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Discover the effect of water concentration on the mass of potato tissue, and investigate the movement of osmosis through potato tissue.

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Biology Coursework Investigation Aim: The aim of this investigation is to discover the effect of water concentration on the mass of potato tissue, and also to investigate the movement of osmosis through potato tissue. Scientific Knowledge: When a substance such as a sugar dissolves in water, the sugar molecules attract some of the water molecules and stop them moving freely. This, in effect, reduces the concentration of water molecules. Water Potential: The water potential of a solution is a measure of whether it is likely to lose or gain water molecules from another solution. A dilute solution, with its high proportion of free water molecules, is said to have higher water potential than a concentrated solution, because water will flow from the dilute to the concentrated solution (from a high potential to a low potential). Pure water has the highest possible water potential because water molecules will flow from pure water to any other aqueous solution, no matter how dilute. When two such solutions (one strong, one weak) are separated by a semi-permeable membrane the water will move from the side with more water to the side with less until both sides are equal (have reached equilibrium). This can be seen in all living cells. The cell membrane in cells is semi-permeable and the vacuole contains a sugar/salt solution. So when a cell is placed in distilled water (high water concentration, lower water potential) water will move across the semi-permeable membrane into the cell (lower water concentration, higher water potential) by osmosis, making the cell swell to carry the extra water. This cell is now referred to as turgid. The opposite of this is where the cell becomes flaccid, where the cell membrane actually can break away from the cell wall. If this process were done with the potato cells I would expect them to increase in length, volume and mass due to the extra water. ...read more.


This was to stop any contamination and made sure I didn't pass on any excess water or sugar from my hands. Apparatus: * Cork borer - diameter of 8mm * Distilled Water * Sucrose solution - 1 molarity * Measuring Cylinder * Cutting tile * Potato - of one type to make the experiment fair (see fair testing) * 6 beakers * Pen - to label the beakers * Ruler - to measure the potato cores * Scalpel - to cut out the potato cores to the correct size * Paper towels - to help keep the work area clear and clean * Digital scales - more accurate than a manual balance * 2 Pipettes - one for the sucrose solution, one for the distilled water so that they do not become contaminated. Diagram: Diagram 1: Diagram 2: Molarity Table: Molarity (M) Water (cm3) Sucrose Solution (cm3) Ratio (water: sucrose) 0.0 100 0.00 100:0 0.2 80.0 20.0 80:20 0.4 60.0 40.0 60:40 0.6 40.0 60.0 40:60 0.8 20.0 80.0 20:80 1.0 0.00 100 0:100 Method: 1. I took one average sized potato, checking it was hard and healthy. 2. Using a borer (size 8) I cut 30, 2cm lengths of the potato. 3. Taking 6 beakers, I labelled each one 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0. 4. Using the measuring cylinder, I measured out each of the correct levels of sucrose solution to distilled water (see molarity table above) and I placed each of the mixtures into the correct beakers. 5. Then I weighed all the potato chips on an electronic balance (see results) and recorded the results. 6. I placed 5 pieces of potato into each beaker and left them for approx 36 hrs. 7. After this time I drained out the solutions from the beakers and I carefully placed them in order of molarity on a paper towel. 8. I dried the potato chips gently and then weighed each potato piece and recorded the results. ...read more.


The higher the concentration of glucose solution, the more the potato chip will lose its weight, width and length. All the evidence I have obtained supports my hypothesis. I have been able to collect sufficient results by repeating the experiment five times. However there are improvements that could be made (see Evaluation). Because water molecules have a form of kinetic energy, they are always moving around in either a gaseous, solid or liquid state, randomly from one place to another. The greater the concentration of water molecules in a solution, the greater the total kinetic energy, and the higher the water potential will be. This means that as the concentration of glucose molecules increases in a solution, the concentration of water decreases, lessening the solution's water potential, and decreasing a solution's ability to move between solutions due to osmosis. Therefore as the concentration of glucose increases in each solution, the water in that solution is less able to move to the potato, causing water from the potato to move into the solution, decreasing the potato's length, mass and width. An osmotic system is set up when a semi-permeable membrane is placed between two solutions. There are many examples in the biological world, in many plants and animals. The plasma membranes of the cell decide the permeability of the membrane. A semi-permeable membrane occurs when some substances with small molecules can pass through, e.g. Water, and some substances cannot pass through the membrane because they have larger molecules, e.g. Sucrose. The permeability of a membrane varies with certain conditions, and can vary due to temperature, and even due to hormonal impulses, e.g. the loop of Henley in the kidney. My investigation shows that, in concentrations above 0.6M, there appears to be no further water loss, suggesting that the cell is completely plasmolysed. However, it is important to realise that this is only an estimate because potato cells will not be uniform in their concentrations. I have enjoyed doing this experiment and I believe it has been very informative. Chiara Catterwell, 10M, Mrs Ince, 10.4. ...read more.

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