Water Potential.

Question 0

What is another way of thinking about water potential?

Answer 0

The tendency of water to leave a system. (E.g a cell)

Question 1

Define water potential.

Answer 1

The potential (free) energy of water molecules in a system (e.g a cell) compared with the potential (free) energy of water molecules in pure water at s.t.p

Question 2

What symbol is water potential given?

Answer 2

Ѱ

Question 3

What will affect water potential?

Answer 3

Anything that slows down or interferes with the free movement of of water. Including temperature, pressure and solute concentration.

Question 4

What is the Ѱ of pure water at s.t.p?

Answer 4

0kPa.

Question 5

In what direction will water move down the water potential gradient?

Answer 5

From higher to lower water potential.

Question 6

What is an isotonic solution?

Answer 6

A solution with the same water potential as another solution.

Question 7

What is a hypertonic solution?

Answer 7

A solution with a lower water potential than another (more solute is present).

Question 8

What is a hypotonic solution?

Answer 8

A solution with a higher water potential (less solute) than another.

Question 9

What is Ѱcell and its equation?

Answer 9

ѰCell is an expression for the water potential of a cell.

ѰCell = Ѱs + Ѱp

Question 10

What is Ѱs?

Answer 10

This is solute potential I.e the contribution made to Ѱcell by dissolved substances.

Question 11

What is Ѱp?

Answer 11

Is pressure potential I.e the contribution made to Ѱcell by internal pressure.

Question 12

What will happen to a plant cell in pure water or a weak solution?

Answer 12

The cell will become turgid.

Question 13

Explain the process by which a cell becomes turgid.

Answer 13

1 - Water enters the cell by osmosis as Ѱoutside is greater than Ѱinside.

2 - As a result the vacuole expands, pushing on the cell wall and causing Ѱp to increase.

3 - as Ѱp increases Ѱs decreases (as internal solution becomes more dilute)

4 - Ѱp and Ѱs reach equal but opposite values and cancel out.

5 - Ѱcell = 0 meaning the net tendency of water to enter the cell is zero. The cell is now in equilibrium with its solution.

Question 14

Describe an experiment to determine Ѱcell.

Answer 14

A solution causing no mass or size change Must be isotonic with the cell. This can be estimated by placing plant tissue in a range of concentrations for about an hour. Afterwards each piece is remeasured or reweighed and change in mass is calculated. A curve may then be produced by plotting this against solution molarity. The Ѱcell value may then be extrapolated.

Question 15

What occurs when Ѱoutside is less than Ѱcell e.g in strong sucrose solution.

Answer 15

Water leaves the cell by osmosis.

Question 16

What happens as water leaves the cell by osmosis?

Answer 16

Ѱs starts to go up and Ѱp starts to go down.

Question 17

What happens when Ѱp=0 (I.e 1atm like outside)?

Answer 17

At this point Ѱcell=Ѱs and the cell reaches incipient plasmolysis.

Question 18

What is incipient plasmolysis?

Answer 18

This is the point at which Ѱp=0 and the membrane is just about to pull away from the cell wall.

Question 19

Why is incipient plasmolysis important?

Answer 19

Because Ѱcell=Ѱs as Ѱp=0 so you can find a value for Ѱs.

Question 20

What is the problem with using a cell at incipient plasmolysis to estimate Ѱcell?

Answer 20

As this event cannot be seen (as it is just about to happen), Ѱs must be estimated by percentage plasmolysis itself. By convention a solution which causes 50% plasmolysis is said to be one that causes incipient plasmolysis and is used to calculate Ѱs.

Question 21

Describe an experiment to estimate Ѱs.

Answer 21

Place uniform onion epidermis samples in a range of sucrose (or other solute) solutions. After one our the sample is viewed and the number of cells plasmolised in a random field of view is expressed as a percentage plasmolysis. This produces a series of results which can be plotted against concentration and the molarity required to give 50% plasmolysis can be extrapolated.