Determination of Water PotentialOf Potato Tissue

EHSEN KHOKHER

Determination of Water Potential

Of Potato Tissue

Introduction:

Osmosis: is the movement of water through a semi permeable membrane from a high water concentration to low concentration.

Osmosis is movement of water, which takes place through the cell membrane that separates the cell contents from the surroundings. Cell membranes allow some substances to pass through and stop other substances from passing: they are partially permeable. In general, substances pass through a membrane if their particles are smaller than the pores in the membrane.

When a partially permeable membrane separates two solutions, water passes through the membrane in both directions. The net flow of water is from the more dilute solution to the more concentrated solution. The flow continues until the two concentrations are equal. The flow of water from a more dilute solution to a more concentrated solution is called osmosis (see diagram). Partially permeable membrane

Concentrated

Sugar Solution Dilute Sugar

Solution

Sugar Molecule

Water Molecule

Water potential is how likely the water is to leave the solution. High water potential is when the water is highly likely to leave. Low water potential is when the water is less likely to leave.

The water potential for water potential is arbitrarily given value 0.

Solute Potential of cell contents (ψs) is only slightly affected by dilution as water enters the cell.

Pressure potential (ψp) is 0 until the protoplast begins to exert pressure on the cellulose cell wall.

If a plant tissue was placed in a hypertonic solution, the plant tissue will increase in size. This happens because this is more concentrated solution, which means that the water will leave the high concentration water and enter the plant tissue, which is low concentrated. This becomes a full plasmolysis.

If you also place the plant tissue in a hypotonic solution, the plant tissue will decrease because there is more water in the plant tissue than the low ...

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Pressure potential (ψp) is 0 until the protoplast begins to exert pressure on the cellulose cell wall.

If you also place the plant tissue in a hypotonic solution, the plant tissue will decrease because there is more water in the plant tissue than the low concentrated water. So the water leaves the tissue into the solution. The tissue becomes turgid.

Then the plant tissue can be placed in an isotonic solution. The length of the tissue will stay the same because they both have the same concentration. This will result in incipient plasmolysis.

Our aim is to find the water potential of the potato tissue by emersing it into different sugar solutions.

The solution where no change in length will happen will be used to find water potential. The water potential can be determined using table 2.

Apparatus:

● 6 similar beakers

● 6 petridishes

● cork borer

● knife

● ruler

● potatoes

● 1M Sucrose Solution

● distilled water

Method:

•First label six similar beakers to make different sugar solutions

•After that put 10cm³ of sugar solution into a beaker

•Then put 90cm³ of water into the solution

•After that use the table of sugar solutions to make the rest of the solutions

•After making the solutions take six petridishes

•Label them with the concentration of sugar solution in each one

•Then cut out 18 potato chips using a cork borer and a knife

•Measure each potato chip 6cm to the nearest millimetre

•Then put 3 chips in each petridish and pour 40cm³ of the solution into each petridish as it says on the label; this is because this is the amount of solution the petridish can take

•Leave potato chips overnight for osmosis to take place.

•Measure potato chips the next day to see which ones increased in length and which decreased or stayed the same.

Table of Sugar Solutions

Results:

Table 1 showing Mean Change

Table 2 Showing Water Potential

Conclusion:

The mean % change in length of potato tissues increased by 2.22% in the 0.1M sucrose solution. This is because the water concentration of the sucrose solution was higher than that of the potato tissue. Water moved from the more concentrated water to the less concentrated potato tissues resulting in full turgor tissues with greater length.

The average % of change in length in the sucrose solution of 0.2M was 1.11%. This was because the water concentration in this solution was more than the one in the potato tissue. This meant that the potato tissue expanded resulting in turgid tissues.

Mean % of change in length, in the 0.3M sucrose solution was -0.56%. This happened because the solution was less concentrated than the potato tissue. The water has moved from the more concentrated solution, the potato tissue, to the less concentrated solution, the sucrose solution resulting nearly in incipient plasmolysis.

The Average % change in length of potato tissue decreased by –1.22% in 0.5M sucrose solution. This happened because the water concentration of the potato tissue was higher than that of the sucrose solution. This resulted in the plasmolysis.

The mean % of change in length, in the 0.75M sucrose solution was –9.45% in the potato tissue. This was because the water concentration of the potato tissue was much more than the sucrose solution. This meant full plasmolysis.

The mean change in length of the potato tissue in the 1M sucrose solution was –9.45%. This was because the sucrose solution was less concentrated than the potato tissue meaning that the water travelled to the sucrose solution. This resulted in full plasmolysis of the potato tissue.

So to summarise this the basic trend was that if it was soaked in a high concentration of sucrose, the potato tissue tended to decrease in length and if it was soaked in a low concentration, the potato tissue increased in length.

The graph shows that the sucrose concentration that will neither make the potato tissue shrink nor increase will be 0.30M.

Therefore by using the water potential graph, the water potential equals to 0.86 kPa.

Evaluation:

The experiment was easy to do, but all the results I had to take had to be accurate and had to change my plan several times. Things like having the potato chip 5cm long changed to 6cm because it was hard to have all the chips 5cm long, and 6cm was sufficient. Also, I changed the amount of solution I would keep the potato chips in because I only needed enough to cover the potato chip. These changes were easy to make, because I did a preliminary experiment, so I could iron out any errors that may have occurred in my actual experiment.

I am fairly confident about my results because they did lie close to the curve, and therefore were fairly accurate except one or two, but if I was doing the experiment again I could make changes. I would do this experiment at least two times just and take the average results. I could weigh each chip on a digital and more accurate scale, e.g. not to 0g but to 0.00g. This is to get more accurate results.

There was one anomalous result, which was the 0.75M sucrose concentration; this was anomalous because it was way off from the line. Looking at my graph it should have been –0.6 or –0.7%. This may have been caused by human error, which was the making of the sucrose concentrations and measuring the potato chips, or one out of my six results could have been more inaccurate, and changed the average drastically. Or perhaps the potato chip was not cut accurately, or that part of the potatoes, cells did not loose/gain mass well. One or two of my results did vary a lot for the same concentration. The fact that a certain part of the potato may not be the same as another was shown as the chips all of the same length but we did not weigh them so they might not have been the same weight, or even close.

I could extend my enquiry by testing the percentage change in mass with molarity using more concentrations and different kinds of potatoes. By this I mean using potatoes of different seasons, perhaps different types of potatoes. Then I could find out whether osmosis occurs with the same patterns and trends with any potato.