Investigating the Water Potential Of Potato Cells

We obtained theses solutions by using a syringe. When we had the correct amounts, we put each solution into a separate test tube and added one potato tubes to each of them, (prepared earlier) making sure that we knew which potato tube weighed which amount.

5. We labelled each of the test tubes according to the solute concentration in them and then we left them over night to allow time for osmosis to occur.
6. On my return the following day, I reweighed each potato sample/tube, re-measured them and recorded my findings.

Diagram-

 Fair testing-

In order to obtain reliable results we had to do several things in order to ensure that the test was fair. Firstly as a group we took all our potato samples from the same potato, in order to ensure that all the potato samples had the same water potential/ solute potential. Secondly we had to use the same amount of solution each time; hence we choose to use a 10mililitre solution for each test tube. Thirdly we used the same temperature of water to make each solution as a change in temperature could cause acceleration in osmosis. Fourthly we kept all the test tubes in the same room during storage, in order to keep the environment the same for each test tube. Finally we made are own concentrations of solutions in order to make sure that the concentrations were accurately measured.

Safety-

1. Wear safety specs at all times, as the scalpel could break during cutting and the blade could come into contact with your eyes.
2. Take care when handling the scalpel, as it is sharp and could cause bodily harm.
3. Make sure that bags etc… aren’t in the walkways between work areas as someone could fall over one of them.
4.  Make sure that surfaces are clean prior to use as they could have dangerous chemicals on them from prior experiments with other classes.
5. Make sure that any glass instruments are handled with care, as you don’t want them to smash.

Results

The following results were obtained by recording the change in length and mass and then using the following formula in order to work out the %change for each (length +mass):

Change in length

Original length

The above results are a collection of all our classes

Results, used in order to get a more reliable average

for changes in mass and length.  

The below graph show has used the averages in mass from the above table.

Conclusion

From the graph I am able to work out the sol potential of the potato. I can do this by following two steps.(1). Firstly I can work out the concentration of sucrose in the potato by reading of my graph were the %change in mass is 0, as this shows that at this point there is no net movement of water hence the two areas have equal solute potentials. (In this case the morality of the potato is 0.14m.) (2). I can then use this to work out the solute potential of the potato by using the below graph, which was made from information from data sheets, showing the solute potential at different moralities.

This shows that at 1.4m the solute potential is –280kPa. Therefore I know that the average solute potential of the potato’s used in class is about –280kPa.

What the %change in mass of the potato tubes graph shows us-

The graph shows that as you increase the sucrose concentration of the solution surrounding a potato,(hence making the solution Hypotonic)that the net movement of water out of the cell increase. This is represented by the downwards-sloping curve of the graph and is due to the increased solute potential of the surroundings meaning that there is more solute in the solution; hence more solutes form bonds with water molecules meaning that the water molecules are no longer free to move or they move more slowly. As a result of this the water enters the solute solution faster than water leaves the solute i.e. there is a net water movement out of the cell through the (permeable) membrane .

This is not true however before 0.14m concentration of solution as the solute concentration is greater in the cell  and therefore water enters the cell by a process of osmosis.

At a concentration of .75m of sucrose there is an anomalous point. This could be due to several reasons as shown below:

• The test tubes could have been stored in a way that the 0.75m of was stored at a hot temperature than the other solutions for example side ways on to a radiator. Hence the atoms got excited and therefore moved round more causing an increased rate of diffusion.
• Other groups could have made errors in their production of the different concentrations of sucrose hence the average was affected.
• Different measuring equipment could have been used for each group hence some groups came up with inaccurate result, which again could have affected the average.
• Different groups could have used different amounts of solutions; hence diffusion is either decreased or increased according to how much solution was used.

In the solutions 0.5m 0.25m and 1m the amount of water particles in the surroundings is less than that inside the cell, meaning that there is a negative concentration gradient from the cell to the surroundings, hence water moves out of the cell. The reason for the curve is due to the decreasing concentration of water in the surroundings as you increase the sucrose concentration of the solution. Therefore there is a greater negative gradient with higher concentrations of sucrose causing water to diffuse faster through a process of osmosis as you increase the concentration of the surroundings. This is why the % mass change for 1.0m of sucrose solution is greater than that of 0.5m of solution. The reason it slopes at the end of the curve is because of increasing time meaning difference between water potential is less.

Evaluation.

There were several factors that could have affected my results that were out of my groups’ control. These include the following:

1. Changes in the temperature and pressure of the surrounding, as this would mean that the water molecules would posses more energy, move faster and therefore diffuse fast (by a process of osmosis). As a result more water particles may diffuse in the time set.
2. The availability of measuring equipment, meaning that there was a limit to how accurate we could record our results, as the equipment wasn’t accurate enough.
3. The stock concentration of the sucrose may not have been accurate and therefore variation in different batches used may alter the amount of diffusion that take place
4. Different groups used different potato’s hence each groups potato had a different water potential hence the amount of diffusion may have been different for each group.
5. As there was no distilled tap water made readily available, tap water was used. Tap water is fortified with different minerals hence the water potential is altered and therefore the solute potential of the water is slightly higher than that of pure water causing a decrease in the amount of osmosis in to the cell.
6. Time allowed was also a factor that affected my test as we were only allowed limited time to let osmosis occur hence some solutions may not have fully diffused hence the solute concentration in the potato and out of the cell remain different as a result the potato may have weighed less than expected, due to not all of the water having time to diffuse properly. (this may be a reason for the anomalous point at 0.75m of solution).

Although I have shown that there are various factors that can affect the investigation, I feel that the overall experiment was successful as it showed the overall trends that I would have expected, showing that amount of osmosis differs with different moles of sucrose solution, pending on the concentration gradient between the two sites of diffusion.