TO FIND THE WATER POTENTIAL OF PLANT TISSUE CELLS.

APPARATUS.

• sucrose solution
• 3 large potatoes of same age and size to make it a fair test.
• Distilled water -Pure with no solutes
• 6 boiling tubes
• 2 syringes-For accurate dilution measurements
• Cork borer-To cut same diameter of potato
• Ruler (15cm) - To the length of the potato before and after the experiment.
• Top pan balance-to take accurate mass of potato
• Scalpel- To remove potato cover
• Tile
• Petri dish-To avoid water from evaporating
• A roll of tissue –to dry the potato tissue

RISK ASESSMENT

• It important to wear goggles whenever one is carrying out an experiment.
• Handle the scalpel with care as it might cause serious injuries.
• Throw away the left over potatoes for health reasons.
• Put boiling tubes in racks to avoid accidental breakages.

HOW CAN YOU MAKE IT A FAIR TEST?

To make this a fair test, the following points must be put into consideration:

• Use same size cork borer to cut all six potatoes.
• Put potatoes in sucrose solution at the same time and for the same period of time.
• Use same species and age of potato.
•  Equal volumes of sucrose solutions should be used all the time.

METHOD

1. Prepare a table in which to record the results.
2. Label 6 test tubes, one for each of the following: distilled water (0M), 0.1M, 0.25M, 0.5M, 0.75M and 1.0M.
3. Use distilled water, molar sucrose solution and a syringe to make 20cubic cm (you are advised to make more than 20cubic cm) of each dilution in a beaker. Record in a table how sucrose and water you’ve used in every solution.
4. Mix the solution fully by using a stirring rod or just by shaking.
5. Pour the solutions into the appropriate test tube, arrange the test tubes in a systematic order and put them on the rack.
6. Carefully use the scalpel to remove the outer skin of the potato as it acts as a barrier.
7. Use a cork borer to cut 6 pieces of potato and a ruler to measure about 3cm in length of each potato piece.
8. Gently blot any excess fluid from each the potato before weighing each potato individually, if weighed with excess water the mass of the potato will be affected. Record this as initial mass.
9. To obtain accurate and reliable results, immerse the 6 potatoes in their respective sucrose solutions for 24hours.
10. After 24hrs remove the potatoes from the test tubes, blot any excess fluid from the out side and reweigh them. Notice the changes and record the new mass as final mass.
11. Plot a graph of average percentage change in mass against morality of sucrose.
12. Read off the molarity of sucrose which causes no change in mass.
13. From the already plotted graph of osmotic pressure against molarity of sucrose solution, determine the osmotic pressure of the solution that caused no change in mass, this shows the water potential of the cells in the tissue.

ANALYSIS

Your table of results should like the one below.

Table showing results obtained.

From the results on the graph that the mass of the potato decreases as the sucrose molarities increase and this shows osmosis takes place. Compare for example the potato tissue that was put in 0.1M sucrose solution and one that was put in 0.75M concentration, you can tell by their final masses that the tissue that was put in 0.1M sucrose concentration had gained water whilst the one in 0.7M concentration had lost water. This is because in 0.1M  there is higher water and so water moves through the plasma membrane into the cell where there is lower water potential  causing the increase in size and mass of the tissue. In the 0.75M sucrose concentration, the water potential is lower than in the tissue and as a result the tissue losses water to the solution leading to decrease of mass referred to as plasmolysis.

EVALUATION

Some results are not on the straight line and these are called anonymous results, they may come up as a result of inaccurate measurements of dilution due to inaccurate apparatus e.g. using a measuring cylinder instead of a syringe, insufficient blotting, the regions of the potato used and how long the potato tissue was left in the solution. To improve the results, the potato tissues should be carefully dried before and after immersing them in the solutions, this is to avoid  affecting the mass .The mass could also be affected if the whole of the potato cell is not  immersed in water  as part of the cell will not be in the solution. The potato should be left in the solution for long enough to the whole osmosis process to take place.

However the results do not support the prediction made earlier as to what would happen to the cell when put in an equally concentrated sucrose solution i.e.  0. 5M and this could possibly be because different specie of potato was used due to limited number of the original specie. This could be improved by preparing enough potatoes of the same test.