Membrane Transport and Osmosis Flashcards
Define membrane potential
a separation of electrical charge across the plasma membrane of most cells in which that force generated can influence the movement of ions through their channels
- aka electric potential difference
Electrochemical gradient
provided by the concentration and electrical differences of the ions on both sides of the membrane which determine the direction and magnitude of ion movement
Resting membrane potential
a cell’s electric potential difference at rest
- NEGATIVE in most cells (~-77mV)
Channels
integral membrane proteins that span across the lipid bilayer that allow charged ions to cross the membrane
- Can be a single protein with a hole (depending on its conformation)
OR several proteins arranged together where a pore is in the middle
- FAST ion transport (millions per second)
Ion Channel Selectivity
Determinants:
- channel diameter
- polarity/charge of the proteins at the walls of the channel
- # of water molecules associated with that ion
Channel Gating
open/closed conformations of ion channels allow ion movement on both sides when open
- regulation of ion movement
Voltage-gated Ion Channels
channel opens/closes in response to changes in membrane potential
Ligand-Gated Ion Channels
channel opens upon binding of a ligand; once ligand binds a conformational change occurs in the channel that opens it up and lets the ion through
Leak (Always Open) Channels
channel stays open; play MAJOR role in resting membrane potential
Mechanically- Gated Ion Channels
channel opens/closes in response to pressure differences of the membrane
Transporters
membrane proteins that mediate movement of a larger variety of particles but exhibit chemical specificity for their ligand; SLOW (10,000s ions per second) d/t bigger conformational change requirement and can become saturated
Limitations to Transporter-Mediated Transport
- # of transporters
- speed of conformational change
- solute concentration
- transporter affinity for the solute
Primary Active Transport
transport of molecules across membrane that requires ATP (3 major ATPases)
Na+/K+ ATPase Pump
Drives 3Na+ OUT of the cell and 2K+ INTO the cell; the reason why the concentrations of these two ions are diff in and out of the cell
3 Main ATPases
Na+/K+ ATPase Pump, H+/K+ ATPase Pump, Ca2+ ATPase Pump
Secondary Active Transport
active transport of molecules that takes advantage of the gradient; INDIRECTLY uses ATP
Co-transport
transport of ion that takes advantage of established electrochemical gradient to drive movement DOWN the concentration gradient; coupled with transport of another molecule (SGLT1 and 2= sodium dependent glucose transporters); INDIRECT use of ATP
Counter transport
transport of ion that takes advantage of established electrochemical gradient to drive movement UP/AGAINST the concentration gradient; INDIRECT use of ATP
ABC Transporters
subset of primary active transporters; have a variety of substrates
Osmosis
the simple diffusion of water from high to low water concentration or low to high solute concentration
Aquaporins
membrane protein that mediates osmosis; specialized channels in all cell types
Semi-permeable membrane
membrane that only allows water to pass through not solutes
Osmotic Pressure ( Π)
the pressure that EXACTLY balances the osmotic water movement that results in a net water flow of ZERO
- the higher the solute concentration the GREATER the osmotic pressure because more water is needed to establish equilibrium between the two compartments
- equation:
Π= (Cs)RT
Hydrostatic pressure (P)
pressure exerted by a fluid at equilibrium due to the force of gravity
- equation:
P=Π= (Cs)RT
Nonpenetrating (effective) solute
a substance that CANNOT cross the cell membrane and is EFFECTIVE at generating osmotic pressure (i.e Na+, glucose, proteins) so DOES affect cell volume
Penetrating (ineffective) solute
a substance that CAN cross the cell membrane and is NOT EFFECTIVE at generating osmotic pressure (i.e urea) so does NOT affect cell volume
Isotonic solution
same concentration of nonpenetrating solutes as normal extracellular fluid
Hypertonic solution
concentration of solute OUTSIDE of the cell is greater so water LEAVES the cell to reach equilibrium and cell SHRINKS
Hypotonic solution
concentration of solute INSIDE of the cell is greater so water ENTERS the cell to reach equilibrium and cell SWELLS
Reflection coefficient (σ)
ability of solute to remain “intact” once they cross the membrane; Πeff = σ RT Cs
- ranges 0 ≤ σ ≤ 1
(i.e σ for a solute that can cross the membrane the easiest =0 because it will diffuse right away- like urea)
Osmolarity
the actual concentration of solute particles
and is independent of any membrane
Tonicity
describes a solution in terms of whether the volume of an immersed cell stays the same (ISOTONIC) or increases (HYPOTONIC) or decreases (HYPERTONIC
