2 - transport across cell membranes

the rate at which a solute crosses a protein free and artificial lipid bilayer by simple diffusion depends on

size and solubility

channel

pore that forms across the bilayer through which specific inorganic ions or polar organic molecules can diffuse

ion channels

exist in open or closed conformations that is controlled by an external stimulus or conditions within the cell

transporter

undergoes a series of conformational changes to transfer small solutes across the lipid bilayer (transfer at slower rate than channels and are more selective)

passive transport

spontaneous, allows solutes to move down their concentration gradients

active transport

against a concentration gradient to requires energy

the concentration gradient and membrane potential…

increase the driving force for movement of solutes

the membrane potential acts against the concentration gradient to…

decrease the electrochemical driving force

aquaporin

tetramer that forms a pore across the bilayer to transport water

tactics to avoid osmotic swelling

• discharging contractile vacuole

• cell wall

• reduce IC solute concentration by pumping out ions

three conformational states of a transporter

• outward open state (binding sites for solute exposed on outside)

• occluded state (sites are not accessible from either side)

• inward open state (sites are exposed on the inside of the bilayer)

• transition btwn states is random and reversible (does not depend on whether the solute-binding site is occupied)

active transport ways in pumps

gradient driven pump

atp driven pump

light driven pumps

Na+ pump

uses energy of ATP hydrolysis to pump Na+ OUT and K+ IN

symports

transfer solutes in the same direction

antiports

transport solutes in opposite directions

uniports

facilitate movement of a solute down its concentration gradient (not pumps bc no energy needed)