The aim of this experiment is to investigate the movement of water in and out of plant cells. The cells chosen for study will be taken from potato tubers. Firstly I will explain what osmosis is. Osmosis is the passage of water from a region of high water concentration through a semi permeable membrane to a region of low water concentration. This definition contains three important statements:
a) It is the passage of water through a semi permeable membrane
b) It is the passage of water from a region of high water concentration
c) It is the passage of water to a region of low water concentration.
All the above statements are included in the definition, but define certain aspects of it.
Semi-permeable membranes are very thin layers of material which allow some things to pass through, but prevent others. A cell membrane is semi permeable. They allow small molecules like oxygen, water, amino acids etc. to pass through but will not allow larger molecules like sucrose, starch, protein etc. through. A region of high concentration of water is either a very dilute solution of something like sucrose or pure water. In each case there is a lot of water: a high concentration of water. A region of low water concentration is the opposite of the above, i.e. a very high concentration of sucrose solution: a low water concentration.
The water content of plants varies depending on environmental conditions. In Land plants this water plays a vital role in the support of tissues and the transport of materials around the organism. Lack of water leads to wilting and eventually death. Water is mainly absorbed through the roots, which are covered in specially adapted root hair cells, with large surface areas and thin cell walls to aid absorption. It is drawn up the plant through xylem vessels by a pull resulting from the evaporation of water through the stomata on the leaves.
This evaporation is called transpiration and the xylem flow resulting is called the transpiration stream. Soluble food substances formed during photosynthesis are transported around the plant in the phloem tubes. This movement of water through the plant in the xylem vessels or phloem tubes is similar to the flow of blood in humans as it transports soluble mineral salts, nutrients and auxins, (plant hormones), from place to place. The evaporation of water from the leaves also removes heat energy from the plant and helps to prevent overheating.
Transpiration pulls water up the plant stem but osmosis is the process whereby water is drawn into or out of cells and tissues. Osmosis is the flow of water by diffusion through a differentially permeable membrane from areas of high water concentration to regions of low water concentration. The diagram below illustrates this:
Water can freely penetrate all membrane. The cellulose cell wall does not act as a semi permeable membrane and will allow most substances that are dissolved in water to freely pass through it.
Whether water enters the cell by osmosis or not will depend on the balance between external and internal solute concentrations and the state of the cell. If the solutions on each side of the differentially permeable membrane are equally concentrated then there will be no net movement of water across the membrane. This is called an equilibrium state and the solutions are referred to as being isotonic. A solution that contains more solute particles than another, and is hence more concentrated, is referred to as being hypertonic. The less concentrated solution is hypotonic. This concentration of solute particles is usually described as a molarity.
Even if the solute concentration external to the cell is hypotonic to the vacuole contents the cell will not continue to take in water by osmosis for ever. The cellulose cell wall provides a rigid barrier to uncontrolled expansion. A cell that is full of water is called turgid and cannot expand further as the outward pressure on the cell wall is balanced by the inward force of the stretched wall. This wall pressure is called turgor pressure and the internal outward force on the wall is called osmotic pressure.
At the other extreme, a cell placed in a solution that is hypertonic to its contents will lose water by osmosis. The cytoplasm will cease to exert a pressure on the cellulose cell wall and the cell, described as flaccid, will lack support.
Water loss can continue to such an extent that the cytoplasm, and attached cell membrane, contracts and detaches from the cell wall. A cell in this condition is said to have undergone plasmolysis. This very rarely, if ever happens in nature.
As osmosis is the diffusion of water molecules and as diffusion is the random movement of particles from areas of high concentration to low concentration it might be expected that any factors that speed up or slow down the movement of these particles would affect the rate of osmosis.
Using knowledge of the process of osmosis and with a good understanding of molarity I should be able to determine the solute concentration of the vacuoles in potato tuber cells. As it would be impossible to measure with any degree of accuracy the expansion or contraction of cells on an individual basis I have decided to look at gain or loss of water in terms of increase or decrease in mass. Mass, I feel, will be a more accurate way of recording the change of the potatoes as when measuring length, it does not take into account the change in diameter of the chip. I will also look at the increase or decrease in length to verify the accuracy of my results and compare the two readings. A cell placed in an isotonic solution should show no change whereas one placed in a hypertonic solution will lose mass.
For this experiment, I will have to choose a factor to vary. These factors are:
ƒá Molarity of the sucrose solution
ƒá Surface area of the potato
ƒá Type of potato used
ƒá Age of the potato
ƒá pH of the sucrose solution
The factor I have chosen to vary is the molarity of sugar solution as I believe this will be easy to regulate as the concentration can be easily altered using distilled water. I will use 1 molar solution and alter the concentrations as shown below:
Molarity of sugar solutionAmount of waterAmount of sucrose solution
For this experiment I will need:
ƒá 1 large potato to produce 18 potato tubers
ƒá cork borer
ƒá distilled water
ƒá 1 molar sugar solution
ƒá 18 test tubes
ƒá ruler to measure length of potato tubers
ƒá electric balance to measure the mass
I have selected the above equipment because I feel it will help me to ensure accurate results. To ensure a fair test I will take all my potato samples from the same potato using the same cork borer and keep all of my apparatus the same. I will try and treat each potato tube the same. I will measure each potato tube separately to ensure accurate measurements and carry out the procedure 3 times for each molarity tested. This will mean that I will need to measure 18 potato tubers. Three results will enable me to take an average result, making the results, hopefully, more precise and reliable. If one of the results seems very different to the others, I shall identify it as an anomalous result and retake the reading.
When I carry out this experiment, I will get a potato and take some tubes from it using a cork borer I will then cut these tubes into shorter lengths and measure the length and mass of each of the 18 lengths. All the lengths will be cut to 25mm. The solutions will be altered according to the molarity required and cm3 of each solution placed in each test tube. Each molarity will occupy three test tubes. The chips will then be put into each test tube and left over night. They will then be taken out of their test tubes, dried lightly with a paper towel and the new mass and lengths recorded. Once the results have been collected, they will be tabulated and analysed. A graph will be drawn and any trends noticed explained.
Prior to the experiment we carried out a short pilot test, using potato chips and solutions of strength 0.0, 1.0 and 2.0 molar solutions. The chips were 25mm in length each, and each chip was placed in 5 cm3 of either distilled water/1.0 molar / 2.0 molar sugar solutions and left for 30 minutes. The potato chips were then measured and the results recorded. They are shown below:
Chip numberOriginal lengthResultant length
These results show that a potato chip placed in water will gain in length, a weak sugar solution will lose length and a strong sugar solution will lose length also. The results from this test will allow me to choose an appropriate range of moralities in order to find out what the concentration is inside the cell vacuole. I am going to investigate 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 molar sugar solutions. I have chosen these concentrations to try and accurately find when there is no net movement of water, hence the concentration of the cell vacuole.
From previous work done on osmosis, I predict that molarity and average change in mass/ length will be indirectly proportional. I think there will be a negative correlation between the two. I think that there will be both loss and gain in mass discovered. I think the graph will look like this but there will be no plasmolysed on my graph, as I do no expect my measurements to go that far. I hope to be able to identify the point when there is no net movement of water.
Analysis of Results
The Consequences of Osmosis in plant cells:
Plant cells always have a strong cell wall surrounding them. When the take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant cells become “turgid” when they are put in dilute solutions. Turgid means swollen and hard. The pressure inside the cell rises, eventually the internal pressure of the cell is so high that no more water can enter the cell. This liquid or hydrostatic pressure works against osmosis. Turgidity is very important to plants because this is what makes the green parts of the plant “stand up” into the sunlight.
When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become “flaccid”; this is the exact opposite of “turgid”. If you put plant cells into concentrated sugar solutions and look at them under a microscope you would see that the contents of the cells have shrunk and pulled away from the cell wall: they are said to be plasmolysed.
When plant cells are placed in a solution which has exactly the same osmotic strength as the cells they are in a state between turgidity and flaccidity. We call this incipient plasmolysis. “Incipient” means “about to be”. When I forget to water the potted plants in my study you will see their leaves droop. Although their cells are not plasmolysed, they are not turgid and so they do not hold the leaves up into the sunlight.
Graph  shows the average percentage change in length of the potato tubers. It shows that as molarity increases the average change in length decreases. The graph drawn looks accurate as the curve did not have to be one of best fit, but went through all of the points plotted showing that all the readings were accurate. The potato tubers gained/ loss length, the molarity increases the sugar solution becomes more concentrated, and more concentrated than inside the cell. At 0.2M solution there is no net movement of water. As the strength of the concentration increases the cells shrink and become flaccid.
Graph  shows the average percentage change in mass of the potato tubers. It shows that as molarity increases the average change in length decreases. This graph is very similar to the graph showing the length loss or gain, but appears less accurate as there is an anomalous result. This is at 0.4 molar, it lies off the best-fit curve drawn by 9.2%. The curve is one of best fit and follows the same trends as graph .
My results seem fairly accurate and although the graph showing length seems to be more accurate as it is a curve that goes through all of the points, it only shows the change in length, and not in mass. The graph showing mass change  gives a more accurate view of what happened as it takes into account the expansion of the potato both ways and has a broader percentage change range. This means that instead of just spanning 30% in total (as does graph ) it spans 80% (as does graph ). This gives a broader field of results and is therefore more accurate, as the mass is a more accurate result than length as the potato chip will get wider as well as longer. My results do seem to be reliable, as the graphs drawn support my prediction and seem accurate as they all lie on a smooth curve.
From the results obtained, I can conclude that the average gain or loss in mass of the potato chip is indirectly proportional to molarity. I can also say that average gain or loss in length of the potato chip is indirectly proportional to molarity. Both of the results show a negative correlation. I can now say that the more concentrated the solution, the more mass/length is lost. This is because the water inside the cell moves out, causing the cell to shrink. When the cells are in a less concentrated solution they gain in length and mass as water is taken into the cell and the cell swells. The results gave enough information to support my original prediction. Both of the graphs cut the x-axis at 0.2, showing that the molarity of the internal solute of a cell is 0.2m. This also shows that my results were very alike and reliable.
My results seem to be very accurate. I can tell this because when the points were plotted they all lay on the curve, apart from one anomalous result, 0.4Mon the graph showing mass. There was however only one anomalous result and the others were all very reliable. This may have been because the results had an average taken so it may not have been accurate. I could increase the accuracy by taking more repetitions which should make the average more accurate. As the potatoes were left over night, the temperature changed which may have affected the results, but it should not have made a drastic difference to the graphs as all of the potatoes were subjected to exactly the same temperature changes.
This could be improved by placing the test tubes into a water bath so they were kept at a constant temperature. The same potato was used in each of the experiments, which may also have contributed to the reliability of my results. The mass was more accurate to measure for many different reasons. Length does not take into account the change in diameter of the chips, and you can not measure fractions of millimetres on a ruler, but the electric balance will record change from 2 decimal places,
e.g. mass: 1?43 “” 1?34length: 25 “” 23
whilst length can only be measured to the nearest millimetre. For the mass, we had to be careful that all the potato chips were dried in the same way as this may have altered the reading. This may have been what caused the anomalous results, as it was lighter that the best fit line i.e. some water may have been lost through harder drying, or squeezing during the drying process. If some of the water evaporated overnight, it would have incresed the molarity of the solutions, thus making the results innaccurate. This could be combatted by putting a bung in the top of the test tubes to stop the evaporation and keeping the sugar slution concentrations the same.
To improve the accuracy of the results I would include more concentrations to find the point of plasmolysis as in my experiment, I did not get to the point of plasmolysis in my experiment, so if I was to extend this experiment, I would investigte a wider rage of concentrations to investigate furthur and increase accuracy. I would also increase the repetitions to 5 per molarity and increase the molarity to try and find the point of plasmolysis. I could also decrease the range between each molarity (every 0.05 for example) to try and find the exact concentration of the potato cells where there is not net gain. This investigation was succesful but could still be made more accurate by some of the above ways.