Lecture 3: Water movement

Question 1

Define osmosis

Answer 1

the diffusional movement of water from a region of greater water concentration/activity to an area of lower water activity/concentration

Question 2

What is the change in water concentration/activity due to in order for osmosis to occur?

Answer 2

due to a difference in the solute concentration

Question 3

As you add solute molecules, water molecules are displaced and so the concentration of water is what?

Answer 3

reduced

Question 4

What is the difference between a solute and a solvent?

Answer 4

A solute is what is added to the solvent - the solute is dissolved in the solvent

Question 5

What does a semipermeable membrane mean? What can pass through the one in the example on the slide?

Answer 5

It means that only a few things can pass through it. In the example on the slides, only water can pass through it

Question 6

Before any more solutes are added to compartment B, what is the net flux of water? Why is this?

Answer 6

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.

Question 7

As more solutes are added to compartment B, what happens to the water?

Answer 7

There is now a higher concentration of solute in compartment B and so water flows from compartment A to compartment B

Question 8

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?

Answer 8

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

Question 9

Define osmotic pressure

Answer 9

This is the external pressure required to prevent the osmotic flux of water

Question 10

What is osmotic pressure a means of?

Answer 10

quantifying the driving force for water movement by osmosis

Question 11

When we apply a pressure to the increased volume of compartment B, what happens?

Answer 11

The increase in pressure pushes water from compartment B to compartment A

Question 12

Osmotic pressure can be defined as

Answer 12

the force required to stop the flow of water from compartment A to B

Question 13

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?

Answer 13

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

Question 14

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?

Answer 14

The chemical nature of the solute is irrelevant. All that is important is the total number of osmotically active solutes

Question 15

What are osmotically active particles. Give examples

Answer 15

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

Question 16

How can we calculate osmolarity? Give an example using 150 mmolL-1 NaCl

Answer 16

the concentration of the solute multiplied by the number of osmotically active particles
eg. 150 mmolL-1 x 2 = 300mOsmolL-1

Question 17

Osmotic pressure only exists when

Answer 17

osmosis can occur (but this always happens due to the semipermeable cell membrane)

Question 18

We can say that osmolarity is the same as what?

Answer 18

osmolality

Question 19

Although we say they are the same, what is the difference between osmolarity and osmolality?

Answer 19

osmolarity has a volume unit and so is therefore temperature dependent

osmolality has a mass unit and so is therefore temperature independent

Question 20

The equation for solute diffusion is Jnet = PΔC. What is the equation for water flux?

Answer 20

Jv = Lp A (ΔP - Δπ)

Question 21

The equation for water flux is Jv = Lp A (ΔP - Δπ). What do each of these things mean?

Answer 21

Lp means hydraulic conductivity
A is area
ΔP is the hydrostatic pressure
Δπ is the osmotic pressure

Question 22

(ΔP - Δπ) forms the what?

Answer 22

driving force for water movement

Question 23

Define hydraulic conductivity

Answer 23

this is how easily fluid moves

Question 24

What is the difference between hydrostatic and osmotic pressure with reference to the U tube and compartment B?

Answer 24

The force we applied to compartment B is the hydrostatic pressure and we use it to find the osmotic pressure

Question 25

Cell membranes cannot withstand a __________ pressure. Why not?
How does moving ions around help?

Answer 25

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

Question 26

Water movement across cell membranes is due to the differences in ________ pressure across the membrane

Answer 26

osmotic

Question 27

Describe the diffusion of water through the cell membrane

Answer 27

It is unregulated and relatively slow

Question 28

Lipid bilayers have quite a high _______ _________

Answer 28

hydraulic conductivity

Question 29

Different _________ have different hydraulic conductivites

Answer 29

membranes

Question 30

How can hydraulic conductivity can be regulated?

Answer 30

Using vasopressin/antidiuretic hormone and aquaporins

Question 31

What are aquaporins and what do they do?

Answer 31

These are small proteins that create pores to increase water permeability 100-fold because they allow regulation of hydraulic conductivity

Question 32

Define osmolarity (2)

Answer 32

It is the number of osmotically active molecules dissolved in solution. It indicates where water wants to go

Question 33

Define tonicity

Answer 33

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

Question 34

Water has _______ and likes it ________

Answer 34

FOMO

salty

Question 35

Define isosmotic

Answer 35

This is when the outside of the cell has the same osmolarity as the inside of the cell

Question 36

Define hyperosmotic

Answer 36

this is when the outside of the cell has a higher osmolarity than the inside of the cell

Question 37

Define hypo-osmotic

Answer 37

this is when the outside of the cell has a lower osmolarity than the inside of the cell

Question 38

What effect does a isotonic solution have on the cell volume?

Answer 38

it has no effect on cell volume

Question 39

What effect does a hypotonic solution have on a cell?

Answer 39

it causes the cells to swell

Question 40

What effect does a hypertonic solution have on the cell?

Answer 40

it causes the cell to shrink

Question 41

What would happen to a cell if placed in pure water?

Answer 41

it causes the cell to swell and eventually swell