Exchange Across Cell Membrane - Osmosis

Question 1

What is osmosis?

Answer 1

Diffusion of water molecules across a partially permeable membrane from an area of a higher water potential (high concentration of water - hypotonic) to an area of lower water potential (low concentration of water - hypertonic)

Question 2

What is water potential?

Answer 2

Likelihood of water molecules to diffuse in or out of the solution

Question 3

What does pure water have?

Answer 3

Highest water potential - 0

All other solutions have lower water potential

Question 4

What is isotonic?

Answer 4

If two solutions have the same water potential - isotonic

Question 5

What are the factors affecting osmosis?

Answer 5

• Water potential gradient
• Thickness of exchange surface
• Surface area of exchange surface

Question 6

What does water potential gradient affect osmois?

Answer 6

Higher the water potential graident

faster rate of osmosis

difference in water potential on either side of membrane decreases

rate of osmosis levels off over time

Question 7

How does thickness of exchange surface affect osmosis?

Answer 7

Thinner the exchange surface , faster rate of osmosis

Question 8

How does the surface area of exchange surface affecting osmosis?

Answer 8

Larger the surface area the faster rate of osmosis

Question 9

Instructions on doing serial dilution technique of different known concentrations

Answer 9

• five test tube in rack
• 10cm^3 of inital 2M sucrose solution to first and add 5cm^3 of distilled water to other
• Use pippette to draw 5cm^3 of solution of first and add to second ND MIX
• Repeat three times

Question 10

How to find water potential of potato cells

Answer 10

• Use cork borer to cut potatoes in identically sized chips
• Divide of three and measure mass
• Leave them in solutions for 20 minutes - same amount of time
• Remove and dry
• Weight each group
• Calculate % change in mass
• Use calibriation curve

Question 11

Potato chip mass

Answer 11

Potato chips will gain water (mass) with solutions of high water potential

lose water with solutions with lower water potential

Question 12

What does the point of the curve crosses x-axis where % change in mass is 0

Answer 12

Point sucrose solution and water potential is same

Find concentration at point and look up water potential for the concentration of the sucrose solution

Question 13

What does selectively permeable?

Answer 13

Permeable to water molecules and few water molecules and few other molecules but not to larger molecules

Question 14

Why aren’t red blood cells placed in pure water?

Answer 14

Absorb water by osmosis because low water potential

Cell surface membrane is thin

Break , bursting the cell and release its contents

Question 15

How to prevent cells from bursting?

Answer 15

They live in liquid with same water potential as cells

Question 16

What happens if cell lives in external solution that is higher water potential ? (hypotonic)

Answer 16

• Net movement of water neters the cell
• Cell swells and bursts

Question 17

What happens if cell lives in an external solution that has equal water potential?

Answer 17

• Net movement neither leaves nor enters cell
• No change

Question 18

What happens if a cell has lower external water potential?

Answer 18

• Net movement of water leaves the cell
• Shrinks

Question 19

Why is table above known as processed results?

Answer 19

Calculations made from raw data

Question 20

Describe how you would use the student’s results in the table above to find the water potential of the potato tissue

Answer 20

• Plot graph with concentration (x)
• percentage change (y)
• Find point at which curve crosses x-axis where there is no percentage change of mass
• Find the concentration at this point and look up water potential for this by using another resource

Question 21

Before reweighing the potato chips the student blotted dry the outside of each cylinder

Explain why:

Answer 21

Water moving in and out by osmosis

If not water affect mass

Ensures the same amount of water on the outside

Question 22

Why use percentage change in mass than change in mass?

Answer 22

Cylinders have different starting weights

Use as a comparison

Question 23

Why repeats are important

Answer 23

Reduce the chance of anomalous results

Line of best fit is more reliable

Question 24

Why giving results as a raito?

Answer 24

Use comparison

Show proportional change

Question 25

Why improve reliability by taking additional readings is important?

Answer 25

Reduce the change of anomalus results

Allows a mean to be calculated for the concentrations

Question 26

How should students use cork borer to cut cylinders be accurate?

Answer 26

Cut down the root

Through the same part

Question 27

Explain why bungs were placed in tubes?

Answer 27

Avoid change in water potential

Solution as water evaporate

Question 28

The water potential of a plant cell is -400kPA.

Cell put in solution of -650kPA

Explain what happens?

Answer 28

Mmebrane/cytoplasm shrinks/pulls away from cell wall

plasmolysed

Question 29

Water moves into the cytoplasm from the surrounding solution

Water moves from cytoplasm into contractile vacuole and is then expelled from the organism

Explain how water moves from cytoplasm into contractile vacuole

Answer 29

Water moves from cytoplasm to contractile vacuole by osmosis

High water potential in cytoplasm then moving into less high water poential in vacuole by diffusing through partially permeable membrane

Question 30

Suggest why contractile vacuoles are not found in these species live in sea

Answer 30

Amobea is surrounded by water as cell is already rigid and maintains pressure

level of water potential is the same as cytoplasm