BI 375 transport of substances across and between cell membranes

throws up

passive transporters (pore-based transporters)

• AQPs

• ion channels

• uniporters

• porins

ALL enable facilitated diffusion

selective transporters

• AQPs

• ion channels

• uniporters

nonselective transporters

porins

AQP (aquaporin)

narrow pore diameter, selective for water

porin

relatively large pore diameter, allows any substance that can fit through, non-gated

ion channel

very small pore diameter with selectivity filter for specific ion & (in most) a gating domain that can conformationally cycle open or closed

uniporters

randomly, spontaneously & reversibly cycle btwn different conformational states

• the transporter is NEVER continuously open to both sides of the membrane

primary and secondary active transporters

active transporters

primary active transporters

• ATP driven pumps

• Redox-driven pumps

• Photon-driven pumps

secondary active transporters

gradient-driven pumps or co-transporters

active transport (give definition)

transport of a substance AGAINST a concentration or electrochemical gradient (low → high)

requires ATP and a carrier protein (ion pumps)

primary active transport

transportation of a substance across PM is directly coupled with hydrolysis of ATP

secondary active transport

the movement of two substances simultaneously across membrane indirectly coupled to ATP hydrolysis

(indirect utilization of ATP as an energy source)

simple diffusion w/ concentration gradient

mechanism for movement of freely bilayer permeant substance

simple diffusion OR facilitated diffusion (
via passive transport protein) [w/ concentration gradient]

mechanism for movement of somewhat bilayer permeant substance

facilitated diffusion (w/concentration or electrochemical gradient)

mechanism for movement of bilayer impermeant substance

active transport (via active transport protein)

mechanism for movement of any substances moving in absence of or against gradient

symporter (explain process)

Up, open side, facing high end of gradient so x wants to bind

Substance that is moving down gradient MUST bind first

Second binding site for y substance ONLY becomes operational AFTER first substance has bound

X binds -> Y then can bind

Only comes off or dissociates when down open side

Follows same rules that uniporter would in isolation

X and Y bind ONTO transporter on same side of membrane and come off same side of the membrane on opposite side

Down open conformation:

X wants to come off

X comes off, Y is forced off as well

X behavior 100% dictated by it’s gradient 

Y is forced to move in same direction as X

antiporter (explain process)

Opposite relationship between two substances

X behavior is SAME, the DIFFERENCE is in y

Y can only bind once x has come OFF and can only come off once x is ON

X still moves in same direction, Y moves in OPPOSITE direction 

using nerst equation to calculate the eq potential

how to predict net direction of inorganic ions down EC gradients?

Efflux occurs: value of membrane potential gets more negative

Influx occurs: value of membrane potential gets more positive/

efflux vs influx for CATIONS

Efflux: value of membrane potential gets more positive

 Influx: value of membrane potential gets more negative

efflux vs influx for ANIONS