Lecture 3: Water movement Flashcards
Define osmosis
the diffusional movement of water from a region of greater water concentration/activity to an area of lower water activity/concentration
What is the change in water concentration/activity due to in order for osmosis to occur?
due to a difference in the solute concentration
As you add solute molecules, water molecules are displaced and so the concentration of water is what?
reduced
What is the difference between a solute and a solvent?
A solute is what is added to the solvent - the solute is dissolved in the solvent
What does a semipermeable membrane mean? What can pass through the one in the example on the slide?
It means that only a few things can pass through it. In the example on the slides, only water can pass through it
Before any more solutes are added to compartment B, what is the net flux of water? Why is this?
The net flux is zero because the concentration of solute molecules is the same in both compartments and so the water molecules can move from compartment A to B and from B to A.
As more solutes are added to compartment B, what happens to the water?
There is now a higher concentration of solute in compartment B and so water flows from compartment A to compartment B
When water is flowing from compartment A to compartment B, when does this stop? What does this mean for the volumes of water in compartment A and B?
There is a driving force pushing water from compartment A to B and this stops when the concentrations in each compartment are the same. Because there are more solute molecules in compartment B, in order for the concentrations to be the same, there would have to be a greater volume of water in compartment B
Define osmotic pressure
This is the external pressure required to prevent the osmotic flux of water
What is osmotic pressure a means of?
quantifying the driving force for water movement by osmosis
When we apply a pressure to the increased volume of compartment B, what happens?
The increase in pressure pushes water from compartment B to compartment A
Osmotic pressure can be defined as
the force required to stop the flow of water from compartment A to B
The van’t Hoff Equation describes the osmotic pressure of a solution and is given by π = RTΣCΦi. What do each these things mean?
What can we simplify it to?
π = osmotic pressure in atmospheres R = Gas constant T = temperature in Kelvin C = total solute concentration Φ = osmotic coefficient i = number of solute molecules formed by dissociation
We can simplify it to π = ΣCi
With regards to the modified van’t Hoff Equation, i refers to the number of solute molecules formed by dissociation. What is irrelevant and what is relevant?
The chemical nature of the solute is irrelevant. All that is important is the total number of osmotically active solutes
What are osmotically active particles. Give examples
These are the particles that a molecule dissociates into. For example, NaCl dissociates into an Na+ ion and a Cl- ion. These two ions are the osmotically active particles. In comparison, glucose doesn’t break up further in water and so its osmotically active particle is just glucose
How can we calculate osmolarity? Give an example using 150 mmolL-1 NaCl
the concentration of the solute multiplied by the number of osmotically active particles
eg. 150 mmolL-1 x 2 = 300mOsmolL-1
Osmotic pressure only exists when
osmosis can occur (but this always happens due to the semipermeable cell membrane)
We can say that osmolarity is the same as what?
osmolality
Although we say they are the same, what is the difference between osmolarity and osmolality?
osmolarity has a volume unit and so is therefore temperature dependent
osmolality has a mass unit and so is therefore temperature independent
The equation for solute diffusion is Jnet = PΔC. What is the equation for water flux?
Jv = Lp A (ΔP - Δπ)
The equation for water flux is Jv = Lp A (ΔP - Δπ). What do each of these things mean?
Lp means hydraulic conductivity
A is area
ΔP is the hydrostatic pressure
Δπ is the osmotic pressure
(ΔP - Δπ) forms the what?
driving force for water movement
Define hydraulic conductivity
this is how easily fluid moves
What is the difference between hydrostatic and osmotic pressure with reference to the U tube and compartment B?
The force we applied to compartment B is the hydrostatic pressure and we use it to find the osmotic pressure
Cell membranes cannot withstand a __________ pressure. Why not?
How does moving ions around help?
hydrostatic
because cells don’t have a cell wall and so an increase in the pressure inside the cell causes the inside of the cell to burst
if we control the inside concentration of the ions relative to the outside, we control water movement
Water movement across cell membranes is due to the differences in ________ pressure across the membrane
osmotic
Describe the diffusion of water through the cell membrane
It is unregulated and relatively slow
Lipid bilayers have quite a high _______ _________
hydraulic conductivity
Different _________ have different hydraulic conductivites
membranes
How can hydraulic conductivity can be regulated?
Using vasopressin/antidiuretic hormone and aquaporins
What are aquaporins and what do they do?
These are small proteins that create pores to increase water permeability 100-fold because they allow regulation of hydraulic conductivity
Define osmolarity (2)
It is the number of osmotically active molecules dissolved in solution. It indicates where water wants to go
Define tonicity
This is the relative solute concentration of two solutions separated by a membrane. If is the effect a solution has on cells and it takes into account the ability of molecules to cross the cell membrane
Water has _______ and likes it ________
FOMO
salty
Define isosmotic
This is when the outside of the cell has the same osmolarity as the inside of the cell
Define hyperosmotic
this is when the outside of the cell has a higher osmolarity than the inside of the cell
Define hypo-osmotic
this is when the outside of the cell has a lower osmolarity than the inside of the cell
What effect does a isotonic solution have on the cell volume?
it has no effect on cell volume
What effect does a hypotonic solution have on a cell?
it causes the cells to swell
What effect does a hypertonic solution have on the cell?
it causes the cell to shrink
What would happen to a cell if placed in pure water?
it causes the cell to swell and eventually swell
