs to be regulated for optimal enzyme function (pH~7.2)
- but excess H+ occurs in the cytosol from acid forming reactions, and leaks out of the lysosome
- transporters maintain cytosolic pH: when there is a drop in cytosolic pH, the transporter activity increases and H+ is transported out of the cell
how is the Na+ electrochemical gradient maintained in animal cells?
Na+-K+ pump (plasma membrane ATP-driven pump)
why does the Na+ electrochemical gradient need to be maintained?
- Na+ going down its electrochemical gradient provides energy for symport and antiport transporters
- continued action of gradient-driven pumps may equalise the Na+ gradient
P-type pumps
- use ATP
- phosphorylated during the pumping cycle
- many P-type pumps transport ions (H+, K+, Na+, Ca2+)
- flippases to transport phospholipids
Na+ and K+ moved —– their electrochemical gradients
Na+ and K+ moved against their electrochemical gradients
sodium gradient used to:
- transport nutrients into cells (eg glucose)
- maintain pH
pumping cycle of the Na+-K+ pump
- 3 Na+ bind from inside the cell
- pump phosphorylates itself, hydrolysing ATP
- phosphorylation triggers conformational change and Na+ is ejected
- 2 K+ bind from the extracellular material
- pump dephosphorylates itself
- pump returns to original conformation and K+ is ejected
what do plant cells use instead of an Na+-K+ pump?
H+ pump
- generate H+ electrochemical gradient used for H+ driven symport/antiport
- leads to membrane potential
- H+ is moved from low (inside cell) to high (outside cell)
- solutes can then be moved into cell with H+
ABC transporters
use 2 ATP molecules to pump small molecules across the cell membrane
eg transport toxins outside of the cell but can also be source of chemotherapy resistance as cancer cells overproduce these transporters
V-type proton pump
- found in lysosome and plant vacuole
- uses ATP to pump H+ into organelles to acidify the lumen
F-type ATP synthase
- structurally related to V-type proton pump, but opposite mode of action
- uses H+ gradient (movement down) to drive the synthesis of ATP
- in mitochondria, chloroplasts, bacteria
compare a V-type proton pump to a F-type ATP synthase
V-type: uses ATP to pump H+ against the electrochemical gradient
F-type: uses the H+ electrochemical gradient to produce ATP.
is the action of the F-type ATP synthase reversible?
yes, it depends on ATP concentration and the H+ electrochemical gradient
how do transporters work together to transfer glucose from the intestine to the bloodstream?
epithelial cells of the villus
top of epithelial cells has microvilli - very high surface area
top of epithelial cell: apical domain
side: lateral domain
bottom: basal domain to face basal lamina
basal + lateral = bas-lateral domain
at top, we have Na+-glucose symporter which uses active transport to transport glucose against its gradient into the epithelial cell
in baso-lateral region, there is an Na+-K+ pump which keeps a low concentration of Na+ in the epithelial cell to maintain electrochemical gradient
GLUT uniporter carries out passive transport down the concentration gradient into extracellular fluid, which then ends up in the bloodstream
tight junctions between epithelial cells
creates a boundary where nothing can get by. proteins can move anywhere on apical surface or basal membrane but not past these junctions
helps keep transporters in their right compartment (eg GLUT uniporter and Na+-K+ pump stay on bottom, Na+-glucose symporter stays on top)
generation of membrane potentials
K+ leak channel (passive): facilitates outward flow of K+
1. K+ leak channels closed; plasma membrane potential = 0 (positive and negative charges balanced exactly)
2. K+ leak channels open; membrane potential exactly balances the tendency of K+ to leave
Na+-K+ pump
~10% of membrane potential
maintains:
- Na+ gradient with low cytosolic [Na+]
- K+ gradient with high cytosolic [K+]
electrogenic:
- 3Na+ pumped out
- 2K+ pumped in
- net 1+ ion pumped out
what is the usual balance in an animal cell?
cells generally balance electrical charges inside and outside of cell:
- extracellular space: high [Na+], high [Cl-], low [K+]
- cytosol: low [Na+], low[Cl-], high [K+], cell’s fixed anions (nucleic acids, proteins, cell metabolites)
but:
- K+ flows out (K+ leak channels), ions diffusing from high to low
- Na+-K+ pump resulting in net 1+ ion pumped out
net result:
- bit more positive on outside (Na+, K+)
- bit more negative on inside (Cl- and fixed anions)
- forms membrane potentials
equilibrium =
resting membrane potential, which in animal cells varies from -20mV to -200mV
draw a diagram to show the generation of membrane potential in animal cells
generation of membrane potential in plant cells
plasma membrane P-type pump
- H+ pump
- generates H+ electrochemical gradient of -120 to -160mV
- used by gradient-driven pumps to carry out active transport
- electrical signaling
- regulating pH
