Osmotic pressure and water potential
Raw data
Results / Data
We put samples of apple and potato tissue in 6 different sucrose solutions of different molarities; we measured the change in mass. We were waiting for half an hour and observing what happened with them. We put our results in a graph and a table, but they do not seem to be accurate.
Observations:
In some concentrations the apple and potato pieces were floating (between 0,6 and 0,8). That is because they lost a lot of their water – the concentrations were too concentrated for them.
The equilibrium point (where the concentration of sucrose did not cause change of mass) for apple was approx. between 0,4 and 0,6
concentration values. For potato
the equilibrium point was approx.
between 0,2 and 0,4 concentration values.
Where are the errors?
But our results have shown that we have done something wrong. Usually two results were accurate enough, but the third one was wrong. That may happen because we did not cut or measure the pieces right. The concentrations may also not be as they should be. The measurements also depend on types ...
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The equilibrium point (where the concentration of sucrose did not cause change of mass) for apple was approx. between 0,4 and 0,6
concentration values. For potato
the equilibrium point was approx.
between 0,2 and 0,4 concentration values.
Where are the errors?
But our results have shown that we have done something wrong. Usually two results were accurate enough, but the third one was wrong. That may happen because we did not cut or measure the pieces right. The concentrations may also not be as they should be. The measurements also depend on types of apples, because we did not use the same apple all the time.
Graph 2. Approx. change of potato mass
Table 1. The average mass and length values (B/A = before/after)
Explanation and conclusion:
Average masses and lengths may not be accurate enough because we did not get accurate results in our measurements. The mass was supposed to be lower at a higher concentration, but it is not always like that. We cannot be sure about these results and we would maybe have to repeat this experiment and take much more representative sample.
The results were supposed to show that the mass gets lower at a higher concentration of solution and it grows at the value 0,0. They do not confirm the theory.
According to theory, osmotic pressure can be regarded as the tendency of solution to gain water across an ideal partially permeable membrane. The osmotic pressure depends on its solute concentration. It follows that a solution with a high osmotic pressure has a low water potential. In our case, the water potential is the highest at 0% concentrated solution.
Plant cells generally have water potential which is lower than of their surroundings. Their plasma membrane is partially permeable, letting the water in, but not solutes. The cell wall is, however, fully permeable to both water and solution.
When such a cell is put into the pure water or a solution whose water potential is bigger than inside the vacuole, the cell swells, but it does not burst.
When a cell is put into a solution whose solute concentration is higher that that of the cell sap, water leaves vacuole by osmosis and the cell shrinks. So, we did not support the theory with our experiment.
However, the graph for change in apple mass was approximately supposed to look like this:
Testing the presence of starch and sugar in potato and apple tissue
We can test for the presence of these important compounds in food by using chemical reagents that react in predictable ways in the presence of these nutrients.
Test 1 Sugar test-Benedict's solution:
Benedict's solution is used to test for simple sugars, such as glucose. It is a clear blue solution of sodium and copper salts. In the presence of simple sugars, the blue solution changes color to green, yellow, and brick red, depending on the amount of sugar.
When we put benedict’s solution into small amount of apple with distilled water and heated the test tubes in hot water it turned out that there is a lot of sugar in the apple. The color changed from orange to very dark orange.
When we put benedict’s solution into small amount of potato with distilled water and heated the test tube in hot water it turned out that there is not so much sugar in it. The color changed from orange to green-yellow.
We can conclude, that benedict reduces sugar and causes a change of color.
Test 2 (determining starch) Iodine:
Iodine solution is used to identify the presence of starch. The solution is yellow-brown but, when it reacts chemically with starch, a blue-black substance called iodide starch is produced.
When we put few drops of iodine solution in each test tube and in apple tissue it turned out to be less starch than in potato. That is because apple has pectin and not starch. The color of apple changed from orange to brown, and in potato it changed from yellow to blue-black.
by Marija Galinec IB1