topic 2 - exchange across cell membranes - osmosis

Question 1

what is osmosis

Answer 1

the diffusion of water molecules across a partially permeable membrane, from an area of high water potential to an area of lower water potential

Question 2

what is water potential

Answer 2

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

Question 3

what has the highest water potential

Answer 3

pure water - all solution have a lower water potential than pure water

Question 4

what is an isotonic solution

Answer 4

the concentration of solutes is balanced on both sides of the membrane, so there is no concentration gradient to drive the movement of water molecules by osmosis

Question 5

what are three factors that the rate of osmosis depends on

Answer 5

(1) water potential gradient
(2) thickness of the exchange surface
(3) surface area of the exhange surface

Question 6

how does the rate of osmosis depend on the water potential gradient

Answer 6

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 7

how does the rate of osmosis depends on the thickness of the exchange surface

Answer 7

the thinner the exchange surface, the faster the rate of osmosis

Question 8

how does the rate of osmosis depends on the surface area of the exchange surface

Answer 8

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

Question 9

what can you use to investigate water potential

Answer 9

serial dilutions

Question 10

what is the first step to investigating water potential

Answer 10

line up five test tubes in a rack

Question 11

what is the second step to investigating water potential

Answer 11

add 10cm3 of the initial 2M sucrose solution to the first test tube and 5cm3 of the distilled water to the other four test tubes

Question 12

what is the third step to investigating water potential

Answer 12

using a pipette, draw 5cm3 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 10cm3 of the solution that’s half as concentrated as the solution in the first test tube

Question 13

what is the fourth step to investigating water potential

Answer 13

repeat this process three more times to create solutions of 0.5M, 0.25M and 0.125M

Question 14

what’s the first step if you want to make 15cm3 of 0.4M sucrose solution

Answer 14

start with a solution of a known concentration, e.g 1M

Question 15

what’s the second step if you want to make 15cm3 of 0.4M sucrose solution

Answer 15

find the scale factor by dividing the concentration of this solution by the concentration of the solution you want to make - so in this case the scale factor = 1M ÷ 0.4M = 2.5

Question 16

what’s the third step if you want to make 15cm3 of 0.4M sucrose solution

Answer 16

this means that the solution you want to make is 2.5 times weaker than the one you have - to make the solution 2.5 times weaker, use 2.5 times less of it, i.e 15cm3 ÷ 2.5 = 6cm3 - transfer this amount to a clean test tube

Question 17

what’s the fourth step if you want to make 15cm3 of 0.4M sucrose solution

Answer 17

top up the test tube with distilled water to get the volume you want to make - in this case you want to make 15cm3 of solution, so you need to add: 15 - 6 = 9cm3 of distilled water

Question 18

what’s the first step to finding the water potential of potato cells

Answer 18

use a cork borer to cut potatoes into identically sized chips, about 1cm in diameter

Question 19

what’s the second step to finding the water potential of potato cells

Answer 19

divide the chips into groups of three and measure the mass of each group using a mass balance

Question 20

what’s the third step to finding the water potential of potato cells

Answer 20

place one group into each of your sucrose solutions

Question 21

what’s the fourth step to finding the water potential of potato cells

Answer 21

leave the chips in the solutions for at least 20 minutes (making sure that they all get the same amount of time)

Question 22

what’s the fifth step to finding the water potential of potato cells

Answer 22

remove the chips and pat dry gently with a paper towel

Question 23

what’s the sixth step to finding the water potential of potato cells

Answer 23

weigh each group again and record your results

Question 24

what’s the seventh step to finding the water potential of potato cells

Answer 24

calculate the % change in mass for each group

Question 25

what’s the eighth step to finding the water potential of potato cells

Answer 25

use the results to make a calibration curve, showing % change in mass against sucrose concentration

Question 26

what is the point at which the curve crosses the x - axis of the water potential of potato cells graph

Answer 26

the point at which the water potential of the sucrose solution is the same as the water potential of the potato cells