Permeability Of Lipid Bilayer Smaller and more hydrophobic molecules diffuse across membrane more rapidly

Membrane Transport Proteins Many types; each transports specific molecule Carrier proteins- bind molecule, conformational change Channel proteins- aqueous pore allows passage

Passive Versus Active Transport Driven by concentration or electrochemical gradient Mediated by carriers and channels Passive Active Transport against gradient Requires energy input Mediated only by carriers

Types Of Carriers Uniport- single molecule Symport- two molecules in same direction Antiport- two molecules in opposite directions

Passive Transport By Carriers Resembles enzyme-substrate reaction Binding affinity reflected by K m V max achieved when binding saturated Binding can be inhibited

Mechanisms Of Active Transport Driven by ATP hydrolysis (primary active transport) Driven by coupling to passive transport of a different molecule (secondary active transport)

Na + -K + Pump [Na + ] higher on outside; [K + ] higher on inside ATP-driven Na + -K + antiporter

Na + influx coupled to active transport across plasma membrane Import of sugars and amino acids by Na + -driven symports Using Na + Gradient To Drive Active Transport

Transcellular Transport Transport through epithelial cells Active transport by Na + -driven symporters on apical surface Diffusion via carriers on basolateral surface

Regulating pH Na + -H + exchanger: Na + gradient drives H + out of the cell ATP-driven H + pumps: H + into intracellular compartments such as lysosomes

Maintaining Low Cytosolic Ca 2+ Levels Ca 2+ ATPase (calcium pump): Ca 2+ out of cell or into intra- cellular compartments Na + -Ca 2+ exchanger: Na + gradient drives Ca 2+ out of the cell

Classes of ATP-Driven Pumps

P-Type Transport ATPases Includes Na + -K + pump and Ca 2+ ATPase Autophosphorylation and subsequent dephosphorylation transmit conformational changes

ABC Transporters ATP binding domains: dimerization from ATP binding, dissociation from ATP hydrolysis ATP binding/hydrolysis transmits conformational changes MDR overexpression causes resistance to cancer chemotherapy drugs

Osmosis Diffusion of H 2 0 across membrane; facilitated by aquaporins H 2 0 moves from side of low to high solute concentration Water enters cells in hypotonic and leaves in hypertonic solutions

Control Of Cell Osmolarity Charged macromolecules and metabolites attract inorganic ions Osmotic gradient pulls in H 2 0 Opposing osmotic gradient generated by Na + (pumped out) and Cl - in extracellular fluid

Ion Channels Passive transport Selective Gated

Gating Of Ion Channels

Resting Membrane Potential Between -20 mV and -200 mV Small flow of ions has large effect

Generating Resting Membrane Potential Depends mostly on K + gradient and K + leak channels K + flows out of cell down its electrochemical gradient Equilibrium condition defines resting membrane potential

Cystic Fibrosis Cystic fibrosis transmembrane conductance regulator (CFTR) is chloride channel Critical function in epithelial cells lining airway CFTR  F508 mutant not transported to plasma membrane; retained in ER and degraded