L3 Flashcards
define osmosis
diffusional movement of water from a region of greater water activity to an area of lower water activity where the change in the water activity is due to a difference in solute concentration
activity of water in low amounts can be considered the same as concentration
if you have pure water and you add solutes to it, what happens to the concentration of that water
it decreases
this is because the more solutes you add, the more water that is displaced (because solutes take up space) therefore decreasing the concentration
if you had pure water in a U tube with sides A and B separated by a semi-periable membrane that was selective to water, what would happen when you added solutes to side B
initially the solutes in side B would take up space so water would travel to side A. this causes an osmotic gradient and water will want to flow back into side B as it goes from high to low conc
this movement causes an increase in volume on side B
what is osmotic pressure
the external pressure required to prevent the osmotic flux of water
how can you measure osmotic pressure in a U tube
by applying a force to side B (where the volume has increased) so that the volumes on side A and B are =
this applied force is = to the osmotic pressure
what is the unit for osmotic pressure
atmospheres
what does the van’t Hoff equation describe
the osmotic pressure of a solution
π = RTECoi
what does osmotic pressure (π) tell us
where the water is going to move to
how do we calc osmolarity
concentration x number of solute molecules formed by disassociation
Ci
when does osmotic pressure occur
only when osmosis exists
in the body this is always because all membranes are selective for water
what is the difference between osmolaRity and osmolaLity
osmolaRity = Osmol/L
is about volume therefore it is temperature dependent
osmolaLity = Osmol/Kg
this is about mass therefore it is temperature independent
in PHSL these mean the same thing
J = P dC is the equation for flux
what is the equation for water flux
Jv = Lp x A x (dP - dπ)
however it is only dπ (osmotic pressure) that is applicable in the human body therefore
water flux is = to osmotic pressure
what does Lp in the equation Jv = Lp x A x (dP - dπ) stand for
hydraulic conductivity (how easily a fluid can move)
what does dP in the equation Jv = Lp x A x (dP - dπ) stand for
hydrostatic pressure
dP - dπ is the driving force for water movement
NOTE in the U tube example this is the applied force
what is important to note about hydrostatic pressure in PHSL
this does not happen in cells as they dont have a cell wall to protect them from this pressure therefore it would cause the cells to burst
to avoid this the cells move ions around (water follows salt)
what are the 2 ways that water can get through the membrane
through the lipid bilayer or through aquaporins
how is water able to diffuse through the lipid bilayer
because of hydraulic conductivity of the membrane however this process is unregulated and slow
do all cells have the same hydraulic conductivity
no
different membranes have different hydraulic conductivities
what can alter a cells hydraulic conductivity
vasopressin/antidiuretic hormone
it does this by altering the number of aquaporins in the membrane
aquaporins increase the hydraulic conductivity of the membrane
what are aquaporins
water channels
small proteins that create pores
these proteins have polar, non polar, negative and positive parts to the protein
how much do aquaporins increase permeability
by 100 fold
however osmotic pressure is still the driving force as water moves from a high to low conc but this can be regulated by the movement of ions
what do aquaporins allow for
regulation of hydraulic conductivity
what is osmolarity
number of osmotically active molecules dissolved in a solution
285 - 300 mOsmol/L in the human body
it indicates to us where the water wants to go
what is tonicity
the relative solute concentration of two solutions separated by a membrane
it describes the effect a solution has on cells
