membrane transport

primary active transport

the free energy of ATP hydrolysis is used to drive the movement of sodium and potassium against their conc gradients maintaining a source of potential energy.

what are porins

• form aqueous channels and accelerate the passive diffusion of small hydrophilic molecules across the membrane

• solute selectivity of a porin is detemined by characteristics of the amino acid side chains at the entrane and interior lining of the pore as well as the size of the opening

• the positively charged regions at the mouth of the pore and at the construction site makes the pore specific for small anions

• they are beta proteins

ion channels

• require more than one subunit to form a membrane passageway

• only open when stimulated

• signal could be through ligand binding to the transporter, changes in membrane potential, changes in pH or covalent modification by a cellular enzyme

• after simulation the blocked gate then opens by structural changes that move a polypeptide segment out of the channel or by a concered conformational rotation of helices that open the pore like the iris of a camera

membrane carriers

• do not have a channel or a pore

• bind molecules selectively and change their structure to allow them to pass to the other side of the membrane

uniport transporter

• liver cells

• shuttle glucose between liver and bloodstream

• only one solute moved

• direction of mov is passive or down conc gradient

symport transporter

• not passive

• potential energy of a steep gradient is dissipated and used to drive the movement of another molecule against its concentration gradient. 

• eg sodium ion glucose transporter in the renl epithelial cells in the kidney

antiport transporter

• also known as secondary active transport

• one solute moves along its electrochemical gradient and results in another to move against its electrochemical gradient

• eg sodium and hydrogen ions in kidney tubules