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# Osmosis Coursework Aim: To determine the water potential of a potato tuba cell.

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Introduction

Osmosis Coursework Aim: To determine the water potential of a potato tuba cell. Water potential is the tendency of water to move from one place to another. It is usually measured in Pascal's. The more water molecules there are per volume of the cell the more likely that by random movement they will collide with the cell's plasma membrane, and travel out of it. Pure water can absorb no more water - it has a defined water potential of zero. Solutions, however, can absorb more water, because all solutions have negative water potentials. The stronger the solution, the more negative it's water potential. It is possible for the water potential to be positive or negative depending on the size of ?p or ??. The movement of water molecules is not totally random. The net movement of water molecules is always from a region of high water potential to one of lower water potential. They move down a water potential gradient until equilibrium is reached. Equilibrium is reached when the water potentials on both sides of the plasma membrane are the same Water potential (?w) = Solute potential (?s) + Pressure potential (?p) (Pictures form Google) Independent Variables My only variable in this investigation is the concentration of the sucrose solution. I will do this by using different volumes of 1 mol dm-3 sucrose solution, with different volumes of distilled water. To get it as accurate as possible I will be using a graduated pipette to get the method as accurate as possible. This method of dilution is the most accurate, as it allows me to chose a wider range of concentrations and there is less likely to be contamination or inaccuracies in the concentrations of sucrose. To make 1M of sucrose solution I need to make 1 mole of sucrose and add it to 1 litre of distilled water as shown in the equations below:- Number of moles of sucrose (n) ...read more.

Middle

It prevents the potato chip coming into contact with skin, and therefore grease. 100cm3 of 1 Molar sucrose solution This is the stock solution from which all other concentrations can be made. The volumes can be measured accurately in a syringe according to the 'dilutions' table. 100cm3 de-ionized water De-ionized water would provide more accurate results as tap water often contains chlorine, fluoride, sulphur, minerals and salts, especially calcium and magnesium minerals that make the water hard. Heavy metals such as copper, lead and iron also often contaminate tap water. 2 ? 250cm3 beakers It is difficult to transfer sucrose or water, using syringes, from a conical flask. Using beakers reduces the risk of spillage or breaking glassware whilst transferring solutions 2 ? 10cm3 pipette One for sucrose solution and one for water preventing contamination between solutions. 10cm3 is the volume of solution to be used. This is a precise way of transferring solutions; the instrument is very precise if it is ensured that there are no bubbles in the solution. It had a better degree of accuracy compared to measuring cylinders. 1 scalpel knife Enables the skin to be removed from the potato easily, can remove very thin slivers of potato making the length more precise and the surface uniform. 1 ruler Measures the length of the chip to the nearest 1mm only having �5% error. 1 white tile Provides hard surface that the potato can be cut on. Suitable background to observe variations in the tissue. Tweezers Enables potato to be held securely without skin contact, increasing accuracy of results. Petri dish (no lid) Protects the balance from the potato and solutions, also prevents the potato picking up debris, affecting the mass measured. Tissue paper Used to blot the sides of the potato chips meaning the mass is not affected by any remaining solution on the surface of the potato. Each chip is blotted once, making the effect on mass uniform across all chips Electronic scales Measures the mass of potato chips, in grams, to two decimal places. ...read more.

Conclusion

0.6M the average change in mass was -33.42% as the water potential was greater inside the cell because there are more water molecules inside then outside, so via osmosis there was be net moment of water towards the outside of the cell as osmosis is from a high water potential to a low water potential. Resulting in the protoplast shrinks and the plasma membrane pulls away from the cell wall. From this we notice that when the potato is placed in a solution with higher water potential, closer to zero, than the potato cells the potato will gain mass as water moves in by osmosis. They will lose mass when placed in one with lower water potential, as in both cases water molecules move down a water potential gradient because there are not at equilibrium. When they are at equilibrium it must mean that the water potential on each side of the cell membrane is the same and there is no net moment of water across the membrane, therefore the water potential of the potato tissue can be determined by balancing it with an external solution which produces no mass or volume change in the potato tissue. And in our case it was sucrose solution. We can find out at what concentration there is no change in mass and the potato cells are plasmolysis using the equation of the line of best fit. There is no change in mass when y=0. So the concentration at which there was no change in mass was 0.21M. So using the other graph about the solute potential against the morality I estimated that the water potential of the potato was -568kPa. As there is no change in mass and the cell is at plasmolysis and there is no osmosis talking place and there is equal water potential on both sides the pressure potential must be 0, so the solute potential must be -568kPa then the same must be the water potential. Water potential (?w) = Solute potential (?s) + Pressure potential (?p) Water potential (?w) = -568kPa + 0 Water potential (?w) = -568kPa ...read more.

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