4.3 - Osmosis

Question 1

what is the definition of 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 is a solute

Answer 2

any substance that is dissolved in a solvent

Question 3

what is water potential usually measured in

Answer 3

ψ - kiloPascals (kPa)
- pure water is 0

Question 4

what is water potential

Answer 4

the pressure created by water molecules

Question 5

what are the rules for water potential

Answer 5

• addition of a solute to pure water will lower its water potential
• the water potential of a solution (water + solute) must always be less than 0, a negative value
• more solute added = lower the water potential
• water will move by osmosis from a region of higher (less negative water potential) to one of a lower (more negative water potential)

Question 6

how could you find out the water potential of cells or tissues

Answer 6

• place them in a series of solution of different known water potentials
• where there’s non et gain or loss of water from the cells or tissues = water potential of solution is the same of the water potential of the cell/tissue

Question 7

what happens to an animal cell and a plant cell if they get too much water from osmosis (include example for red blood cell)

Answer 7

• animal cell eventually bursts = lysis
• plant cell: swelling of cytoplasm and vacuole, push against the cell wall which will stop the cell getting any larger = turgid
• red blood cell: CSM is thin and not flexible, so when it absorbs water it will break and burst - releasing its contents = haemolysis

Question 8

why does blood plasma have the same water potential as red blood cells

Answer 8

prevent haemolysis

Question 9

what happens to an animal cell and a plant cell if they get placed in a hypertonic solution (include example for red blood cell)

Answer 9

• animal cell: cell contents shrink and membrane wrinkles = crenated
• plant cell: cytoplasm + vacuole shrink and CSM pulls away from cell wall = plasmolysis

Question 10

what is a isotonic solution

Answer 10

solution of equal water potential to a cell

Question 11

what is a hypertonic solution

Answer 11

solution of lower water potential then a cell

Question 12

How do you describe 2 solutions that have the same water potential

Answer 12

Isotonic

Question 13

What factors affect the rate of osmosis

Answer 13

• the water potential gradient
• the thickness of the exchange surface
• the surface area of the exchange surface

Question 14

How does the water potential gradient affect the rate of osmosis

Answer 14

• the higher the water potential 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 over time

Question 15

How does the thickness of the exchange surface affect the rate of osmosis

Answer 15

• the thinner the exchange surface = the faster the rate of osmosis
• because the water molecules have less distance to travel

Question 16

How does the surface area of the exchange surface affect the rate of osmosis

Answer 16

• the larger the surface area, the faster the rate of osmosis

Question 17

How can investigate the water potential: method for serial dilution

Answer 17

• line up 5 test tubes in a rack
• add 10cm3 of the initial 2 M sucrose solution to the first test tube and add 5cm3 of distilled water to the other 4 test tubes
• (using a pipette), draw 5cm3 of the solution from the first test tube, add it to the distilled water in the 2nd test tube and mix thoroughly
• you now have 10cm3 of solution that’s half as concentrated as the solution in the first test tube
• repeat this process 3 more times to create solutions of 0.5M, 0.25M and 0.125M

Question 18

How do find the water potential of a potato cell

Answer 18

1) cut potatoes into identically sized chips, about 1cm 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 chips in solution for 20 mins (make sure they all get the same amount of time)
5) remove the chips and pat dry gently with a paper towel
6) weigh each group 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 19

Describe what you should find for the masses of each potato chip

Answer 19

• potato chips gain water (therefore mass) in solutions with a higher water potential than the chip
• potato chips lose water (therefore mass) in solution with a lower water potential than the chip

Question 20

In the graph created from the potato chip experiment, what does the point at which the curve crosses the x-axis show

Answer 20

• its where the % change in mass = 0
• its the point where the water potential of the sucrose solution is the same as the as the water potential of the potato cells