To investigate how varying the concentration of sucrose solutions affects the rate of osmosis between the solution and potato cylinders immersed in it.

Osmosis in potato cells

Section 1: Planning

Aim:

Hypothesis:

A substance, when dissolved in water, has a natural desire to dilute itself by bonding with surrounding water molecules. When a semi permeable membrane, such as a cell membrane separates two solutions, pure water flows from the weaker solution to dilute the stronger one until they are both the same strength. The term used to describe the flow of water through a semi permeable membrane separating two solutions of different concentrations is osmosis.

In other words, osmosis is the process in which water passes in and out of plant cells thus controlling their turgidity. It is in fact the diffusion of water from places of high water concentration to places of low water concentration and can take place only across a selectively permeable membrane – such as the cell membrane. The cell membrane allows small molecules such as those of water to diffuse through it. However, larger molecules such as those of sucrose can not pass through its pores. This movement of water will continue until equilibrium is attained and the concentration is equal on both sides of the cell membrane. An example of a region with high water concentration is distilled water or very dilute sucrose solution where there is a large number of free water molecules.

Either of three things can happen if a plant cell is immersed in a solution. These are:

1. The net (overall) amount of water entering the cell may increase causing the cell to swell up and become turgid. This will happen if the medium surrounding the cell has a higher water concentration than the cell sap. As a result of this, the cell will gain water by osmosis. This is so because even though, the water molecules are free to pass across the cell membrane in both directions, more water comes into the cell than leave. This observation can be explained by what happens inside the plant cells. When a plant cell takes up water by osmosis it starts to swell, but the cell wall which surrounds it, prevents it from bursting. This causes the pressure inside the cell to rise, until it becomes so high that no more water can enter the cell. This is so because the liquid or hydrostatic pressure works against osmosis. This turgidity is very important to plants because it allow their green parts to stand up and absorb maximum sunlight which in turn is required for osmosis.

2. If the medium has exactly the same water concentration as the cell sap, there is be no net movement of water across the cell membrane. Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The cell stays the same size and its turgidity is not affected. This observation can be explained by the fact that when plant cells are placed in a solution which has exactly the same osmotic strength as their sap, they are in a state between turgidity and flaccidity called incipient plasmolysis. If the cell is taken out at this point and placed in pure water it will still recover.

3. If the medium has a lower concentration of water than the cell (a very concentrated solution) the cell will lose water by osmosis.

Again, water crosses the cell membrane in both directions, but this time more water leaves the cell than enters it. Therefore the cell will shrink.

When a plant cell is placed in concentrated sugar solution it loses water by osmosis and becomes flaccid.

Observing plant cells in concentrated sugar solutions under a microscope, we can see that the contents of the cells have shrunk and pulled away from the cell wall causing the cells to become plasmolysed. This is so because water continues to leave the cell, the cell membrane gradually draws away from the cell wall, the cell becomes in a state of complete plasmolysis as maximum amount of water leaves it.

However, osmosis does not affect plants alone- it is important to fish as their body fluids are more concentrated that the surrounding water. Water moves by osmosis through the semi-permeable membrane of the fishes skin and gills and constantly dilutes their body fluids. Freshwater fish must excrete wastes to prevent them becoming waterlogged. Marine fish have the reverse problem in that sea water has a higher concentration of dissolved salts than the body fluids of the fish. They must therefore drink plentifully to avoid drying out. Fish control the intake of and expulsion of fluids by osmoregulation.

From my theory on osmosis, I can conclude that when I immerse the potato cylinders in a sugar solution, water will diffuse out of their cells and into the solution. This will be because when sugar is dissolved in water, water molecules are attracted to the sugar molecules resulting in less free water molecules. Consequently, water molecules will move out of the cell in order to balance the number of free water molecules on both sides of the cell membrane. Since the cell experiences water loss, the cells will shrink and the plant will become flaccid. The more concentrated the sugar solution, the lesser the free water molecules will be and hence the water loss by the cell will be at its maximum.

On the other hand, I think that if the solution outside the plant cells is more dilute than the cell sap, there will be more free water molecules present outside the cells. Because of this, water will diffuse into the cells and the latter will become more turgid. According to this theory, I expect that if I place the plant cells in distilled water, the maximum amount of water will diffuse into them. This is because no matter how dilute the cell sap is, it will always have less free water molecules as it contains salts, proteins and sugars. Because of this, I expect the vacuole to gain water and expand, thus exerting more turgor pressure on the cell wall. At the same time, I do not expect this swelling of the cell to be indefinite. If it was indefinite, than the cells will burst thus killing the plant. I think that what will prevent this from happening is the strong cell wall that will be able to withstand the pressure exerted on it, keeping the plant turgid and upright yet preventing its cells from bursting.

The diagram below illustrates the process of osmosis

To test whether my predictions are true, I will conduct an experiment using potato cylinders immersed in sugar solutions of different concentrations, as well as in distilled water. I will note down any changes in the length and mass of the potato cylinders after being immersed in the solutions for 24 hours. I will plot my results in tables such as those shown below:

Solution concentration: ________

I think that several factors will affect the outcome of my investigation. Hence, I will keep them invariable throughout my experiment. These factors include:

The thickness and permeability of the cell membrane. (I think that the thinner the cell membrane, the faster and greater the rate at which water moves through osmosis will be. To avoid errors due to these variations, I will use potato cylinders from the same potato only).
The amount of solution used. (I think that varying the volume of solutions used in each experiment will vary the outcome of my investigation, since there will be different numbers of molecules available to diffuse. To avoid the possibility of errors due to this, I will use the same volume of solution (30cm3) through out the investigation. I will use 30cm3 since it will be enough to completely immerse the potato cylinders)
Temperature at which the experiment is conducted. (I think that temperature will affect the kinetic energy of the molecules. The higher the temperature, the faster they will move, and hence more molecules will move across the cell membrane in a shorter period. Hence, I will carry the entire investigation at constant temperature (room temperature)).
Duration of the experiment. (By changing the amount of time each potato cylinder is immersed in solution, I may obtain erroneous readings since if equilibrium has not been reached by the time I remove them from the solution, osmosis will continue to take place. To prevent this from happening, I will allocate an equal amount of time for each set of solutions.)
Initial lengths of the potato cylinders used. (By varying the lengths of the potato solutions, I will also be varying the surface area across which osmosis can take place. This in turn will cause difference in the percentage change in the length and mass of the potato cylinders before and after the experiment. To avoid this I will use potato strips of the same length).

Hence, the only factor that I will vary for each experiment is the concentration of the solution in which the potatoes will be immersed.

Planned method:

I first carried out a prior test to obtain a more accurate idea of what the outcome of my actual experiment would be. This test also served to allow me to make any changes to the procedures I plan to use for my actual experiment, as well as to introduce any precautions that would make my actual test a fair and safe experiment.

Prior test:

Materials:

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Teacher Reviews

Here's what a teacher thought of this essay

Ross Robertson

10th of May 2013

In this lengthy report (42 pages!) into an investigation concerning the effect of changing sucrose concentration on osmosis in potato tissue, the writer demonstrates a good understanding of the process of osmosis (at GCSE level) but undermines the good science carried out by using a very repetitive writing style. Both the method and data collected were of a good standard. The problem here is that in an attempt to be extremely thorough, the logical flow of ideas has been lost. A GCSE investigation of this nature can be adequately written up in around 1500 words (6 pages). Good scientific writing is concise and relevant and tells the reader exactly what he/she needs to know and no more. More is very often less! In this case, it certainly is. 3 stars

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