How are Potatoes CellsAffected by Osmosis?

How are Potatoes Cells

Affected by Osmosis?

Biology Coursework

Osmosis in Plant Cells

Aim:

The aim of this experiment is to investigate the process of osmosis in plant cells taken from potatoes and to observe what changes which take place in the potato when left in varying glucose solutions.

Background:

The porous nature of the cell membrane means that only those molecules that are small enough will diffuse through it unimpeded. Larger molecules penetrate either slowly or not at all. Thus, the cell membrane is considered to be semi-permeable. In the diagram shown below, the cell has a higher concentration of glucose on the right-hand side compared to the left-hand side, as there are more glucose molecules per water molecules.

Osmosis involves the passage of water molecules from a region of high water concentration (the left-hand side) to a region of lower water concentration (the right-hand side), through a semi-permeable membrane. This is why the water molecules will diffuse from the left-hand side, through the semi-permeable membrane and into the right-hand side. However, there will be no diffusion of glucose molecules across the semi-permeable membrane from the right to the left, as the glucose molecules are too large to pass.

A solution consists of the molecules or ions of one substance (the solute) dissolved in another (the solvent). If a cell is surrounded by pure water, or a solution whose solute concentration is lower than that of the cell’s contents, water flows into the cell by osmosis, and the cell swells up. In this case, the concentration of glucose in the left-hand side is lower than that of the right-hand side, and accordingly the solution is said to be hypotonic to the right-hand side. On the other hand, the right-hand side is said to be hypertonic to the left-hand side as the concentration of glucose is higher than the left.

The following diagram further illustrates the act of osmosis and its effects on fluid levels in a beaker of solutions separated by a semi-permeable membrane. As the water molecules from the weak solution on the left, diffuse to the strong solution on the right, the fluid level will fall in the weak solution and rise in the strong solution. Eventually the concentrations will be the same and the solvent molecules will diffuse equally in both directions.

In plant cells, the concentration is markedly higher than that of their surroundings. When the cell is surrounded by a weak solution, water is drawn into the vacuole by osmosis, resulting in the cell swelling up. The cell does not burst because of the cellulose wall which, once fully stretched, resists any further expansion of the cell. This is referred to as turgor. Turgidity is at its maximum when the cell wall can be stretched no more and it is now fully turgid. Turgidity plays a vital role in supporting plants and maintaining their shape and form.

When a plant cell is placed is immersed in a solution which has a higher solute concentration than it does, the volume of that cell decreases as water passes out. The cell membrane will then shrink to such an extent that it pulls away from the cell wall, leaving a gap between the wall and the membrane. This is referred to as plasmolysis.

Many key facts and terminology were found by reading various books on cellular biology and human biology relating to osmosis. A range of websites proved exceedingly useful in simplifying the explanation of osmosis and its effects on cells from various organisms. All of the information obtained was able to serve as my secondary sources; my primary source having been what I previously knew.

I have also performed a very similar experiment in which I was able to observe the effects of osmosis in potato cells. Forty millimetre potato chips were placed in three different glucose solutions which were 0% glucose concentration (pure water), 50% glucose concentration, and 100% glucose concentration. The potato chips in the pure water increased in length and the chips from the 50% concentration decreased in length. The chips in the 100% concentration also increased in length, but only marginally more than those of the 50% concentration do. This straightforward experiment was able to serve as my preliminary work so that I ...

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Hypothesis:

Based on all of the data and information from my secondary sources and preliminary work, I predict that as the glucose concentration is increased the potatoes will decrease in size. I also predict that the potatoes will reach a point where they will not be able to expand any more. A minimum amount of water molecules will have diffused out of the cells in the potato, the cells will become plasmolysed, and the potato chips will reach a minimum length.

Apparatus:

Having performed the preliminary experiment, I am well aware of what equipment I will need to use so that I will have accurate results and a fair test.

Potatoes
Glucose Solution
Distilled Water
4 mm Cork Borer
Knife
Pipette

Planned Method:

Having performed the preliminary work, I have a sound plan as to what measurements I will use for a beneficial experiment. In the preliminary experiment, I used 50 cm3 glucose solutions. For this experiment, I will again use 50 cm3 glucose solutions of different concentrations, this time ranging from 0% glucose (de-ionised water) to 100% glucose solution. I have chosen to use 20% intervals between each solution as I feel that this would give me a wide range of results without having an impractical number of potatoes to cut and solutions to make. I will use 50 cm3 of solution for each concentration of glucose because it should be sufficient to submerse the potatoes whilst in the Petri dishes.

In the preliminary experiment, I cut the potato chips to forty millimetres and four millimetres in diameter. For this experiment, I will cut the potatoes so that they are 4 mm in diameter, but this time they will be fifty millimetres in length. I chose to increase the length because I feel they would allow a wider range of results, yet fit in to the Petri dishes and still be fully submersed in the glucose solutions. I will then place the potatoes into each Petri dish of solution and wait for twenty-four hours. I feel that twenty-four hours should be enough time to allow any possible changes to occur. Lastly, I will remove the potatoes from the solutions and measure them using the ruler, so that they are accurate to one millimetre.

Fair Testing:

Whenever carrying out any experiment, it is important that a fair test was given. When doing an experiment like this one, I’ll need to make sure that every step is performed in exactly the same way. In this particular experiment, I will ensure that the length of each potato is cut to exactly fifty millimetres with the ruler and not one millimetre more or less. The exact same cork borer will be used to cut the potatoes so the diameter will always be four millimetres. Each solution will be exactly 50 cm3 using the pipette to add water or glucose with precise accuracy. The temperature at which I will perform the experiment will remain constant as it is the Petri dishes will be left in a climate controlled environment. The length of time that the samples are left will be exactly twenty-four hours, accurate to one minute as I will remove all of the potato chips within one minute of each other. All of the Petri dishes will be covered as to not contaminate the solutions with impurities that might alter the results. The Petri dishes will also be placed in a way so that they all receive the same amount of light (natural or un-natural), which might affect the temperature of the Petri dish.

The only variable in this experiment will be the concentrations of the glucose solutions.

Repeats:

I will put three potato chips into each Petri dish, enabling me to allow for experimental error and providing me with a wider range of results which will ensure that I have much more firm conclusion.

Safety:

Every experiment needs to be done safely. With this experiment, a knife was used to cut the potatoes. Care needs to be taken not to cut one’s fingers during this step of the experiment.

Actual Method:

First, the 6 different 50 cm3 solutions were made consisting of 100% glucose, 80% glucose, 60% glucose 40% glucose, 20% glucose and 0% glucose (distilled water). The glucose solutions were made using the following table.

The measuring cylinder was used to obtain the correct volume and therefore percentages of water and glucose for each glucose solution. The pipette was used to add water or glucose with precise accuracy, as required.

Next, the solutions were poured into six labelled Petri dishes. 18 potato chips were cut out using the cork borer. Care was taken not to bore into the hole left by other samples which were taken from the potato. Every chip was full in diameter for the entire length of its cylindrical shape. The tile was used as a base for cutting the potatoes.

All the chips were cut to exactly 50 mm using a ruler and a knife. The ends of every single chip were removed of its skin so that it would not affect the effects of osmosis. Three potato chips were placed into each of the six solutions and left for a period of twenty-four hours. Every Petri dish was covered and placed in way so that they all received the same amount of light (natural or un-natural), which might have affected the temperature of the Petri dish.

Results:

Length Before & After Soaking in Glucose Solutions

Analysis:

After twenty-four hours, the potato chips were then removed from the Petri dishes and measured one-by-one with the ruler. The following observations were made:

The potato chips from the 0% solution (deionised water) were measured and two out of the three potato chips increased by 5 millimetres (10.0%) while one increased by 4 (8.0%) millimetres. The potato chips together have increased in size by an average of 9.3%. This is because the glucose concentration in the potato cells was stronger than that in the Petri dish, therefore the water molecules diffused into the cells through the semi-permeable cell membrane by osmosis. The vacuole in the cell has filled with water molecules and is swelling and pressing the cytoplasm and cell membrane against the cell wall. The cell has done this to the point where the cell wall can expand no further and the cell is now considered to be fully turgid.
The potato chips from the 20% glucose solution were measured and two out of the three potato chips increased by 3 millimetres (3.0%) while one specimen neither increased, nor decreased in size; it remained at 50 millimetres (0.0%). The potato chips together have increased in size by an average of 4.0%. The concentration of glucose in the potato cells was only marginally stronger than that in the Petri dish, therefore not as many water molecules needed to diffuse into the cells to make the concentrations equal. These potato cells are of a slightly lower turgidity.
However, all three potato chips in the 40% glucose solution decreased by 1 millimetre. That is 2.0% less than the original size of 50 millimetres. Though this is only a small decrease in length, it does show that water molecules diffused out of the potato cells by osmosis. This is because the glucose concentration in the potato cells was weaker than that in the Petri dish. So the water molecules diffused from the weaker concentration through the semi-permeable cell membrane, into the stronger concentration of the glucose solution in the Petri dish, making the potato chips shrink. These potato cells are starting to become flaccid.
Two out of the three potato chips in the 60% glucose solution decreased by 4 millimetres (-8.0%), while one decreased by 5 millimetres (-10.0%). The potato chips together have decreased by an average of 8.7% less than their original size. The length of the chip is now steadily decreasing. The decrease in size indicates that the glucose concentration of the solution in the Petri dish is much stronger than that in the potato cells.
Two out of the three potato chips in the 80% glucose solution decreased by 5 millimetres (-10.0%), while one decreased by 4 millimetres (-8.0%). The potato chips together have decreased by an average of 9.3% less than their original size. That is only 0.6% less than the 60% solution, indicating that the potato chips are beginning to become plasmolysed. In other words, almost all of the water molecules have diffused out of the cells in the potato chips.
Two out of the three potato chips in the 100% glucose solution decreased by 6 millimetres (-12.0%) while one decreased by 7 millimetres (-14.0%). The potato chips together have decreased by an average of 12.7% less than their original size. This indicates that osmosis is still taking place, with water molecules diffusing from the potato cells to the glucose solution in the Petri dish. Of all of the specimens, these potato cells were of the highest flaccidity. Had these specimens been left in the solutions for any further length of time, it would be accurate to suggest that the potato cells would have become fully plasmolysed. The cell membrane would have pulled away from the cell wall and the potato chip would have reached its minimum length possible.

My overall trend shows that as the glucose solution is made stronger, more water is diffused from the potato cells. This is evident by looking at both graphs and observing the fact that there is an obviously close negative correlation between the change in length of the potato chip and the concentration of glucose solution. However, the last two readings indicate that the loss of water molecules is starting to decline. This is due to the fact that the cell wall is still holding the shape of the cell, even as the water molecules diffuse out of the cells and the cell membrane pulls away from the wall. The cells begin to become flaccid as they get plasmolysed. This doesn’t mean however, that water is not passing through the cell; it simply means that water is diffusing in and out of the potato’s cells through the semi-permeable membrane, equally in both directions, allowing it to maintain its original size.

By looking at the graphs, we can see that the potato chips neither grew nor shrank when the glucose solution was between a 20% and 40% concentration. When looking at the trend line, we can see that the length of the potato chip would stay the same if the concentration was approximately 33% glucose. If the concentration of a cell and its surroundings are the same, there is no net flow in either direction and the external solution is said to be isotonic with the cell. We can conclude from this that the potato has a concentration of approximately 33% as well.

After considering all of the data, I have concluded that these findings do support my hypothesis that as the glucose concentration is increased the potatoes will decrease in size.

Evaluation:

I feel that performing the experiment using this method was highly effective and proved to be very accurate. Setting up this experiment was very straight forward and I had no difficulties. Care was taken when cutting the potato chips and making up the glucose solutions, so that all the measurements were precise. This was the most difficult step of the setup, though it was still quite simple to perform.

This method is a very simple, yet effective way of testing for osmosis. Most of the results I gathered from the experiment appeared to be very accurate. Four out of the six results followed the trend-line very closely. The two anomalous results I found were the specimens from the 80% glucose solution and the 100% glucose solution. Having taken so much care in setting up the experiment and keeping the variable to an absolute minimum, I feel that a fair test was give and that the accuracy of my results can be fully relied upon. Having used three potato chips for each solution, I increased the repetition of my experiment. Taking the average of the repeats would rule out any anomalous results that might have occurred due to any natural imperfections or irregularities in the potato chips themselves.

If I had more time, I would have liked to do the experiment again so that I could record a wider range of data. I would have liked to record the mass of each potato chip before and after soaking in the solution. If I had performed the experiment at different temperatures, I could have observed how osmosis is affected by temperature. I would have also liked to use smaller intervals between the glucose solution concentrations, possibly 10% increments of concentrations of the solutions. I could have also used four or five potato chips per solution. This would have produced a much wider range of data and allowed me to have a stronger conclusion.

Using different types of cells from various different sources would have provided an accurate account of how the effects of osmosis affect different types of organisms. When researching secondary sources, I came across an experiment in which osmosis was observed in human cells. Human red blood cells were placed in hypertonic, 1.2% salt solution and they were found to shrink perceptibly. If the cell was placed in a weaker (hypotonic) solution, they would swell and burst, a phenomenon known as haemolysis. Had I have had access to the proper equipment and facilities, I would have been especially interested to observe just how important it is to have an isotonic environment for living creatures.

Overall, I am pleased with how the experiment turned out and I feel that I have gained a sound understanding of how osmosis occurs.