ILA 12 Diffusion and Osmosis Flashcards
Describe the relationship between passive solute permeability and lipid solubility of the solute. Define the distinction between water transport by diffusion through lipid bilayers and through aquaporins. Explain the relationship between osmotic pressure and the concentrations of impermeant solute on each side of a membrane. Define serum osmotic gap and stool osmotic gap. Define colloid osmotic (oncotic) pressure.
What is diffusion?
the random movement of atoms and molecules, powered by thermal energy
Diffusion is most efficient in long or short distances?
Short Distances
What is the driving force for the diffusion of uncharged solutes across a membrane?
Concentration difference (gradient)
What is the equation for net transport?
Net transport = PA([C1]-[C2])
P=permeability coefficient
A=area of the membrane
What is used as the standard measure of hydrophobicity?
octanol:water partition coefficient (Kow)
How is the octanol:water partition coefficent determined?
- by dissolving the compound in a 1:1 mixture of n-octanol and water
- Octanol and water spontaneously separate into upper (mainly octanol) and lower (mainly water) phases
- Kow = [Concentration in Octanol]/[Concentration in H2O]
A compound with a high Kow means what?
very hydrophobic
A compound with a low Kow means what?
very hydrophilic
Equilibrium is reached faster with compounds that have a (higher or lower) Kow?
Higher
What is Overton’s rule?
- describes the relationship b/w passive membrane permeability and hydrophobicity
- P = Kow/(MW)^(1/2)
- for compounds of the same MW, pemeability is directly proportional to Kow
- permeability is also inversely proportional to the square root of the molecular weight
How does pH affect lipid solubility?
adding a net charge to an otherwise hydrophobic molecule drastically lowers its lipid solubility
Diffusion of weak acids and bases across the cell membrane is dependent upon what?
Whether they are charged or uncharged
uncharged moves across, but charged does not
What is the exception to the rule that charged molecules do not diffuse rapidly across lipid bilayers?
in the charge is delocalized then it can
Example: 2,4-DNP that alternates b/w charge/uncharged to cross membranes
What is the main pathway for water transport across membranes?
water channels (aquaporins)
What was the first aquaporin discovered?
AQP1
What drives the net transport of water across membranes?
through aquaporins
Osmosis
Describe osmosis
water flows to areas of higher solute concentration
think: if cooking and its to salty, you add h2o
What is the red blood cell osmotic fragility test?
- Red blood cell osmotic fragility is measured by varying the osmolality of the medium and measuring the percent lysis
- If the [NaCl] outside the cell is lowered, the cell swells b/c of the inward osmotic force ([solute] higher in the cell)
What is osmolarity?
the number of osmoles per liter of solution
What is an isosmotic solution?
one that has the same osmolality as normal human plasma (275-299 mOsm/kg)
0.9% NaCl solution
What is an isosmotic solution?
one that has the same osmolality as normal human plasma (275-299 mOsm/kg)
0.9% NaCl solution
What is a hyperosmotic solution?
has osmolality higher than 299 mOsm/kg
What is a hyposmotic solution?
has osmolality lower than 275 mOsm/kg
How do you calculate serum osmolality?
=2([Na]+[K]) +[glucose] + [urea]
all in mM
What is an osmol gap?
When there is a larger difference between calculated and measured osmolality of serum/plasma
caused by abn high levels of ethanol, methanol, ethylene glycol & others
How is the stool osmotic gap calculated?
= 290 - 2([Na] + [K])
