Required Practical 3

Question 1

What is the process of calibration curves?

Answer 1

They are used to determine the concentration of a unknown sample by comparing it to a set of standard values with known concentrations

Question 2

How is a calibration curve used to find the concentration of plant tissue?

Answer 2

Plot a calibration curve of percentage change in mass against concentration. Find the x-intercept where the plant tissue is isotonic to the sucrose solution

Question 3

What happens when the plant tissue is placed in a hypotonic solution?

Answer 3

Water moves into the plant tissue by osmosis. plant tissue increases in mass

Question 3

What happens when the plant tissue is placed in a hypotonic solution?

Answer 3

Water moves into the plant tissue by osmosis. plant tissue increases in mass

Question 4

What happens when the plant tissue is placed in a hypertonic solution?

Answer 4

Water moves out of the plant tissue by osmosis, plant tissue decreases in mass

Question 5

Why are the potato discs left in the solution for 20 minutes?

Answer 5

To allow time for osmosis until the plant tissue reaches equilibrium with its surrounding solution

Question 6

What is water potential determined by?

Answer 6

• The concentration of solutes
• The higher the solute concentration then the lower the water potential

Question 7

Outline the procedure of investigating osmosis using potato tissue

Answer 7

1. Make a simple dilution of 1M sucrose to produce 5 concentrations. Add 5cm3 to five different test tubes
2. Cut potato into equal size and weigh
3. Dry to remove any excess water
4. Place a chip in each test tube and leave for 20 minutes
5. Take out, dab the excess water and weight them again
6. Calculate the percentage change in mass

Question 8

Why is the percentage change used rather than the actual change in mass?

Answer 8

• Potato chips may not all have the same starting mass
• Percentage change allows comparison

Question 9

What is the x-intercept of the calibration curve?

Answer 9

The concentration that is isotonic to the solution tested

Question 10

Explain the change in mass in the potato chips

Answer 10

• The potato chips with concentration lower than the sucrose solution (higher water potential) loses mass as there is a net movement of water out of the cells
• The potato chips with concentration higher than the sucrose solution (lower water potential) gain mass as there is a net movement of water into the cells

Question 11

Why are the potato chips dabbed dry after removed from the sucrose solution?

Answer 11

To remove any excess water clinging to its surface

Question 12

What are the control variables of this practical?

Answer 12

• Volume of sucrose solution
• Size of potato chips
• Length of time left in solution
• Dab each potato disc with paper towels

Question 13

Potato skin has been removed from the cylinders. Suggest one reason why the potato skin could have affected osmosis.

Answer 13

• Potato skin is impermeable to water
• Skin is a different tissue
• Cells of skin have different water potential (to rest of tissue)

Question 14

Suggest why the potato cylinders were all the same length

Answer 14

• Same/similar surface area
• (Otherwise) affects rate of osmosis