Membrane transport Flashcards
Permeability through synthetic lipid bilayers is determined by
- molecular size
- electrical charge
- lipid solubility of molecule
- hydrophobicity and hydrophilicity
What are the least and most easily permeable molecules?
Most to Least
hydrophobic molecules (O2, CO2, N2)
Small, uncharged, polar molecules (H20, glycerol, NH3)
large, uncharged polar molecules (glucose, sucrose)
ions (Na+, K+, Cl-, etc)
What causes ions to be so impermeable?
they have strong electrical charges and hydration shells that form as the charges associated with water.
What are ion channels?
transmembrane proteins that conduct ions at high rates. they possess aqueous pore that is shielded from the bilayer. ion channels can have gates and be highly selective and mediate passive transport by simple diffusion
What are transporters?
membrane proteins that transport solutes across bilayers. They bind more strongly than channels but weaker than pumps. Slower than channels because they toggle between two conformations in order to transport solutes. (passive transport moves along gradient)
define passive and active transport
passive: solute moves along electrochemical gradient
active: solute moves against electrochemical gradient, must have energy input either directly or indirectly
Primary active transport
solutes are transported against their gradient by a transporter protein (pump) that consumes energy (directly uses energy)
secondary active transport
solutes are transported against their gradient using a concentration or electrochemical gradient established by a pump (indirectly uses energy)
how does coupled (secondary active) transport work?
one solute moves downhill along its gradient and the energy release of this action drives transport of a second molecule uphill against its gradient. phosphorylation of a pump transporter protein using ATP drives a conformational change that transports solutes against gradient.
types of coupled transporter proteins
symporter: two molecular sublates are transported in the same direction
antipoert: two molecular substrates transported in opposite directions
P-type pumps
use autophosphorylation (using ATP) to power transport of ions agains gradient
F-type pumps
operate in opposite direction as P-type, uses electrochemical gradient for H+ to synthesize ATP fro ADP and Pi
ABC transporters
uses ATP hydrolysis to transport small molecules against gradients (doesn’t actually phosphorylate itself, just causes hydrolysis)
What type of pump is the Na+/K+ pump? What are three important features of the pump?
P-type pump
- rheogenic: generates electric current across PM
- electrogenic: changes the voltage across the PM by -5 mV
- ubiquitous: present in nearly every eukaryotic cell
how much ATP consumption is due to the Na+/K+ pump?
about 33%
describe the pumping cycle of the Na+/K+ pump
- 3 intracellular Na+ ions bind to the pump and trigger autophosphorylation. One ATP is consumed
- phosphorylation triggers conformational change, ejecting the 3 Na+ ions to extracellular solution
- 2 K+ extracellular ions bind the pump, triggering dephosphorylation
- dephosphorylation triggers release of K+ into cytosol
what does Ouabain do?
Ouabain is a poisonous glycoside that at high concentrations will block the Na+/K+ APTase, but at low concentrations will stimulate it.
what does digitoxin do?
(comes from foxglove plant)
digitoxin inhibits the Na+/K+ ATPase and produces a decrease in intracellular K+ and increase in intracellular Na+. This reduces efficiency of an secondary active transporter protein known as Na+/Ca2+ exchanger and increases resting intracellular Ca2+, producing an increased cardiac output
ABC proteins are also known as
MDR (multiple drug resistant) proteins
What diseases are associated with ABC proteins?
Cystic Fibrosis is a result of a mutated cystic fibrosis transmembrane regulator (type of ABC protein)
Cancer cells with over expressed MDR proteins (ABC transporters) can convey resistance to anti-cancer drugs and foil effective chemotherapy.
