3.2.3 Transport 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 higher 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 is the water potential of pure water

Answer 3

Zero

Question 4

How does the water potential of a solution compare to pure water

Answer 4

All solutions have a lower water potential than pure water

Question 5

What happens if two solutions have the same water potential

Answer 5

They are said to be isotonic

Question 6

How does the water potential gradient affect the rate of osmosis

Answer 6

The higher the water potential gradient the faster the rate of osmosis

Question 7

What happens to the rate of osmosis as osmosis takes place over time

Answer 7

The rate of osmosis levels off over time as the difference in water potential decreases

Question 8

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

Answer 8

The thinner the exchange surface the faster the rate of osmosis

Question 9

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

Answer 9

The larger the surface area the faster the rate of osmosis

Question 10

What is the purpose of using serial dilutions in an osmosis experiment

Answer 10

To make solutions of different known concentrations to test the water potential of plant tissue

Question 11

What is the first step in making serial dilutions of sucrose solution

Answer 11

Line up five test tubes in a rack

Question 12

What is the second step in making serial dilutions of sucrose solution

Answer 12

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

Question 13

What is the third step in making serial dilutions of sucrose solution

Answer 13

Using a pipette draw 5 cm3 of the solution from the first test tube and add it to the distilled water in the second test tube then mix thoroughly

Question 14

What concentration do you get in the second test tube when you make serial dilutions by a factor of 2

Answer 14

1 M solution

Question 15

What concentration do you get in the third test tube when you make serial dilutions by a factor of 2

Answer 15

0.5 M solution

Question 16

What concentration do you get in the fourth test tube when you make serial dilutions by a factor of 2

Answer 16

0.25 M solution

Question 17

What concentration do you get in the fifth test tube when you make serial dilutions by a factor of 2

Answer 17

0.125 M solution

Question 18

What is the first step in finding the water potential of potato cells

Answer 18

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

Question 19

What is the second step in 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 is the third step in finding the water potential of potato cells

Answer 20

Place one group into each of your sucrose solutions

Question 21

What is the fourth step in finding the water potential of potato cells

Answer 21

Leave the chips in the solutions for at least 20 minutes ensuring they all get the same amount of time

Question 22

What is the fifth step in finding the water potential of potato cells

Answer 22

Remove the chips and pat them dry gently with a paper towel

Question 23

What is the sixth step in finding the water potential of potato cells

Answer 23

Weigh each group again and record your results

Question 24

How do you calculate the percentage change in mass of the potato chips

Answer 24

Use the formula percentage change in mass equals (change in mass divided by original mass) times 100

Question 25

How do you determine the water potential of potato cells after calculating the percentage change in mass

Answer 25

Plot the percentage change in mass on a graph to make a calibration curve and find the sucrose concentration where the percentage change in mass is zero

Question 26

What does it mean when the percentage change in mass of the potato chips is zero

Answer 26

The water potential of the sucrose solution is the same as the water potential of the potato cells

Question 27

What happens to potato chips in a solution with a higher water potential

Answer 27

The chips gain water and increase in mass

Question 28

What happens to potato chips in a solution with a lower water potential

Answer 28

The chips lose water and decrease in mass

Question 29

How can you make sucrose solutions of different concentrations using a scale factor

Answer 29

Divide the concentration of the solution you want to make by the concentration of the solution you have then multiply the result by the volume you want to make

Question 30

If you need to make 15 cm3 of 0.4 M sucrose solution from a 1 M stock solution how do you find the volume of stock solution needed

Answer 30

Divide 0.4 M by 1 M to get 0.4 then multiply by 15 cm3 to get 6 cm3 of stock solution

Question 31

What do you do after calculating the volume of stock solution needed to make a new concentration

Answer 31

Transfer the calculated amount to a clean test tube and top up with distilled water to the desired volume

Question 32

Molecules and ions can move into or out of cells using diffusion or osmosis.

Describe two similarities between these methods of transport.

Answer 32

both involve movement down/along a concentration gradient

OR

both move from a high concentration to a low concentration

both are passive processes

OR

both are not active processes

OR

neither use energy from respiration/ATP

Question 33

Describe the similarities and differences between the processes by which water and inorganic ions enter eukaryotic cells.

Answer 33

Similarities:

both can move down/along a concentration gradient

both can move through protein channels (in the membrane)

Differences:

ions can move against a concentration gradient by active transport

Question 34

Euglena viridis is a unicellular algae found in freshwater pools, ponds, and streams.

These algal cells contain structural components known as contractile vacuoles which collect water from the cytoplasm and contract to expel water from the cell.

Explain why Euglena viridis would expel water when placed in a freshwater environment.

Answer 34

to maintain optimum/normal water potential of the cell/cytoplasm

OR

to stop the cell from bursting

water moves into the cell by osmosis

(as the) water potential is higher outside the cell