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Diffusion, Osmosis & Water Potential Worksheet #2

Water potential = Osmotic (solute) potential + Pressure potential

Ψw = Ψs + Ψp

ψs = -iCRT

i = Ionization constant (e.g. 1.0 for sucrose [no ionization]); assume that i = 1 unless otherwise indicated

C = Molar concentration

R = Pressure constant (R = 0.0831 liter bars/mole oK)

T = Temperature oK (273 + oC of solution)

Calculation Question:1. Determine the water potential of a cell if Ψp = 0.3 MPa and Ψs = -0.5 MPa.

2. Calculate the pressure in a cell if Ψw = -0.1 and Ψs = -0.2.

3. Calculate the osmotic potential of a cell (Ψs) is Ψw = -0.1 and Ψp = 0.8

Osmometer Question: The diagram represents a simple osmometer. The membrane is freely permeable to water but not solute (sucrose). Inside the membrane is a solution of 0.1 M sucrose. In the beaker is distlled water. For the questions below, please circle the best answer.

1. There will be a net movement of water ________ the membrane sac. a. into b. out of

2. The water level in the tube will: a. fall b. rise c. not change

3. The pressure in the sac will: a. decrease b. increase c. not change

4. On the back of this worksheet, sketch the shape of the line/curve plotting osmotic potential in the sac vs. time

5. On the back of this worksheet, sketch the shape of the line/curve plotting pressure in the sac vs. time.

Short Answer Questions:   Using water potential terminology, explain why:

1. Over-fertilizing your lawn will cause it to “burn” (the leaves wilt, become dry and fall off).

2. Strawberries become mushy when you add sugar to them.

3. Cooks put wilted lettuce in cold water.