RP3: Osmosis

Question 1

What is the aim of this experiment?

Answer 1

• immerse plant tissue in a range of conc of salt or sugar solution to find the change in length or mass to investigate the movement of water by osmosis

Question 2

Name the variables

Answer 2

• IV: concentration of solution
• DV percentage change in mass
• CV: initial length and diameter, species of potato, amount of skin, volume of water, shape of potato, temperature, time, type of sugar used

Question 3

Describe how you would carry out an investigation into osmosis using potato tissue

Answer 3

1) use a cork borer to cut 3 potato cylinders of same diameter
2) trim cylinders so that they are same length (about 3cm)
3) accurately measure and record length and mass of each cylinder
4) measure out 10cm³ of 1 M sugar solution and place into first boiling tube (labelled)
5) measure out 10cm³ of 0.5 M sugar solution and place into second boiling tube (labelled)
6) measure out 10cm³ of distilled water into third boiling tube (labelled)
7) add one potato cylinder in each tube (ensure you know which one is which in terms of length and mass)
8) record lengths and masses in a table
9) leave cylinders in boiling tubes overnight in test tube rack
10) remove cylinders from boiling tubes and carefully blot them dry with paper towels
11) re-measure length and mass of each cylinder (ensure you know which is which)
12) draw a graph with ‘change in mass in g’ on y-axis against ‘concentration of sugar solution’ on x-axis

Question 4

How to calculate percentage change in mass?

Answer 4

• do change in value/original value
• ensure that if value increases, percentage change is positive
• if value decreases, percentage change is negative

Question 5

Describe the results and how to calculate conc of solution

Answer 5

• as concentration outside cylinder increases, the percentage change in mass decreases
• to calculate concentration of solution inside the potato cells, find where the line of best fit meets the x-axis

Question 6

How would you determine the validity of a practical?

Answer 6

• based on control variables

Question 7

How would you increase validity of this practical?

Answer 7

• use a cork borer, a scalpel and a ruler to ensure surface area is the same, temperature constant, volume of solution constant, etc

Question 8

Why is it necessary to blot the cylinders with paper after removing them?

Answer 8

• Each cylinder may have a different amount of water on its surface, so masses may vary
• Blotting cylinders allows masses to be comparable

Question 9

Why is it necessary to use a cork borer to cut the cylinders?

Answer 9

• results in equal sized sample so changes in length and mass can be compared easily

Question 10

Why should you calculate percentage change in mass rather than just using final mass?

Answer 10

• because the potato cylinders have different initial masses; this makes the comparison more accurate

Question 11

Name some sources of error

Answer 11

• Discs taken from different parts of the potato may have different water potentials.
• Potato discs may have different surface areas which affect the rate of osmosis.

Question 12

What safety precautions should be taken?

Answer 12

• Handle cork borer and scalpel with care