Find the concentration of Potato Cell Sap.

-The strength of the sucrose solutions; I used only five solutions with concentrations of 0, 0.5, 1.0, 1.0, 2.5 and 5.0. There was a too big gap between the concentrations, preventing accuracy. The concentrations need to be smaller, e.g. 0, 0.1, 0.2, 0.3, 0.4 and 0.5.

Part 2; Plan of Investigation

The factor (or variable) that I am going to change is:

• The concentration of the sucrose solution; 0, 0.1, 0.2, 0.3, 0.4 and 0.5 M Sucrose.

To keep it a fair test, I shall keep the following factors the same:

• The starting size of the potato; 50mm

• The width of the potato; using a size 3 corer (5mm diameter)
• Test tubes will be used for every strip of potato
• The same amount of solution will be used in each test tube; 10cm³
• Each piece of potato will be left for the same amount of time; 24 hours.

Factor’s that I have no control over are:

• The temperature of the room; this however will have no varied affect on each of my  18 experiments as each different test tube will be exposed to any change at the same time.

My Prediction is:

• The stronger the sucrose solution is, the smaller the potato strips will be finally.
• The strips in the very weak solutions may grow a bit, with water growing the most.
• The concentration of the cell sap will be somewhere between the concentrations of 0.2 and 0.3.

Risk Assessment:

There is very little risk in this experiment, as I am not using any dangerous chemicals or gases, fire or especially sharp instruments. I will however, take extra care when I use the scalpel to cut the potato. As this is a scientific experiment I will take the usual safety precautions to prevent contamination, I will tie my hair back and wear a lab coat and safety goggles. I will also make sure that all bags and stools are safely underneath the desk.

Measurements which I shall take:

• Solution – 10cm³ using a measuring cylinder (accurate to 1cm³)
• Length of potato – 50 mm, using a ruler (accurate to 1mm)
• Width of potato – 5mm, using a size 3 corer which has a diameter of 5mm)
• Final length of potato – using a ruler (accurate to 1mm)
• Time that the potato will be left for – 24 hours, using a clock (accurate to 1 second)
• Number of pieces of potato per concentration – 3

Accuracy of results:

• To ensure that my results are as accurate as possible, I have decided to use 3 pieces of potato per concentration. This will enable me to take an average at the end of the experiment.
• Measuring in mm instead of cm will give me a greater degree of accuracy where the length of the potatoes concerned.

List of Apparatus:

• 18 test tubes (3 per each of the 6 concentrations)
• 1 Scalpel
• 1 30cm ruler
• 1 number 3 ( 5mm diameter) potato corer
• 1 test tube rack ( 20 test tube holes)
• 1 clock
• 1 large potato
• 30 cm³ of distilled water (0 MS)
• 30 cm³ of the 5 concentrations (0.1, 0.2, 0.3, 0.4 + 0.5) of sucrose solution
• 1 50 cm³ measuring cylinder
• 18 labels for the test tubes
• 1 beaker
• 1 tile
• 1 pair of tweezers

Plan:

1. Set up a test tube rack with 18 test tubes, in groups of threes.
2. Using the measuring cylinder, measure 10cm³ of distilled water (0.0  Msucrose) into each of the first three test tubes.
3. Repeat this for the further five concentrations (0.1, 0.2, 0.3, 0.4 + 0.s Msucrose), making sure that the measuring cylinder is washed out each time the concentration is changed.
4. Label each test tube according to which solution is inside it.
5. Using the size 3 corer cut 18 strips from the potato.
6. Cut each one down to 50mm in length using a ruler and a scalpel. Do this on the tile to prevent ruining the surface.
7. Place one piece of potato into each test tube.
8. Record the time from when the last piece of potato is in it’s test tube.
9. Leave the potatoes in the bathing solutions for 24 hours.
10. Draw a table ready to record your results
11. After 24 hours take the first test tube from the stand and tip the contents into a beaker.
12. Using the tweezers, take the strip of potato carefully from the beaker and lay it on the tile.
13. Measure the new length with a ruler and record this in the table.
14. Repeat steps 10, 11 + 12 for the remaining 17 pieces of potato.
15. When all the potatoes have been measured record the time from the clock.
16. Work out the average change in length for each concentration.
17. Draw a graph following the sketch below;

Modification to the plan;

• The time that the potato was left for was 20 hours and 5 minutes as opposed to the planned 24 hours. This was due to the timings of lessons.
• Instead of using a measuring cylinder, I used a 10cm³ Syringe. I found this easier, quicker and more accurate.
• I also set up the apparatus the day before I conducted my experiment. This was to ensure that all my apparatus was ready so that all my experiments could be performed at the same time.

Part 3; Results

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Part 4; Analysis;

The Rseults suggest the following conclusion;

Where the line crosses the x-axis, there is no change in the length of the potato and therefor there is no movement of the water molecules. If there is no movement of the water molecules, then the concentration of the cell sap must be equal to the concentration of that bathing solution. My line of best fit crosses the x-axis at 0.2 M Sucrose. Therefor, form my results, I can see that the concentration of the cell sap is 0.2 M Sucrose.

When I look at the results more closely, I can see the following patterns or trends;

As the concentration of the bathing solution increases in strength, the pieces of potato get smaller. At 0 and 0.1 M Sucrose, the potato grows, with 0 growing the most. At 0.2 M Sucrose, there is no change in length of potato and at 0.3, 0.5 and 0.5 m Sucrose, the potato decreases in size, with 0.5 becoming the smallest. The line of best fit on the graph is straight, indicating that there is a directly proportional relationship between the concentration of the sucrose solution and the change in length of the potato.

My Scientific Knowledge supports this conclusion by explaining;

In osmosis, water molecules travel from where there is a high concentration where there is a low concentration of water molecules, i.e. Down a concentration gradient. !n the bathing solutions of 0 and 0.1 M Sucrose, the solutions are very weak so there are a lot of water molecules. Some of these move into the potato, where there are less molecules than in the bathing solution. This causes the potato to grow. At 0.3, 0.4 and 0.5 M Sucrose, the solutions are becoming more and more concentrated, meaning there are a lot less water molecules. The potato has more water molecules than the bathing solution so some of the potato’s water molecules move into the bathing solution, diluting it and causing the potato to decrease in length. There is no movement, at 0.2 M Sucrose because the concentration of the bathing solution, is identical to that of the potato. My result support this knowledge.

This is my explanation as to whether my results support my original prediction;

I predicted that,

• The stronger the sucrose solution is, the smaller the potato strips will be finally.
• The strips in the very weak solutions may grow a bit, with water growing the most.
• The concentration of the cell sap will be somewhere between the concentrations of 0.2 and 0.3.

The potato did grow in the weak solutions, e.g. 0M Sucrose = 2.7 mm growth.

The potato also shrank in the strong solutions, e.g. 0.5 M Sucrose = -4 mm decrease in length.

The cell sap concentration was 0.2  as there was no change in length.

Part 5; Evaluation

I consider my results to be accurate because;

Four of my six points are on the line and the other two points are extremely close to the line, ( 0, 0.2, 0.3 and 0.5 are are on the line, whilst, 0.1 and 0.4 are next to the line). The line of my graph is straight and crosses the x axis, so there is a region where water is entering the potato above the line and a region where water is leaving the potato below the line. This enables you to pinpoint exactly a value of x when there is no overall water in or out, giving you the cell sap concentration.

My particular results that seem to anomalous are;

I don not have any anomalous results.

I consider my results to be accurate because;

I had three readings for each concentration which together gave an average change in length. The three readings for each concentration were very similar, e.g., 0 M sucrose = +2, +3 and +3 and 0.5 M Sucrose = -4, -4 and –4. The fact that these results are so close, means that my experiment was accurate.

Did I collect sufficient evidence to make a conclusion?;

Yes, my experiment gave me an exact value for the concentration of the cell sap (where the line crosses the x axis). I f I hadn’t enough evidence, I would not have been able to find this.

My investigation could be improved by;

I took enough reading and I used the correct range (0.0 – 0.5 M Sucrose instead of 0.4 – 0.5 M Sucrose). I did not have any difficulties in doing the experiment. I kept all the other variables constant, by making sure that each experiment was left for the same period of time. I also used the same amount of bathing solution each time. Therefor my results came form a fair test. However if I did this experiment again, I would use a thermostatically controlled water bath, to keep the temperature constant. I would also weigh the potatoes at the beginning and at the end of each experiment in order to obtain a mass reading as well as a length reading.