Practical #2- Leek Experiment

Practical: Estimationg of the osmotic concentration of leek cells

Aim: to estimate the approximate osmotic concentration of leek stem cells

Apparatus:

• Razor blade
• White tile
• 5 petrid dishes
• Dorceps
• Distilled water
• 1%, 2%, 4%, 7%, 10% salt solution
• Chinese leeks
• Stop watch
• Vernier calipers
• Thread
• Labels
• Electronic balance
•  

Procedure

1. Cut the inflorescence stalk of the Chinese leek tp obtain a length of 3.0cm

2. Cut the strip longitudanally as shown in the diagram below to obtain 4 equal quarters. Repeat till you have obtained 6 strips.

Using length

3. Measure the length of the inner surface of thw strip and record the length

4. Place one strip in a petri dish of distilled water, one each in 1%,2%, 4%, 7%, 10% salt solution.

5. After 25 minutes, record the length of the inner surface and weight. Record the results in a table and suse the data to plot the graph. Explain how the length was measured.

Discussion Questions:

At which concentrations are the salt solution isotonic to the leek cells?

There is no percentage change of length. This means that the water potential of the cell sap is similar to that of its surroundings

Explain the gain or loss in the length of the stems.

The epidermal layer has a waxy surface and thicker wall. Hence it takes in less water then the outer layer where the walls are thinner. Thus the strip bend inwards.

Texture of a Potato strip in different environments

Hypotonic: high water potential, hard, rigid, coarse

Hypertonic: low water potential, soft limp, smooth

                 

Gain in mass: water potential

                                    Net movement

                                    Across partially permeable membrane

                                    Osmosis

Instead of measuring the change in length of the stem, is there another variable you can measure as an indication to osmosis?

Thickness (but length is still better)

Suggest a way to improve this experiment.

• Experimental errors:
• Repeat à take the average to reduce the random error à reliable results
• Eg: use 3 leeks instead of one
• Using longer leeks will yield more reliable results
• Cut off the ends à they might have dried out
• Do the experiment with more solutions of different concentrations, and at equal intervals
• Water evaprates from petri dish à cover with petri dish cover
• Using thread to measure à a lot of error (mass is more accurate)
• Measure curvature instead à graph paper