2B - Exchange across cell membranes (osmosis)

Question 1

What is osmosis?

Answer 1

The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane.

Question 2

What does the rate of osmosis depend on?

Answer 2

The water potential gradient - the higher the gradient, the faster the rate of osmosis. As osmosis takes place, the difference in water potential on either side of the membrane decreases, so the rate of osmosis levels off.

The thickness of the exchange surface - the thinner the exchange surface, the faster the rate of osmosis.

The surface area of the exchange surface - the larger the SA, the faster the rate of osmosis.

Question 3

What is water potential?

Answer 3

The potential (likelihood) of water molecules to diffuse out of or into a solution.

Question 4

What is the water potential of pure water?

Answer 4

0 KPa (This is the highest water potential).

Question 5

What does the addition of solutes to a solution do to water potential?

Answer 5

Reduces water potential (to a more negative value).

Question 6

What units are water potential measured in?

Answer 6

Units of pressure such as KPa.

Question 7

What can osmosis be quantified using?

Answer 7

Water potential (the symbol for water potential is the greek letter psi which looks like the devils fork!)

Ψ

Question 8

If 2 solutions have the same water potential they are said to be what?

Answer 8

Isotonic.

Question 9

What happens in plant cells if the water potential of the external solution is higher than the cell solution?

Answer 9

Water enters the cell, cell swells and becomes turgid (hypotonic).

Question 10

What happens in plant cells if the water potential of the external solution is lower than the cell solution?

Answer 10

Water leaves the cell, cell shrinks and becomes plasmolyzed (hypertonic).

Question 11

What happens in plant cells if the water potential of the external solution is equal to the cell solution?

Answer 11

Water neither enters or leaves the cell, cell remains flaccid (incipient plasmolysis) (isotonic).

(no net movement of water)

Question 12

What happens in animal cells if the water potential of the external solution is higher than the cell solution?

Answer 12

Water enters the cell, cell swells and bursts (this is called lysis) (hypotonic).

Question 13

What happens in animal cells if the water potential of the external solution is lower than the cell solution?

Answer 13

Water leaves the cell, cell shrinks (this is called crenation) (hypertonic).

Question 14

What happens in animal cells if the water potential of the external solution is equal to the cell solution?

Answer 14

Water neither enters or leaves the cell (no net movement of water) (isotonic).

Question 15

What can you use to investigate water potential?

Answer 15

Serial dilutions

Question 16

How do you use the serial dilution technique to make several Solutions of different, known concentrations to test for water potential?

Answer 16

This is how you make 5 serial dilutions or is sucrose solution, starting with an initial sucrose concentration of 2 M and diluting it solution by a factor of 2.

1. Line up 5 test tubes in a rack.
2. Add 10 cm^3 of the initial 2 M secure solution to the first test tube and 5 cm^3 of distilled water to the other for test tubes.
3. Then, using a pipette,, draw 5 cm^3 of the solution from the first test tube, add it to the distilled water in the second test tube and mix the solution thoroughly. You now have 10 cm^3 of the solution that’s half as concentrated as the solution in the first test tube (it’s 1 M).
4. Repeat this process three more times to create Solutions of 0.5 M, 0.25 M and 0.125 M.

Question 17

Explain how you can use solutions made from serial dilution to find the water potential of potato cells

Answer 17

1. Use a cork borer to cut potatoes into identically sized chips, about 1 cm in diameter.
2. Divide the chips into groups of 3 and measure the mass of each group using a mass balance.
3. Place one group into each of your sucrose solutions.
4. Leave the chips in the solutions for at least 20 minutes (making sure that they all get the same amount of time).
5. Remove the chips and pat dry gently with a paper towel.
6. Weigh each group again and record your results.
7. Calculate the % change in mass for each group.
8. Use the results to make a calibration curve, showing % change in mass against sucrose concentration.

Question 18

When investigating the water potential of potato cells, what will happen to the weight of the potato chips?

Answer 18

The potato chips will gain water (and therefore gain mass) in solutions with a higher water potential than the chips, and lose water in solutions with a lower water potential.