Mechanisms of Solute and Water Transport Flashcards
What are the triggers the transition from open (conducting) to closed (non-conducting) states for gated channels?
- Depolarization of the cell membrane (voltage-gated)
- Binding of hormone
- Binding of ligand
- Tension on the cell membrane (stretch activated)
Can carriers also transition from open to closed randomly?
Yes, due to thermal energy
What is facilitated diffusion?
Movement of a (usually impermeable) solute by a carrier channel across the lipid bilayer
Describe the kinetics of facilitated diffusion via the carrier-mediated system compared to simple diffusion.
see pic
What are P-class ATPases?
Active transporters that function via alternating phosphorylation/dephosphorylation steps.
What are 3 major P-class active transporters? What is their nickname?
- Na/K ATP ase
- Ca2+ ATPase
- H+ ATPase
Nicknamed primary active transporters because they are directly linked to the cell’s metabolic energy
How many ions does a Na/K ATPase move per molecule of ATP hydrolyzed?
3 Na+ out of the cell, 2 K+ in.
What is the function of the Na/K ATPase?
Maintain low Na+ and high K+ levels in the cytoplasm (i.e. membrane potential)
How does the Ca2+ ATPase function?
Transports 1 (PMCA, PM) or 2 (SERCA, ER) Ca+ ions in exchange for H+ ions.
What is the function of the Ca2+ ATPase?
It maintains very low free calcium concentration in the cytoplasm.
How does the H+ (and H+K+) ATPase work?
Secretes protons and/or exchanges H+ for K+.
What are ABC transporters?
ATP-binding cassette transporters that bind and hydrolyze ATP (or just bind!) to actively transport their substrates. Have 6 membrane-spanning segments and 2 extracellular binding domains.
What are 2 common ABC transporters?
- MDR1
2. CFTR
What are the 2 classes of coupled transporters?
Symport/Co-transport
Exchanger/antiport
What are secondary active transporters and what is a major example?
Co-transporters/Antiporters that do not directly use ATP but rely on the exchange of a solute whose concentration is maintained by ATP. Major example = Na+/glucose cotransporter
What is the vant Hoff expression and what does it describe?
Describes the osmotic pressure of a solution:
pi = RTCs
How do you calculate osmotic pressure when a membrane is permeable to a solute?
Pi effective = sigma RTCs
sigma is a reflection coefficient
What does a reflection coefficient represent?
The fraction of solute molecules colliding with the membrane that are reflected (do not cross it).
Thus for impermeable solutes, sigma = 1
What effect would a very permeable solutes (sigma
Osmotic pressure would be nearly zero
What is oncotic pressure?
The colloid osmotic pressure in the plasma as a result of proteins.
Where in the body do you find the lowest reflection coefficients?
Discontinuous capillaries such as the liver.
Where in the body do you find the highest reflection coefficients?
Tight continuous capillaries such as the brain.
Describe aquaporins.
Tetramer channel with a high selectivity for water.
Can you reach a maximum level of carrier flux?
Yes! When all facilitators are saturated. It is based on the substrates’ affinity for the carrier
The osmotic pressure created by a permeable solute is ______.
Lower
