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Discover how potato pieces are affected when placed in varying concentrations of sucrose solution and to find the concentration of cell sap in a potato piece.

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Introduction

Osmosis Coursework Osmosis is the net movement of water particles through a selectively permeable membrane, which is so called because it lets certain molecules through and not others depending on their size. The water particles may pass through but dissolved substances cannot. Osmosis occurs when there are two different levels of concentration of solution either side of the membrane. Water will move from a high concentration of water to a strong concentration, from the weak solution to the strong. It is a passive incidence meaning it does not need energy from respiration to take place. Energy comes from movements of particles. Aim: The main aim of this investigation is to discover how potato pieces are affected when placed in varying concentrations of sucrose solution and to find the concentration of cell sap in a potato piece. Hypothesis: When the concentration of water is lower in the potato tissue, the water from the sucrose solutions will enter the cell, meaning the potato piece gains mass. This occurs due to the way water particles in higher concentration gradients diffuse into areas of low concentrations. This happens in hypertonic conditions where the salts are more concentrated than water. The particles will continue to diffuse through the semi permeable membrane of the potato piece until a concentration is reached where water molecules are in equilibrium. This is where the amount of water entering and leaving the cell are equal, thus the potato neither gains nor loses mass. Here it is possible to estimate the concentration of cell sap. However, I predict that as the concentrations of sucrose solution gradually become higher, because concentrations of water will become higher in the potato tissue, the plant tissue will lose water as they diffuse into the lower concentration of water in the sucrose solution. If the concentration of sucrose solution doubles, then the amount of water in the potato cells will decrease by half. ...read more.

Middle

sucrose 1.946 1.074 -0.872 +44.817 Graph: Because averaged results are more accurate, these are the figures I will be working with to draw my graph. A Line Graph to Show Changes in Mass of Potato Cylinders Conclusion: The results that I obtained support my hypothesis. I found that the potato cells took in and let out water particles at varying degrees depending on how concentrated the solution was. All three sets of data follow the same pattern of mass loss and gain as concentrations got higher. However, my prediction that the amount of mass gain or loss would be directly proportional to change in concentration was incorrect. When 50 millilitres of 0.4M sucrose solution was doubled to become 50 millilitres of 0.8M sucrose solution, the decrease in the mass of the potato cylinder was not doubled, -0.162 to become -0.872. This is a decrease of over 538 %. However, if I calculate the mass change from 0.2M to 0.4M sucrose solutions I find that there is a decrease of 570%. This therefore indicates to me, that although the mass does not change in direct proportion, it does change in indirect proportion. My graph shows a curved line of best fit which slopes downwards as the concentrations get higher. This proves the trend that the higher the concentration, the more water particles are lost from the potato cylinders. The gradient of the line is not constant. This is because the two ends which curve slightly are at the two extremes, completely water with distilled water, and then a very high concentration of sucrose solution. At the end with a very high concentration the cells have to become as flaccid as possible so the change in mass of each molar concentration becomes close together with each increase. At the distilled water end, the line seems to tail off. There appears to be no further water gain which suggests that a gain of 0.146 grams is close to the maximum a potato cylinder of this size can let in. ...read more.

Conclusion

It would also make anomalies more apparent so that I can be more certain that the data is correct and accurate. To make this experiment more exact then each concentration and each individual test tube could have been tested at a time so that the time it takes to get them out of the solution, dry them and weigh them is cut down. However, if this happens then they must also be put in later to ensure that the potato cylinders have been in immersed in the solution for the same amount of time. If the cylinders are not then some get less time to absorb and let out water particles so the change is less. During the experiment, human error may have caused further inaccuracies when weighing and drying the potato pieces. The scales may not have been set to 0 prior to the experiment. This would not have made a difference to the overall conclusions as it would be consistent throughout. However, it is impossible to account for different scales being used. This distorts the raw data. When cutting the potatoes into equal lengths I found that often they were cut lopsided so some cylinders got more potato than others meaning some have more surface area to absorb water than others, so there is a larger or smaller change in mass compared to other sized cylinders, again, affecting reliability. To improve on this a machine could be found to cut the potato into pieces with no angle at the ends. Using a measuring cylinder to measure the amount of distilled water or sucrose solution may not have been very accurate because sometimes I was in a rush and may not have looked very carefully at the amount. Therefore this increases the amount of miscalculation, hence increasing the dependability of the experiment. Also, when the liquid is poured into a beaker there is always approximately a millilitre remaining. However, because this would happen each time, it is not particularly worrying. To ensure accurate measurements, a burette could have been used. ...read more.

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