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Investigate the water potential of a potato tuber.

Extracts from this document...

Introduction

My aim in this investigation is investigate the water potential of a potato tuber. This investigation needs a lot of planning and prediction with the use of scientific knowledge involving preliminary experiments, which will guide me for the main experiment to be successful and for it to produce concise and accurate results to prove the hypothesis, explained in the prediction. The main theory used in this experiment is the theory of osmosis, which is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration (http://www.purchon.com/biology/osmosis.htm). Osmosis controls the exchange of molecules through the semi permeable membrane. It allows small molecules like oxygen, water, carbon dioxide, ammonia, glucose, amino acids, etc. to pass through. Cell membranes will not allow larger molecules like sucrose, starch, protein, etc. to pass through. If the solution surrounding the cell has a higher water concentration than the cell (a very dilute solution) the cell will gain water by osmosis and vice versa. A key feature of osmosis is that only water molecules move across the membrane to bring the two solutions to and equilibrium. This equilibrium is reached when the water potential in one region is the same as the other region. Water potential is the chemical potential (i.e. free energy per mole) of water in plants. Water moves within plants from regions of high water potential to regions of lower water potential, in other word down a water potential gradient. It is this tendency to move which is called water potential (Cambridge Advanced Sciences Biology 1, Mary Jones et al, 2000). It is affected by two factors, which are solute potential (?s) and pressure potential (?p). The solute potential is a measure of the number of dissolved particles in water, for example the amount of dissolved sugar or salt. In pure water the solute potential is zero because there are no solute molecules at all. ...read more.

Middle

error because it will allow osmosis to occur more or less so this will affect the water potential in those few cells (independent variable) I will leave the all the tubers in for sufficient time, that is, 24 hours and will note the exact of putting the cylinders in solution and taking them out. (Independent variable) Before completing the actual experiment I did a preliminary experiment similar to what my actual experiment be like but instead I chose to use an onion. In this experiment the aim was to observe the process of plasmolysis and to record the plasmolysed state in the epidermal cells of the onion. If a plant cell is in contact with a hyper tonic solution, that is a solution that has a higher solute concentration than the cell contents, water leaves the cell by osmosis via the cell membrane. Water is lost first from the cytoplasm and then the sap vacuole through the tonoplast. The protoplast the living part of the cell (cytoplasm and the nucleus) shrinks and eventually pulls away form the cell wall - this process is called plasmolysis, and the cell is said to be plasmolysed. In the experiment I used 6 different concentrations of sucrose solution varying from 0.1 molar to 1.0 molar shown in the table below: Ratio of water Ratio of sucrose Molarity of sucrose solution 8 2 0 7 3 0.3 6 4 0.4 3 7 0.7 2 8 0.8 0 10 1.0 I removed a strip of epidermis from the inner surface of one of the fleshy storage leaves of the onion bulb. First slitting it with a scalpel, and tearing back the single layer of cells with forceps can remove the epidermis. I cut up this epidermis into seven 5 x 5 mm (approximately) pieces. I put each of these in the different concentrations and waited for about roughly 20 minutes. ...read more.

Conclusion

This shows that I had carried out the experiment fairly and accurately, although they are not all exactly fitted on the line of best fit, this small difference may be due to many reasons, as it affected all the different concentration From this graph it is possible to work out the point at which it has cut the x-axis accurately using the equation of the line. At this point incipient plasmolysis occurs, the pressure potential is zero. From the use of excel I have drawn a trend line which is more accurate then the one that I have done previously by hand (-74.429 being the gradient and 27.381 being the y-intercept following the rules of mathematic where y = mx +c). If it is not accurate it is not possible to see where the line cuts the x - axis. Below I have worked out this point: The equation of the line is Y = -74.429x + 27.381 At this point y = 0 ? 0 = -74.429x + 27.381 -27.381 = -74.429x x = -27.381/-74.429 = 0.367880799 This is the molarity of sucrose solution at which point incipient plasmolysis occurs. This is a very accurate number because it is up to 9 decimal places, which is not necessary but is very accurate. On my and drawn graph you can clearly see that this same point is at 0. 38, which is not accurately because the line of best fit has been drawn approximately. As we found out from the experiment even small changes in concentration can have an effect in the percentage change in mass. Here is a table of solute potentials of sucrose solutions (at 20?C). This will help me work out the water potential Concentration of sucrose solution (Molarity) Solute potential Kpa 0.05 -130 0.10 -260 0.15 -410 0.20 -540 0.25 -680 0.30 -820 0.35 -970 0.40 -1120 0.45 -1280 0.50 -1450 0.55 -1620 0.60 -1800 0.65 -1980 0.70 -2180 0.75 -2370 0.80 -2580 0.85 -2790 0.90 -3010 0.95 -3250 1. ...read more.

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