Practical lesson #2: Leek cells Lab Report

Homeostasis

 Practical 1 - Estimation of the osmotic concentration of Leek cells
29th January 2014

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

Materials:

1. Scalpel
2. White tile (for cutting)
3. 5 Petri dishes
4. Forceps
5. Distilled water
6. 1%, 2%, 4%, 7%, 10% salt solution
7. Chinese leeks (Allium tubersosum)
8. Stop watch
9. Vernier calipers
10. Thread
11. Labels
12. Electronic balance

Procedure:

You will be investigating the effects of different concentration of salt solution on leek stem. The effects observed in the stem can be recorded quantitatively and used to approximate the osmotic concentration of the leek stem. 

1. Cut the stalk of the Chinese leek to baton a length of 3.0cm.
2. Cut the strip longitudinally as shown in the diagram below to obtain 4 equal quarters. Repeat till you have battened 6 strips.
3. Using the mass of the leek stem to estimate the osmotic concentration
• Wight the individual strips and record the masses.
• Place one strip in a Petri dish of distilled water, one each in 1%, 2%, 4%, 7% and 10% salt solution. 
• After 20-22 minutes, remove the strips from the solution and dab and the strips dry with paper towel. Weigh and record the mass of the individual strips in a table and use the use the data to plot a graph. 

Results: 

Effect of concentration of salt solution (%) on the percentage change in mass (%) of the leek stems

Concentration of salt solution (%)	Initial mass of leek (g)	Final mass of leek  (g)	Difference in mass (g)	Percentage change in mass (%)
0	0.30	0.37	+ 0.07	+ 23.3
1	0.32	0.35	+ 0.03	+ 9.38
2	0.26	0.27	+ 0.01	+ 3.85
4	0.28	0.29	+ 0.01	+ 3.57
7	0.29	0.30	+ 0.01	+ 3.45
10	0.33	0.32	- 0.01	- 3.03

Discussion questions:

• At which concentrations are the salt solution isotonic to the leek cells? Explain how you arrived at your answer. 

Based on the data I have gathered, the salt solution is isotonic to the leek cells at slightly less than 10% concentration. This is because the overall percentage change in mass of the leeks is the least when placed in the 10% salt solution. If a solution is isotonic to the leek cells, then there will not be an overall net movement through the partially permeable membrane of the cells, as their water potential are the same. Since the leek strip in the 10% salt solution experience the least mass change, which means that there is little movement of the water movement leaving the cells into the solution through the partially permeable membranes through osmosis. This accounts for the least percent change in mass, which is a negative -3.03%, which means that the water potential of the surrounding solution that that of the leek cells which is why water left the cell, resulting in a decrease in mass. Hence, the osmotic concentration of the leek cells is slight below 10%.

• Explain the gain or loss in mass of the stems

The leek strips in all the petri dishes except for the one in the 10% salt solution experienced a gain in mass. This is because they were placed in a hypotonic solution, where the water potential of the surrounding solutions is higher than that of the leek cells. This results in an overall net movement of water molecules entering the leek cells through a partially permeable membrane, through the process of osmosis. The water gain caused the gain in mass of the stems. 

This water gain also caused the stem to curve outwards and turn rigid and coarse. This is because the epidermis has a waxy cuticle and thick as cells there have a thicker cell wall as compared to those in the cortex, while the cortex of the stem is softer as the cells there have a thinner cell wall as compared to those in the epidermis. The waxy surface on the epidermis minimises water entry and loss, coupled with the fact that the epidermis is much thicker than the cortex, very little water is able to enter the cell through the epidermis when placed in a hypotonic solution. However, the cortex is quite the opposite, as it is softer and has thinner cell walls, more water is able to enter the cells easily, hence the inner layer of the stem gains much more water than the epidermis. As the inner cortex is also more flexible than the epidermis, the difference in flexibility and water gain, causes the stem to bend outwards. The water gain of the leek cells causes them to be turgid and rigid and coarse as they are expanded.

The strip in the 10% salt solution is the only one that decreased in mass. The solution the leek strip is placed is hypertonic to the leek cells, where the water potential of the surrounding solution is lower than that of the leek cells. This results in an overall net movement of water molecules leaving the leek cells and entering the surrounding salt solution through a partially permeable membrane, through the process of osmosis. This water loss caused the decrease in the mass of the stem. 

The water loss caused the stem to bend inwards in a hypertonic solution and turn soft and a little slimy, the opposite of what happens when placed in a hypotonic solution. This is also due to the difference in properties between the epidermis and the cortex. As mentioned, the waxy cuticle on the epidermis and its hard layer minimises water loss and makes it harder for water molecules to leave the cells. However, the cortex is much thinner and has no waxy layer, more water would be able to escape through the cortex than the epidermis, and because the cortex is more flexible than the epidermis, the stem would bend inwards. The water loss from the leek cells caused it to be plasmolysed and turn soft. 

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

The concentration of salt in the solution can be used as an indication to osmosis. If there osmosis occurring, for example, water molecules leaving the cells and entering the surrounding salt solution, the surrounding salt solution would then be diluted to the increased amount of the solution. Vice versa, if there is water molecules entering the cells, then the amount of water in the salt solution decreases, and the solution would be more concentrated. This can be used an indication of water movement between the salt solution and the leek cells.

• Suggest a way to improve the experiment.
• Firstly, as the experiment with the petri dishes uncovered, one way to improve the experiment is to cover the petri dishes with a cover to prevent the evaporation of the water in the salt solution. This ensures a fair test, as the results will be reliable because the solutions will not be subjected to evaporation or contamination that might affect the results. 
• Secondly, the experiment could be repeated, this time with 3 leeks in one petri dish, and the average of the results could be obtained and calculated, so that the results would be reliable as the human errors would be reduced. 
• Thirdly, the experiment could be conducted with more intermediate concentrations of salt solutions (e.g. 3%, 5%, 6%,8%,9%), so more data could be gathered and when plotting the graph, the gradual change in mass can be observed more clearly.

Graph:

Effect of concentration of salt solution (%) on the percentage change in mass (%)

Photos: