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Investigation to find the water potential of a plant cell.

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Introduction

Investigation to find the water potential of a plant cell. Introduction: - Theory: Plant cells are made up of a phospholipid bilayer (fig. 1). There are hydrophilic heads, which face the liquid medium in and around the cells. A hydrophobic tail attached to the head, which face inwards. These tails are non-polar. Non-polar molecules like oxygen can diffuse through the membrane easily. Water, despite being a polar molecule, can also diffuse rapidly across the membrane because it is so small. Big polar molecules and ions can not pass straight through the membrane, e.g. Na+, Cl- and glucose. These molecules can only pass the membrane through protein channels, which only allow specific molecules in. (fig. 1) Diagram of cell membane. 7nm phospholipid width bi-layer. Water molecules posses kinetic energy in that, when together in liquid or gas form, they move about very rapidly in random directions. When water is surrounded by a membrane the water molecules will tend to hit it as they move randomly, and in doing so, they will generate pressure (fig.2). This pressure is known as water potential. The more times the water molecules hit the membrane in a unit of time then the higher the pressure, i.e. the higher the water potential. (fig. 2) Diagram showing Water potential. One definition for water potential could be 'the tendency for water molecules to move from one place to another- with or without a net movement'. Water always moves from an area of high water potential to an area of low water potential, meaning it always moves down its water potential gradient. ...read more.

Middle

Fig. 5 shows what this graph may look like. We will change the solutions around by adding concentrations of either different amounts of distilled water to a 1 molar solution or different concentrations of solvent to distilled water. This means that it will be easier to plot the graph and find the exact water potential. (fig. 5) A table to show the variables that must be controlled. Variable. Why it must be controlled. How it will be controlled. Temperature. At higher temps., the water molecules move faster than at lower temps., i.e. diffusion will take place quicker I shall do the experiment at room temp., so if there is a change in temp., all the specimens will experience the same change. Surface area. The more surface area there is the more area for diffusion to take place and the faster diffusion can occur. I shall cut the potato with a cork borer and cut the cyliders to the same length (nearest mm) so they all have the same suface area. Concentration gadient. The bigger the difference in concentrations, the more molecules pass through the membrane, which means a bigger net rate of diffusion. The concentration of the water and glucose solutions will be the variable I will change in the experiment. Cells water potential. Two different potatoes may contain cells with slightly different water potentials, which could give anomolous results. I shall take all my potato samples from the same potato. Measurements. Wrong measurments of the different molar solutions could mean the osmotic potential will be wrong and my results will not be accurate. ...read more.

Conclusion

This may be because I only left these samples in for ten minutes rather the thirty that I will do in the experiment. I found using the cork borer a lot easier to use than I thought it would be, but cutting the cylinders with the scalpel to exactly the right length was hard. The concentrations that I have used are good and easy to make up. However, I feel they do not give me a wide enough range for the experiment, so I will use the following, 0, 0.2, 0.4, 0.6, 0.8 and 1 molar solutions. I think at least three tests will have to be done to get an average, if not more, so that anomaly can be recognised like the one in this preliminary investigation (highlighted). A table to show how the new concentrations of the solutions can be made End concentration. (Mol dm) % of distilled water. % of 1M glucose solution. 0.00 molar solution 100 (20ml) 0 (0ml) 0.20 molar solution 80 (16ml) 20 (4ml) 0.04 molar solution 60 (12ml) 40 (8ml) 0.06 molar solution 40 (8ml) 60 (12ml) 0.08 molar solution 20 (4ml) 80 (16ml) 1.00 molar solution 0 (0ml) 100 (20ml) Safety precautions: - * Care most be taken with glasswear to prevent any breakages. * Spillages of the solutions must be mopped up quickly. * Care must be taken when using the sharp cork borer by using it on a hard surface like a tile. * Care must be taken when using the sharp scalpel, and also using it on a hard surface like a tile. * Attention must be given to the experiment at all times so that optimum results are obtained safely. ...read more.

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