Osmotic pressures Flashcards
What is osmosis?
The process of the diffusion of water molecules across a membrane
Give an example of when osmosis is used in the body?
In the kidney reabsorption of water from urine depends on establihsing a chemical potential gradient that favours spontaneous movement of water from lumen to renal tube
Why is osmosis generally used in biological membranes?
Most biological membranes are largely impermeable to ions and other solutes but are slightly permeable to water.
Water moves spontaneously across a membrane, driven by the chemical potential of water.
What is the movement of water in osmosis driven by?
The chemical potential of water
Describe the water inside and outside of a membrane using equations
- NW= mole fraction of water
- T= temperature, Kelvink always a positive number
- NW= negative so overall RTlogNw is negative
Chemical potential is lower on the inside of the membrane so to equilibrate potentials, water enters the cell. This could lead to cell swelling and bursting
Permanent driving force which gets lower and lower, but water will always want to go in
Tell me the direction in which water moves in osmosis?
What is the name of this process?
Water moves spontaneously across a membrane from a solution of high-water concentration to one of a low water concentration.
In other words, water will move from a solution of low solute concentration to one of higher solute concentration.
This process is known as osmotic flow
Define osmotic pressure?
Osmotic pressure is defined as the hydrostatic pressure required to stop the net flow of water across a membrane separating solutions of different compositions.
Complete the blanks…
The flow of water will ___ pressure inside the water ___ membrane by an amout ___
Flow of water will increase pressure inside the water permeable membrane by an amount ∏
Whats ∏?
∏ is the amount of pressure increase (pressure build up/ osmotic pressure)
Use pressure dependence of chemical potential to form an equation…
Use pressure dependence of chemical potential, from G = H – TS = U + PV – TS.
Leads to ∏V = njRT
nj is the number of moles of solute
V is the volume of solution
Whats the Van’t Hoff equation?
∏V = njRT rearranges to ∏ = cjRT
cj the concentration of the solute
What is osmotic pressure proportional to?
Solute concentration
Osmostic pressure is a Colligative property, what does this mean?
Osmotic pressure is a colligative property, meaning that the osmotic pressure depends on the molar concentration of the solute but not on its identity.
It is the total number of solute molecules that is important. For example, a 0.5 M NaCl solution is actually 0.5 M Na+ ions and 0.5 M Cl− ions and has approximately the same osmotic pressure as a 1 M solution of glucose or lactose.
Whats an Isotonic solution?
Equal solute concentration in and out of the cell
Tell me 3 properties of an isotonic solution?
- No net water diffusion
- Animal cell optimum
- Plants flaccid
Whats a Hypotonic solution?
Low solute (High water) concentration outside of the cell
Tell me 3 properties of an Hypotonic solution?
- Net water diffusion into cell
- Animal cells swell
- Plants turgid; plant cell optimum
Whats an Hypertonic solution?
High solute (low water) concentration outside the cell
Tell me 3 properties of an Hypertonic solution
- Net water diffusion out of cell
- Animal cells shrivel
- Plant cell membranes shrink away from their cell wall
Iso, Hypo and Hypertonic solutions
What can cause animal cells to shrink or swell rapidly?
What animal doesn’t this occurs in?
Even though a phospholipid bilayer is only slightly permeable to water, small changes in extracellular osmotic strength cause most animal cells to swell or shrink rapidly.
This does not happen in Frogs
Frog cells don’t change size rapidly. This observation led scientists to suspect what?
That plasma membranes of erythrocytes and other cell types contain water-channel proteins that accelerate the osmotic flow of water
These channel proteins are therefore absent in the membranes of frogs
In the 1980s, what did Peter Agre study?
What did he discover?
studied various membrane proteins from red blood cells. He discovered a protein he thought must the cellular water channel.
What did Agre do to come to the conclusion that he did?
Agre compared cells which contained the protein with cells which did not have it. When the cells were placed in a water solution, those that had the protein in their membranes absorbed water and swelled up while those that lacked the protein were not affected at all.
Agre also ran trials with liposomes, He found that the liposomes became permeable to water if the protein was planted in their membranes.
What are aquaporins?
Aquaporins are channel proteins present in cell membranes that lower the energy barrier for the translocation of water and some small solutes across the lipid bilayer?
What organisms are aquaporins conserved in?
Bacteria, plants and animals
What do aquaporins allow?
The passage of thousands of millions of water molecules per second passing through one single channel
What has the structural analysis of aquaporins revealsed?
The molecules have a pore in the centre of each aquaporin moelecule
Whats the equation that relates chemical potential of solute and solvent?
µj= µ˚j + RTloge[j]
Knowing concentrations, we can compare chemical potentials on different sides of a membrane and judge thermodynamic drive for flow of solute across membrane.
flow from region of higher chemical potential to region of lower chemical potential.
Whats the Donnan effect?
a name for the behaviour of charged particles near a semi-permeable membrane that sometimes fail to distribute evenly across the two sides of the membrane.
consider ions equilibrating across a membrane, in the presence of charged macromolecules that cannot cross membrane.
Here, we have Na+ and Cl- and an anionic macromolecule with charge –Zm
The initial concentration of the NaCl in the outer compartment is x2M and concentration of protein is x1 M (it has charge -Zm)
Equilibration occurs by the movement of equal numbers of Na+ and Cl- in the same direction, to preserve electroneutrality.
We end up with a concentration gradient of the ions. For Cl-, we get the ratio?
