Lecture 7: Membrane Transport Flashcards
Name four classes of molecules
1- small hydrophobic (O2, CO2, N2)
2- small uncharged polar (H2O, glycerol, ethanol)
4- large uncharged polar (amino acids, glucose, nucleotides)
4- charged (H+, Na+, K+, Ca2+, Mg2+, Cl-, HCO3-)
Define a synthetic bilayer
protein-free artificial, impermeable to large uncharged polar molecules and charge ions.
Cell membranes are permeable to
Large uncharged polar molecules and charged ions. These water-soluble molecules are transferred through transport proteins.
Transport proteins are
substrate specific and responsible for maintaining cells specific chemical composition.
Every membrane has its own characteristic set of transport proteins and why(T/F)
True, and therefor organelles perform unique functions due to their unique chemical composition.
The concentrations of cations in and out of cell
no electrical forces
- low [Na+] in
- high [K+] in
- [Na+]in + [K+]in=[Na+]out + [K+]out
The concentrations of anions in and out of cell
no electrical forces
- high [protein-] in
- low [Cl-] in
- low [HCO3-] in
The quantity of positive charge ___________ must be ______ by an almost exactly equal quantity of negative charge.
- inside the cell
- balanced
Name three mechanisms by which small molecules cross the bilayer
- passive diffusion
- facilitated diffusion
- active transport
Define passive diffusion
- hydrophobic and small polar uncharged molecules dissolve in lipid bilayer
- membrane protein not required
- no energy required
- hydrophobic molecules diffuse down their concentration gradient
The direction of transport of molecules in passive and facilitated diffusion is determined
only by the relative concentrations of the molecule in and out of cell, at equilibrium there is no transport.
Difference between passive and facilitated diffusion
In facilitated diffusion membrane transport proteins are involved, used to transport large polar molecules and charged ions without interacting with hydrophobic interior of the membrane.
Define active transport
- membrane transport proteins required
- energy driven
- molecules move against their concentration gradient through the hydrolysis of ATP
Name three classes of transport proteins
1) ATP-powered pump
2) channel proteins (ion channels)
3) carrier proteins (transporters)
Relative rate of transport of transport proteins in order from most to least molecules transferred
Ion channels (10^7-10^8) > transporters (10^2-10^4) > ATP- powered pump (10^0-10^3)
Why does ATP-powered pump have a lower rate of transport than ion channels?
- ATP-powered pumps transport a) one molecule at a time, b) limited reserve of ATP, c) require hydrolysis of ATP.
- whereas, ion channels transport a) many ions at once, b) movement is thermodynamically favoured, c) doesn’t require external use of energy.
Define ion channels
- transport many specific ions at once down their concentration gradient.
- hydrophilic pore.
- exist in open or closed conformation as polypeptide chain relocates.
Key difference between ion channels are carrier proteins (transporters)
-the binding of a molecule changes the conformation of the carrier protein.
Name three types of transporters
1) uniporter
2) symporter
3) antiporter
Describe a uniporter
- transport single type of molecule down its concentration gradient.
- moves glucose or amino acids across plasma membrane into mammalian cell.
Describe a symporter
- couple movement of an ion or polar molecule against its concentration gradient and different molecule down its concentration gradient.
- both ions or polar molecules move in the same direction.
- The couple movement of energetically favoured reaction drives the other energetically unfavoured movement.
Describe a antiporter
- couple movement of an ion or polar molecule against its concentration gradient and different molecule down its concentration gradient.
- move in opposite direction.
- The couple movement of energetically favoured reaction drives the other energetically unfavoured movement.
Symporters and antiporters are called
cotransporters
What role do Na+/K+ ATPase play in mammalian cells
- maintain intracellular K+ and Na+ concentrations.
- responsible for the couple movement of Na+ and K+ in and out of cell.
Steps in the mechanism of Na+/ K+ ATPase
step 1: binding of 3 Na+ ions (E1)
- E1 conformation, 3 high affinity Na+ binding sites, 2 low affinity K+ binding sites on cytosolic face.
- low conc of Na+ inside cell, high affinity of Na+ binding sites.
- high conc of K+ inside cell, low affinity of K+ binding sites.
step 2: binding of ATP, phosphorylation of aspartate (E1~P)
- ATP binds to cytosolic surface.
- ATP hydrolyzed to ADP, and the liberated phosphate is transferred to aspartate forming high energy acyl phosphate bond.
step 3: conformation change, transport of 3 Na+ ions (E2~P)
- 3 Na+ ions move outward through protein.
- generates 2 high affinity K+ binding sites on extracellular face.
step 4: dissociation of 3 Na+ ions, binding of 2 K+ ions
- 3Na+ ions dissociate into extracellular medium from low affinity Na+ binding site.
- 2 K+ ions bind to high affinity sites despite low conc of K+ in extracellular medium.
step 5: hydrolysis of aspartate phosphate (E2~P –> E1)
- back to E1 conformation.
- during conformation transition, 2 K+ move inward through protein.
step 6: dissociation of 2 K+ ions
- 2 K+ ions dissociate into cytosol from low affinity K+ binding sites on cytosolic surface, despite high conc of K+ in cytosol.
Define two properties of ion channels
1) ion selectivity,because of narrow pores.
2) gating, because ion channels are not permanently opened. open and closed state are regulated by gates that open in a response to specific stimuli.
Define ion selectivity of Na+ channels
permit the passage of Na+ bound to a single water molecule, but interferes with passage of K+.
Name and define three type of gated ion channels
1) voltage-gated: electric potential
2) ligand-gated: binding of chemical ligand in or out of cell
3) stress-activated: mechanical force
Medically important ligand-gated ion channel
Cl- in epithelial cells (CFTR)
CFTR= Cystic Fibrosis Transmembrane conductance Regulator - is unusual in that it requires both ATP hydrolysis and phosphorylation in order to open.
Cystic fibrosis caused by and is
by: recessive mutation in the CFTR
is:
- most common lethal inherited disease of Caucasians.
- production of thick mucus by cells in respiratory and gastrointestinal tracts.
- respiratory disease.
- diagnosed if presence of excessive salt in sweat.
- is fatal, along with lung disease with 95% mortality rate.
