Investigate the water potential of potato.

Isabel Murtough Biology coursework MR

Aim: to investigate the water potential of potato.

Research:

Osmosis in plant cells

Osmosis is the net movement of water molecules from a region of their higher concentration to a region of their lower concentration through a partially permeable membrane.

If the concentration of water molecules is the same in the plant cell and the surrounding liquid, there will be no net movement of water into or out of the cell; and if the concentration of water molecules is greater in the cell than the surrounding liquid, there will be a net movement of water from the plant cell to the surrounding solution. Osmosis will occur whenever a partially permeable membrane separates two solutions containing different concentrations of water molecules. If the cell is surrounded by pure water, or by a solution whose solute concentration is lower, and water potential higher, than that of the cell’s contents, water flows into the cell by osmosis and swells up. The external solution is said to be hypotonic to the solution in the cell. This means the solute concentration is lower than that of the cell. If the cell is surrounded by a solution whose solute concentration is higher and water potential lower than that of the cell, then water will flow out of the cell causing the cell to shrink. When the external solution is of higher concentration it is said to be hypertonic to the solution in the cell. If the cell has the same solute concentration as the surrounding solution there will be no net flow of water in or out of the cell, in this instance the external solution is said to be isotonic.

Mary Jones and Geoff Jones Biology

The effect of osmosis on plant cells

Plant cells generally have a lower water potential than that of their surroundings, this is because of their high concentration of solutes in their vacuole. The plasma membrane and the tonoplast are both partially permeable, letting water through but not solutes. The cell wall however is fully permeable to both water and solutes. When a plant cell is put into a pure water solution whose water potential is higher than that of the cell, the net movement of water is into the cell causing it to swell. However, due to the cell wall it does not burst causing hydrostatic pressure to develop. This is called pressure potential which opposes the continued uptake of water into the cell through osmosis. The pressure potential reaches a maximum and the cell becomes fully turgid. Biology third edition, Campell. If the cell is out into a solution with a lower water potential, the net movement will be from the cell to the surrounding solution. This would cause the cell to collapse, when the cytoplasm withdrawals from the cell wall plasmolysis occurs.

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Results of an experiment to determine the water potential of potato tuber cells. Samples of potato tissue were placed in a series of sucrose solutions of different molarities, and the change in mass was measured. As you can see from the graph, the concentration of sucrose causing no change in mass has a molarity of 0.27M.

Water potential

The net direction of water movement in a cell depends on the water potential of the cell and whether it is more negative or positive than the water potential of the surrounding solution.

C.J Clegg

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Water potential

The net direction of water movement in a cell depends on the water potential of the cell and whether it is more negative or positive than the water potential of the surrounding solution.

C.J Clegg

Prediction:

I predict that the when the potato tubers are placed in solutions with different molarities their mass will change accordingly. I think that the potato samples will gain water and mass when placed in solutions with higher water potential; and will lose mass and water when in a solution of lower water potential. My aim is to find the water potential of the potato tuber, I will be able to obtain the water potential of the potato when there is no net movement of water in or out of the potato. Finally the cells will not reach equilibrium (become iso-osmotic). This is because plants have a cellulose cell wall which cannot expand beyond a certain point as it is turgid, therefore the contents will never be able to be equal to the outside or it will not be able to give out all its water as it has a rigid structure. It is the pressure created by the cell wall that stops the cell reaching equilibrium.

Method:

I will firstly take 18 test tubes and label each one with the corresponding solution that will be placed in them (18 will be needed so I can test each solution three times); 0M, 0.1M, 0.2M, 0.3M, 0.4M and 0.5M and place the tubes in a rack. I will then make up the six different solutions by mixing distilled water with a certain amount of sucrose solution:

Then after the solutions have be prepared I will put them into the relevant test tube, I will prepare each solution one at a time as to not get them mixed up. Once the solutions are in the test tubes I will seal the top with a bung as to prevent any evaporation of the liquid solution. I will allow the solutions to stand in the tubes for about 10 minutes so they can settle to room temperature. The potato samples will be the next thing to prepare. I will chose a hard and healthy potato for my experiment, the potato will be peeled so the cuttings will remain as fair as possible without any hard skin. I will use a corer to take samples from the potato and cut each piece to a length of 2cm using a scalpel and a white tile, along with an accurate ruler. A corer will allow me to take pieces of potato with the same diameter. I will cut 18 different pieces of potato so I can use 3 chips for each solution. This part of the preparation must be done very accurately as a change in the surface area may allow more or less osmosis to occur. I will then use an electric balance to weigh the potato tubers, once a tuber has been weighed the mass will be recorded as well as the test tube it has been placed into. The potato is going to be left in solution for 24 hours to allow the processes to occur and to allow the water movement to settle. After 24 hours I will return to the experiment and take each piece of potato out of the test tube with tweezers, dry them off by dabbing them with tissue on by one and weighing them on the electric balance and record their final mass.

Apparatus:

18 test tubes and bungs
Test tube rack
White tile
Scalpel
Potato corer
Thermometer
Tissue paper
Electronic balance
Sucrose solutions
Distilled water
Ruler
Measuring cylinder
Tweezers

Variables:

To create a fair test certain aspects of the experiment will have to be kept the same whilst one key variable will change. I have chosen to vary the concentration of the sucrose solution. This will give me a varied set of results from which I hope to draw a firm conclusion. If any of the non-variables are not kept constant it would mean an unfair test. If one of the potato tubers were either larger or smaller than another, the difference in surface areas would mean more or less osmosis occurring.

In my experiment I am going to keep the temperatures the same at room temperature. If different temperatures were used it would mean that osmosis would occur more rapidly in some test tubes than others, leading to an unfair test.
To keep the initial water potential of the potato, I will use tubers from the same potato and make sure they are all prepared and measured in the same way.
The mass of the potato is the dependent variable which means it will be measured throughout the experiment. I will measure the mass in grams. The potato tuber will be measured before and after it has been in the solution; this will allow me to see whether osmosis has taken place and to what extent.
I am going to use the same balance to weigh the tubers; this is because the measurements may vary slightly between different scales.
The volume of the solution that the potato tubers are kept in must be fair. The solution must cover all of the potato in order for osmosis to occur over the whole of the tuber.

Safety considerations:

The experiment involves the use of a scalpel which is very sharp and potentially dangerous if used carelessly. To avoid any accidents the blade should never be pointing directly at anybody and should be placed in a safe area when not being used.
Hair should be tied back to avoid being caught in any apparatus.
Lab coats need to be worn, even though the solution is harmless lab coats prevent the sticky solution from getting on clothes.
Apparatus should always be handled appropriately especially when there are glass instruments around

RESULTS

The experiment has yielded a graph, (see attached) this graph gives me a good set of data. The graph shows that in the distilled water the potato gains in mass of approximately 0.195g then it gains 0g in the 0.25M solutions. This concludes the water potential for the potato.

The first observation I made was after the potato cylinders were placed in their solutions I could see a difference. The ones in the 0.0M and 0.25M solutions were floating and the potatoes in the 0.5M, 0.75M and 1.0M solutions were at the bottom of the test tube, this lead me to drawing my next conclusion.

The graph shows that the potato in the 0.0M solution and in the 0.25M is hyper osmotic, as I said in my prediction, this means that there is a higher water potential in the distilled water and 0.25M solution than in the potato, which has a high concentration of solutes. Therefore this is why the water diffuses by osmosis down the concentration gradient from high concentration to low concentration, resulting in the potato gaining mass.

The opposite occurs in the solutions where the molarity is higher 0.5M, 0.75M and 1.0M, the potato in these is hypo-osmotic because there is a higher concentration of water inside the potato than in the solutions, therefore the water diffused out of the potatoes by osmosis, down the concentration gradient and into the solutions outside, this resulted in the loss of mass.

Another important fact is that the graph makes a shape that will result in the potato not being able to take in any more water or lose any more. Like I said in my prediction the cell wall causes the pressure that prevents this from happening, therefore the cell will never become iso-osmotic.

On my graph I have marked the point where the graph line crosses the place on the axis where the potato neither loses mass nor gains mass. This happens at 0.25M; therefore the contents of the potato cells in molar strength are 0.25M.

I think that the experiment went very well, there was only one result that really stands out which is the mass for 0.30 molar concentration. This anomalous result could be explained by numerous errors, for example the balance may not have been very precise; water from previous potatoes may still have been on the balance. Also it may have been measured in accurately from the other cuttings of potato. The precision of measuring the concentration of the solution may also have changed for this result. However there were a few areas where there could be improvement. Firstly, when I dried off the excess water on the potato cylinders after the experiment and before I weighed them, I used a paper towel. This might have either taken some water out of the potato or it might of left some excess water on the potato. This part of the experiment is difficult to come up with an accurate and fair method, as other ways would also lead to some slight mistakes.

Also the potato itself was not from the same potato and was not exactly the same size, although I did try to cut them to 1.5cm each, this could have effected the amount of water gained or lost.

Finally, I could extend the experiment to a more exact level by looking at the potato cylinders under a microscope, then I would be able to see the cells in greater detail and draw some more observational results.