Agenda9/27---Day 5

• Take out water potential problems• Take out enduring understanding work for

check• Notes if time

Calculating Water Potential

• Things to remember!Water potential (Ψw) is the physical property

predicting the direction in which water will flowThis is governed by solute potential (Ψs)and

pressure potential (Ψp)

Ψw= Ψs + Ψp

What we know

Plant cells have a cell wall that will exert pressure =to the pressure entering the vacuole

Vacuole pressure is exerting pressure on the wall ONLY if the cell is taking in more water than it can hold

SO if the plant cell is flaccid (limp) there is 0 (zero) pressure inside the plant cell

To calculate you use the Ψw=Ψs+Ψp

Solute Potential

• The more negative the solute potential, the lower the water potential is, therefor movement of water out of that solution is lower

• You must know the solute concentration to calculate solute potential

Calculate using this formula

• Ψs=-iCRT

• Sugar molecules such as sucrose and glucose are 1 particle, they do not dissociate in water

What is next

• It’s a matter of thinking and working through the problems, each time trying to make sense of it.

• This requires time and effort

1

• Calculate the water potential of a solution of 0.15 sucrose assuming the RT=2.478

• Ψs= -1(0.15)(2.478)= -0.37

2

• Cell is flaccid (pressure = 0), with Ψs=-0.65Is placed in #1 solution with a Ψs = -0.37At equilibrium, what is the pressure potential?

At equilibrium, the water potential is zero SO

0= -0.37 + Ψp0=-0.37 +0.370=0

3

• Put cell in 0.35 M solution• Calculate • Ψs = -1(0.35)(2.478)=-0.87• Compared to the -0.65 inside cell• Which has MORE solute?• Which way will water move?• Therefore, what is the pressure inside the cell?

4

• Ψw=Ψp+Ψs• -0.24= 0.46 + Ψs -0.24-0.46=ΨsΨs=-0.7

5

• Cell Ψs= -0.35• In distilled water, the water will move into the

cell until the pressure potential = that of the solute potential

• BECAUSE, at equilibrium the water potential is zero

• -0.35 + 0.35 = 0

6

• Ψw=Ψp+Ψs• Ψw=-0.67 + 0.43• Ψw= -0.24

7

• The Water potential outside the cell goes from -1.0 to -0.2

• Does water continue to move in this case?• So what is the pressure potential inside the

cell?

8

• Hypertonic has a low water potential because?

9.

• The water will move out of cell, why?