Investigate what factors would have most effect on osmosis in plant cells.

OSMOSIS INVESTIGATION

Introduction

Osmosis is the diffusion of water molecules across a partially permeable membrane, from a region of high concentration to a region of low concentration. The partially permeable membrane contains a series of small holes, allowing only water molecules to pass through, as shown in the diagram below. The glucose molecules (represented by red circles) are too big to fit through the membrane. As there are a greater number of water molecules (represented by blue circles) on the left side, there is a steady net flow into the right side with fewer water molecules, i.e. into the stronger solution.

Water moves into and out of plant cells by osmosis, depending on the concentration of the surrounding solutions. When water moves into a plant cell, the vacuole increases in size, pushing the cell membrane against the cell wall. The cell wall makes sure that too much water doesn’t enter, which would cause the cell to burst. The cell becomes turgid or firm when the cell membrane pushes against the cell wall. It gives the cell support and keeps the plant upright. Plant cells which do not receive enough water cannot stay turgid and so wilting occurs. Cells which are not turgid are described as flaccid. If a plant cell loses too much water by osmosis, plasmolysis occurs, and plasmolysed cells are unlikely to survive.

Aim of investigation

In my investigation, I aim to investigate what factors would have most effect on osmosis in plant cells. There are several factors which could have an impact on osmosis in plant cells, which include:

Surface area of the plant

Sucrose concentration of the water
Age of the plant
Type or variety of plant
Atmospheric temperature
Mass of the plant

From these options, the option I will be investigating is the sucrose concentration of the water. I can have full control over this factor. My aim is to investigate the effect of varying concentration of a certain sugar solution on the amount of osmotic activity between the solution and a potato plug of a given size.

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Prediction

I predict that the higher the water potential of the water outside the plant, the quicker osmosis will take place. This is because water flows in from a higher concentration of water molecules to a lower concentration of water molecules in the cell sap of plant cells, and hence the mass of the plant will increase.

Apparatus

Borer
Potato
White tile
Scalpel
Ruler
Syringe
Scales
Measuring cylinder
Paper towels
6 test tubes
6 wooden corks
Sucrose solution
Distilled water

Method

Use borer to cut 6 potato plugs from the same potato. ...

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Prediction

Apparatus

Borer
Potato
White tile
Scalpel
Ruler
Syringe
Scales
Measuring cylinder
Paper towels
6 test tubes
6 wooden corks
Sucrose solution
Distilled water

Method

Use borer to cut 6 potato plugs from the same potato. Make sure potato is on the white tile.
Using the ruler and scalpel, cut each potato plug to 2cm in length. Name the plugs 1, 2, 3, 4, 5 and 6.
Dry the potato plugs using a paper towel and then weigh each one. Record each plug’s mass.
Name each of the 6 test tubes 1, 2, 3, 4, 5 and 6. Put the corresponding potato plug beside each.
Measure the correct concentration of sucrose solutions for each of the six test tubes with the measuring cylinder and the syringe, using the table below.

This will ensure that 20cm³ of solution will be used for each. Pour the solutions into the corresponding test tube, i.e. solution 1 into test tube 1, etc.
Place each plug into the corresponding test tube, and try to do so at the same time. Leave for the desired period of time (in my experiment’s case, one week).

Put a wooden cork on the top of each test tube to prevent evaporation of water into the surrounding air.
After this period of time, take all plugs out of the test tubes carefully. Place on separate paper towels, and dry each to remove excess water,
Weigh the potato plugs for a second time, and record the results, making it possible to compare the masses before and after the experiment, i.e. being able to show whether the mass of the plug has increased or decreased.

Identifying variables

(a) Independent variable. The independent variable is the factor that needs to be changed to experiment. In this case, the independent variable is the sucrose concentration of the water.

(b) Dependent variable. The dependent variable is what is measured in the experiment, and here it is the change in mass of the potato plug.

(c) Control variables. The control variables are factors that need to be kept the same in order to make it a fair test. There are many control variables in this experiment, including volume of water, atmospheric temperature, shape of container, time elapsed, initial mass of potato plug, same potato used, same scales used, etc.

It is also important to repeat the test a number of times and get the average results, in order to make my results as precise as possible. Instead of doing the test again myself a number of times, I will get the results from other groups doing the experiment.

Safety factors

In order to keep my experiment safe, I will: do all cutting on a white tile; be careful not to spill any sucrose solution or distilled water; have paper towels near at hand to clean up any spillages which may occur; take care while using the scalpel.

My group’s results

A graph for these results is shown on the next page (page 6).

Average results of all groups

A graph for these results is shown on page 7.

Explanation of results

When comparing my group’s results to the overall average results, we can see that there is a difference in the trend of the graph. My group’s line graph went up, and then decreased, and went back up again; while the average results’ line graph just gradually decreased.

From the average results we see that the solutions with sucrose concentrations of 0%, 5% and 10% made the potato plug increase in mass. We also see that the solutions with sucrose concentrations of 15%, 20% and 25% made the potato plug decrease in mass.

From the average results we also see that the least concentrated solution had the biggest increase in mass, and the most concentrated solution had the biggest decrease in mass. The graph showing the average results show this, as there is a pattern of the points plotted decreasing.

Interpretation of results

The difference in trends in my group’s results and the average results may be explained by an inaccurate reading, or maybe by not removing all excess water.

The solutions with sucrose concentrations of 0%, 5% and 10% were hypotonic solutions. Endo-osmosis took place here (water travelled into the plug). The solutions with sucrose concentrations of 15%, 20% and 25% were hypertonic solutions. Ex-osmosis took place here (water travelled out of the plug).

From the fact that the least concentrated solution had the biggest increase in mass and the most concentrated solution had the biggest decrease in mass, we can say that the higher the water potential outside the plant, the quicker osmosis will take place. The graph of the average results shows this too, as the biggest increase in mass was for 0% sucrose concentration solution, which gradually decreased.

Evaluation of results

The experiment was quite successful in my opinion. The fact that I got results from five groups and found the average results made my results more reliable and accurate. I think I obtained a sufficient number of results from which I was able to create an informative and more accurate graph.

I think the time that I used for the experiment to last was enough to allow sufficient osmosis to occur. However if I was to repeat the experiment I might well increase the length of time to allow more osmosis to happen and possibly find out the saturation point of the chips.

The range of concentrations was adequate, but if I were to do the experiment again I would possibly create more concentrations so that I would have more varied results. This way would have allowed me to also find out the isotonic point far more accurately as the one that I estimated (page 5) is rather approximate.

I could have also made the results more accurate, i.e. to records my results to more than only two decimal places.

There were not many out of the ordinary results, but some were not as close to the line as others. When the potato chips were removed from the test tubes and dried some groups may well have dried some potatoes more thoroughly than others and so some would have more excess water, which would add to the mass. However with all this said I think that the experiment was quite successful and I was very pleased with the complete comparison of my results with my initial prediction.

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