water potential of potato tuber cells

EXPERIMENT 2:

METHOD

1. Put 3 ml of 0.1, 0.15, 0.2, 0.25, 0.3, 0.5, 1 mol dm3 sucrose  in different test tubes and label them.
2. Make potato discs as in experiment 1.
3. Put the discs in the solutions and leave them for 30 min.
4. Put 2 ml of same solutions again in test tubes and put 3 drops of blue ink in them and label them.
5. After 30 min remove the potato discs from solutions.
6. With a syringe take some coloured solution (e.g. 0.1 mol dm3) and carefully put one drop of it in a middle of a same solution of 3 ml. Repeat it for all solutions.
7. If the coloured drop sinks potatoes have lost water so solution has lower water potential than potato. If the drop rises potatoes are gaining water so solution has higher water potential than potato. If the drop doesn’t rise of sink solution has the same potential as potato.

JUSTIFICATION OF THE APPARATUS:

Sucrose solution is used because it is easily made and available. Potato is rigid, cheap and it is easy to detect any changes in its mass.

Digital balance has smaller uncertainty than analog ones so it is better to use.

LIST OF VARIABLES AND THEIR CONTROL:

• Time: all times are used accurately and measured with stop watch
• Potato size: accurate measurements
• Volume of solutions: accurate measurements
• Ph:  use distilled water
• Same potato: different potato may have different water potentials

RELIABILITY OF VARIABLES:

Same potato has to be used because different potato may have different water potential so results wouldn’t be reliable. Tap water is ionised so water potential can’t be 0 so distilled water is   used because it doesn’t have any ions in it. Also Ph of distilled water is neutral so there is no change in permeability of membrane.

SAFETY:

• Carefully use cork borer and scalpel to prevent injuries by cutting
• Watch out that you don’t break any glass apparatus to prevent cutting

RESULTS:

EXPERIMENT 2

ANALYSIS:

When potato was immersed in solution it gained or lost water. Sinking in this table means that potato gained water so coloured solution had bigger density and it fell on the bottom of test tube when it was immersed in the solution no1. Rising in chart means that coloured solution has smaller density than solution no1.

Anomalies are visible in results. Error occurred between 0.1 and 0.25 mol dm3 Solution 0.15mol dm3 can’t have bigger density than solution 0.2 mol dm3 and from experiment number 1 is visible that potato water potential is somewhere between water potentials of 0.2 and 0.3 mol dm3 sucrose solutions. Error is probably in the 0.15 mol dm3 solution.  

EXPERIMENT 1

 MASSES BEFORE IMMERSED INTO SOLUTION

MASSES AFTER IMMERSED INTO SOLUTION

SUMMARY TABLE

ANALYSIS:

Graph of potato mass against sucrose solution shows that there is constant decrease in mass along the x-axis. When concentration of solution grows mass of potato decreases. When concentration of sucrose decreases  potato gains water. That is visible between 0 and 0.2 mol dm3 Then somewhere between 0.2 and 0.3 mol dm3 potato starts to loose water, which is visible between 0.3 and 0.5 mol dm3 It is visible from the graph that water potential of potato is between–540kPa and –860kPa (water potentials of 0.2 and 0.3 mol dm3 sucrose solutions).

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CONCLUSION:

 My conclusion based on results of both experiment is that water potential of potato tuber cells is between –540kPa and –860kPa.

EVALUATION:

According to the graph result shows that experiment was successful because all results could be reliable. Parts of results from experiment 2 are little out of order but experiment 1 clearly shows in what range could water potential of potato be. Experiment 1 is more reliable because electronic gear was used which has less uncertainty than human eye and skill in experiment 2. Errors appeared because of human errors.

• There was problem to cut same cylinders of potato because any difference in length can affect the result.
• Making appropriate solutions was easy but there is also some kind of uncertainty in them because we are never sure that exactly correct solution is made.
• There is potential error in weighting the potato after it stayed in solutions for 24 hours. If the potato hasn’t been dried when it was weighted, mass of water also affects the result because masses are small and measured in grams with numbers smaller than 0.
• The trickiest part of experiment 2 was to put the drop of coloured solution in other without dispersing it. In my attempts to put it in the solution I had little success. I squeezed the syringe too hard and drop just popped out of it causing the current in the solution so I couldn’t see if colour was sinking or rising. That’s why experiment 2 hasn’t been successful.

The best reliability for experiment would be to repeat it and to take average of solutions. For experiment 2, repetition is also needed so I would improve my skill of immersing a drop into other solution without dispersing it.