Investigation Osmosis in Potato Tissue

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Investigation Osmosis in Potato Tissue

Planning

Scientific Knowledge and Understanding

Osmosis is the movement of water molecules across a partially permeable membrane from a region of high water potential (dilute solution) to a region of low water potential (concentrated solution).

If the water potential is greater inside the cell than outside the cell, then there will be a movement of water out of the cell. If however, the water potential is greater outside the cell than inside the cell, then osmosis will be a movement into the cell. If the water potential on each side of a cell membrane is the same, there should be no movement of water across the membrane. In equilibrium, the individual molecules travel across the membrane. The movements into the cell are balanced by movements out of the cell. This results in there once again being no movement of water at equilibrium.      

A cell is surrounded by a partially permeable membrane, and water may cross this membrane easily. If a cell is placed in a solution of lower water potential, water leaves the cell by osmosis. If the cell is placed in a solution of higher water potential, water enters by osmosis. When water enters a plant cell by osmosis the cytoplasm will swell, until it pushes against the cellulose wall. When water leaves a plant cell by osmosis, the cytoplasm will shrink, however the cellulose wall will continue to give some support.

The stomata consist of two guard cells surrounding a central pore. The stomata open in the light and close in the dark. Each guard cell has a thick inner wall and a thinner outer wall. In the light the guard cells absorb water by osmosis and become turgid. As the cell expands, the thin outer wall bulges outwards and the guard cell curves, opening the pore. In the dark, guard cells lose water by osmosis and become flaccid, this closes the pore.    

     

 

The presence of solutes dissolved in the water lowers the water potential. Water then moves from an area where the solute concentration is lower to an area where the solute concentration is higher. Distilled water is the purest water and water with dissolved salts has lower water potential.


Key Variables

There are three different variables that will affect my experiments; Independent variable, Dependent variable and Controlled Variable.

The Independent Variable is the variable that I have control over; the one that I change. In my experiments, the independent variable is going to be the concentration of the sugar solution.

The Dependant variable is the variable that I measure. In my experiment there are several dependant variables; the mass, the size of the potato – length/diameter, volume of solution, turgidity. For my experiment I am going to measure the change in mass for the potato, the turgidity of the potato and possibly the change in volume of the solution and the change in length and diameter.

The final type variable is the Controlled variable. The controlled variable stays the same all the time. In my experiments the controlled variable will be the temperature, volume of liquid to begin with, the initial length and diameter of the potato and the length of time the potatoes are in the solution.


Predictions

I am using five different concentrations for my experiments.

I predict that for experiment 1- where the concentration of my mixture is 0%, the mass will increase and so will the surface area of the potato cylinders. However, the volume of the solution will decrease and the angle of the potato cylinder to the cork will be 90°. This is because:

 

For experiment 2, concentration 25%, the mass will also increase but not as significantly as it will for experiment 1. The surface area will also increase, but again, the change will not be as significant as in experiment 1. The volume of the solution will decrease but I do not think that it will decrease to a great extent.  The angle will be smaller than in experiment 1 but I do not feel it will curve greatly as the solution still has a higher percentage of water than sucrose solution.

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In experiment 3, there is a concentration of 50%, which means that there is an equal concentration of water and sucrose solution, equilibrium. This means that the mass should not increase, nor should it decrease. The surface area of the potato and the volume of solution should also stay the same as;

             

     


Experiment 4, concentration 75%, has a higher concentration of sucrose solution than water solution; this means that the mass will decrease, as will the surface area of the potato cylinder. However, in this case, the volume ...

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This is a very well written and detailed report. 1. The scientific knowledge section is well researched and contains relevant information. 2. The variables section is good but the dependent variable needs to be clarified. 3. The method section is well written, although the use of two potato sections in one boiling tube may affect results. 4. The conclusion is well explained. 5. The evaluation shows a good understanding of scientific processes. 6. There are no references in this report and some of the information is clearly taken from sources. 7. Begin to give diagrams reference numbers so that they can be referred to. **** (4 stars)