Osmotic pressures

Question 1

What is osmosis?

Answer 1

The process of the diffusion of water molecules across a membrane

Question 2

Give an example of when osmosis is used in the body?

Answer 2

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

Question 4

Why is osmosis generally used in biological membranes?

Answer 4

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.

Question 5

What is the movement of water in osmosis driven by?

Answer 5

The chemical potential of water

Question 6

Describe the water inside and outside of a membrane using equations

Answer 6

• N_W= mole fraction of water
• T= temperature, Kelvink always a positive number
• N_W= 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

Question 7

Tell me the direction in which water moves in osmosis?

What is the name of this process?

Answer 7

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

Question 8

Define osmotic pressure?

Answer 8

Osmotic pressure is defined as the hydrostatic pressure required to stop the net flow of water across a membrane separating solutions of different compositions.

Question 9

Complete the blanks…

The flow of water will ___ pressure inside the water ___ membrane by an amout ___

Answer 9

Flow of water will increase pressure inside the water permeable membrane by an amount ∏

Question 10

Whats ∏?

Answer 10

∏ is the amount of pressure increase (pressure build up/ osmotic pressure)

Question 11

Use pressure dependence of chemical potential to form an equation…

Answer 11

Use pressure dependence of chemical potential, from G = H – TS = U + PV – TS.

Leads to ∏V = n_jRT

n_j is the number of moles of solute

V is the volume of solution

Question 12

Whats the Van’t Hoff equation?

Answer 12

∏V = njRT rearranges to ∏ = cjRT

c_j the concentration of the solute

Question 13

What is osmotic pressure proportional to?

Answer 13

Solute concentration

Question 14

Osmostic pressure is a Colligative property, what does this mean?

Answer 14

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.

Question 15

Whats an Isotonic solution?

Answer 15

Equal solute concentration in and out of the cell

Question 16

Tell me 3 properties of an isotonic solution?

Answer 16

• No net water diffusion
• Animal cell optimum
• Plants flaccid

Question 17

Whats a Hypotonic solution?

Answer 17

Low solute (High water) concentration outside of the cell

Question 18

Tell me 3 properties of an Hypotonic solution?

Answer 18

• Net water diffusion into cell
• Animal cells swell
• Plants turgid; plant cell optimum

Question 19

Whats an Hypertonic solution?

Answer 19

High solute (low water) concentration outside the cell

Question 20

Tell me 3 properties of an Hypertonic solution

Answer 20

• Net water diffusion out of cell
• Animal cells shrivel
• Plant cell membranes shrink away from their cell wall

Question 21

Iso, Hypo and Hypertonic solutions

Answer 21

Question 22

What can cause animal cells to shrink or swell rapidly?

What animal doesn’t this occurs in?

Answer 22

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

Question 23

Frog cells don’t change size rapidly. This observation led scientists to suspect what?

Answer 23

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

Question 24

In the 1980s, what did Peter Agre study?

What did he discover?

Answer 24

studied various membrane proteins from red blood cells. He discovered a protein he thought must the cellular water channel.

Question 25

What did Agre do to come to the conclusion that he did?

Answer 25

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.

Question 26

What are aquaporins?

Answer 26

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?

Question 27

What organisms are aquaporins conserved in?

Answer 27

Bacteria, plants and animals

Question 28

What do aquaporins allow?

Answer 28

The passage of thousands of millions of water molecules per second passing through one single channel

Question 29

What has the structural analysis of aquaporins revealsed?

Answer 29

The molecules have a pore in the centre of each aquaporin moelecule

Question 30

Whats the equation that relates chemical potential of solute and solvent?

Answer 30

µ_j= µ˚_j + RTlog_e[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.

Question 31

Whats the Donnan effect?

Answer 31

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.

Question 32

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?

Answer 32