Osmosis in potato cells.

Equipment-

1.  5 x 250cm³ beakers for each concentration of sugar solution.
2.  5 petri dish lids to cover the beakers
3.  5 labels for the beakers
4.  7mm diameter cork borer
5.  Cutting tile- to cut the potatoes on
6.  Scalpel for cutting potato samples
7.  Digital balance- accurate to 0.1g
8.  Different concentrations of sugar solutions- 0%, 8.5%, 25.5%, 34%, 51%
9.  Tissue paper for drying potato samples

10. Beaker to measure sugar solution

Safety-

In general, this experiment was quite safe to carry out, however, careful use of the cork borer and scalpel is required as they are sharp objects. Care is also needed when handling sugar solutions to avoid spillages.

Diagram-

               

Method-

1. I will set up five beakers making sure they are correctly and clearly labelled with the different concentrations of sugar solutions.
2. I will then prepare the potato samples, cutting out pieces of potato using a uniform cross-section cork borer of diameter 7mm.
3. I will cut several pieces of potato of same lengths, using a scalpel, taking care that the pieces are of same length.
4. All the pieces will then be weighed to check their weight.
5. I will then add 100cm³ of the different concentrations of sugar solutions into their labelled beakers.
6. Next I will place four pieces of potato in each beaker.
7. Each beaker will then covered with a petri dish lid, to avoid evaporation of water, which would otherwise change the solute concentration.
8. The beakers will be left for approximately 24 hours.
9. The potato samples will then be removed from the beakers and I will wipe off any excess water, gently, on a dry tissue paper, taking care to dry all pieces equally. This will be done to obtain more accurate weight of the potato pieces.
10. Finally I will re-weigh the samples and measure the lengths of the pieces and record the results.

Fair Test-

In this experiment there was only one factor that was varied:

• The different concentrations of sugar solutions.

Other variables were controlled to enable me to make this a fair test:

1. The diameter and the length of the potato samples. This is because if one of the samples was longer it will have a larger surface area and osmosis will occur quicker.

2. Temperature- the temperature may affect the reliability of the experiment as large variations of temperatures will affect the cell walls of the potato pieces because the cells would dilate differently at different temperatures, thus affecting the surface area. The movement of water molecules will also vary at different temperatures. To prevent this from happening, all the beakers were kept in the same place, at room temperature. Evaporation of water will also be controlled by covering the beakers with petri dishes, thus maintaining the concentration level of the solution.

3. Use the same potato- because there could be a variation in the texture and sugar content between different potatoes.

4. Contamination- the beaker used to measure out the sugar solution should be washed and dried each time, as different concentrations of sugar solution could get mixed up.

There were five different concentrations of sugar solutions:

0%, 8.5%, 25.5%, 34% and 51%.

The results were also taken four times and an average was then taken to provide more accurate results.

Average- to make the experiment as accurate as possible, an average will be taken from the four results from each concentration of sugar solution. Also, any anomalous results will be removed before the averages are made.

Prediction-

Osmosis is the movement of water molecules from a region of higher water concentration to a region of lower water concentration across a semi-permeable membrane.

In a higher concentration of water the amount of solute (sugar in this experiment) is low. This means a weak or dilute solution. In this type of solution there would be a net flow of water molecules into the cell and the cell would swell, and because of the extra water it would also result in an increase in mass. I would expect the potato pieces to shrink and loose mass when submerged in lower concentration of water, because in this case there would be net movement of water molecules out of the cell. In between, I expect to work out a concentration level where there is little or no movement of water molecules in either direction across the cell membrane of the potato cell.

From my research an average level of sugar concentration in a potato is approximately 10%. However, this is likely to vary from potato to potato.

I have based my range of solution around this 10% - 0%, 8.5%, 25.5%, 34% and 51% and I anticipate the solution to be isotonic at 8.5% concentration hypotonic at 0% and hypertonic at 25%, 34% and 51%.

Section O (Obtaining Evidence)

Experimental results: (Effect of various levels of sugar concentrations on the length of the potato pieces)

Table 1

Experimental results: (Effect of various levels of sugar concentrations on the mass of the potato pieces)

Table 2

Section A (Analysing and considering evidence)

From my results on the four pieces of equal diameter and lengths of a potato, I calculated the average change in length and mass and worked out % changes in length and mass using the given formula:

                         

 

x is the initial value of length or mass

y is the final average value of the length or mass.

This formula gives an increase in length or mass as a negative % change and a decrease in length as a positive % change.

Two graphs of % solute concentration against % change in length and mass were then drawn.

My graph has a negative % change for 0% solute concentration and a positive % change for higher solute concentrations. The point at which the graph crosses the x-axis is the isotonic point. At this point there is no net flow in either direction of the cell membrane. This is the point at which sugar concentration in the potato is the same as the % sugar concentration of the solution. The reading on the x-axis gives the % sugar level in the potato.

From graph 1, graph showing % solute concentration against % change in length, the % sugar in my potato is approximately 6% and from graph 2, graph showing % solute concentration against % change in mass, the % sugar in my potato is approximately 7%

I have drawn a line of best fit for readings of 0%, 25.5% and 34% because these points are almost on a straight line. The gradient after the 34% solute concentrations is much slower representing small % changes at higher solute concentrates.

Section E (Evaluation)

Having taken steps to make this a fair test I think my results were relatively accurate. I have obtained very close figures for sugar content in the potato from both the graphs.

The following should be considered to improve this experiment:

1. Human error- errors made in reading the correct lengths and mass and obtaining correct % solute concentrations etc.

2. Care should be taken not to allow the potato pieces to dry out excessively or gain moisture from the surrounding.

3. Use a digital balance accurate to 0.01g