Membranes and cellular transport Flashcards
Membrane functions
-Boundary and permeability barrier
-Organisation and localisation of function (specific reaction occurs in specific organelle)
-Transport Processes
-Signak Detection
-Cell-to-cell interactions
Passive transport
Exergonic movement down the concentration gradient
-Simple diffusion
-Facillitated diffusion
Active Transport
Endergonic movment up concentration gradient away from equilibrium
-Primary active transport
Electrochemical Potential
Movement of ion is determined by electrochemical potential
-This is the combined effect of its concentration gradient and the charge gradient across the membrane
Membrane potential
Charge gradient across the membrane
-Created by active transport of ions across the membrane
Simple diffusion
-Unassisted movement
-High to low
-Movement towards equilibrium
E. simple diffusion
Typically gases, nonpolar molecules, small polar molecules (H2O, ethanol and glycerol)
-Includes: O2 and CO2 in and out of RBC
Facilitated diffusion
-Protein-mediated movement down gradient
-Limited number of transport proteins => can become saturated
E. facillitated diffusion
Polar or charged molecules like glucose
Transport proteins
Large, integral membrane proteins with multiple transmembrane segments
Carrier proteins
Bind solute molecules on one side of a membrane, undergo a conformational change and release the solute on other side of membrane
-Include transporters and permeases
Channel proteins
Hydrophilic channels through membrane to provide passage route for solutes
Alternating conformational model of carrier proteins
-Facilitated diffusion involves binding a substrate on a specific solute-binding site
-The carrier protein and solute form an intermediate
-After conformational change, the “product” is released (transport solute)
-Carrier proteins are regulated by external factors
Carrier protein characteristics
-High specificty
-Can become saturated
-Competitive inhibition is possible (because they can become saturated; useful for drug design)
Uniporter
-A carrier protein that transports a single solute across the membrane
Coupled transport
Two solutes are simultaneously transported
Symport/cotransport
2 solutes are moved across a membrane in the same direction
Antiport
Solutes are moved in opposite directions
E. uniport
Glucose transporter (GLUT1) in erythrocytes
GLUT1 mechanism
-Glucose binds to GLUT1 transporter protein that has a binding site open to the outside of the cell (T1 conformation)
-Glucose binding causes the GLUT1 transporter to shift to its T2 conformation with the binding site open to the inside of the cell
-Glucose is released to the interior of the cell, initiating a second conformational change in GLUT1
-Loss of bound glucose causes GLUT1 to return to its original (T1) conformation, ready for a further transport cycle
Anion exchange protein example
Chloride bicarbonate exchanger
-Antiport carrier
-Reciprocal exchange of Cl and HCO3- ions in 1:1 ratio
-Follows a ping pong model
Chloride bicarboante exchanger mechanism
-Cl binds to the protein on one side of membrane
-Binding causes a conformational change in the protein allowing the release on other side
-Cl release facilitates HCO3 binding, triggering another conformational change, releasing HCO3 on the other side
Channel proteins
Forms hydrophilic channels through membranes
-Allows direct movement
Highly selective
-Includes ion channels, porins and aquaporins
Types of channel proteins
-Ligand-gated
-Mechanically-gated
-Always open
-Voltage-gated
where is mechanical gated found
Mainly found in plant cells where high water pressure causes channels to open or close
Active transport use
-Uptake of essential nutrients
-Waste removal
-Maintenance of nonequilibrium of certain ions
Direct active transport
Involves a transport system coupled to an exergonic chemical reaction
-most commonly hydrolysis of ATP
Most common type of direct active transport
Hydrolysis of ATP drives the outward transport of protons which creates an electrochemical potential across the membrane
Indirect active transport
Involves the coupled transport of a solute and ions
-The exergonic inward movement of protons provides the energy to move a transported solute against its concentration or electrochemicl gradient
Na+/K+ ATPase pump overview
Direct active transport
-Most cells have high K+ and low Na+ outward
-Pumps K+ inward and Na+ outward
-Allosteric protein with 2 alternate conformational states: E1 and E2
Na+/K+ ATPase mechanism
-3 Na+ from inside the cell bind to E1
-Na+ binding triggers autophosphorylation of the alpha subunit using ATP and ADP is released
-A conformational change to E2 expels 3 Na+ to the outside of the cell
-2 K+ from outside bind to E2
-K+ binding triggers dephosphorylation causing a conformational change back to E1
-2 K+ expelled to inside as ATP binds and the pump returns to initial stae
E1 conformation
Pump open to inside
initial state
E2
Pump open to outside
ready to start second half of cycle
Na+/glucose symporter mechanism
-2 Na+ from outside the cell are bound
-Binding of Na+ allows glucose binding and a subsequent conformational change
-Symporter opens to inside
-Na+ are released inside, but are continually extruded to outside by a seperate Na+/K+ pump
-Loss of Na+ is followed by glucose release to inside
-Release of glucose allows the empty symporter to return to initial state
Importance of transporters for commercial use
-Drug targets
-Drug resistance mechanisms
Aquaporin
Aquaporin
