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To find out the internal concentration of a potato cell.

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Introduction

Osmosis in Potato Cells Coursework Plan Aim: To find out the internal concentration of a potato cell. Hypothesis: Osmosis is defined as the diffusion of water across a partially permeable membrane "from the dilute to the concentrated solution" (Mackean GCSE Biology textbook). There are more water molecules in a dilute solution than in a concentrated solution so the diffusion gradient favours water passing across the membrane from the dilute to the concentrated solution. A concentrated solution e.g. sugar solution has a lower concentration of water molecules because the glucose molecules attract some of the water molecules when they dissolve in the water, preventing them from moving freely. Therefore there are more free water molecules in the dilute solution which can move through to the concentrated solution by osmosis. Sugar molecules are larger because the water molecules cluster around them, so they move more slowly and some may pass through the membrane pores slowly in the other direction. The diagram below shows osmosis across a partially permeable membrane: (diagram from Mackean GCSE Biology textbook) The water molecules actually pass both ways through the membrane but because there are more on one side than the other, there is a steady net movement of water molecules flowing from the weaker solution to the stronger solution by osmosis. The more concentrated sugar solution fills up with water molecules, which act in an attempt to dilute it, until there is an equal concentration either side of the membrane. ...read more.

Middle

Potato skin will be removed because it is not a partially permeable membrane, so osmosis cannot occur across it. If the potato chip floats to the surface of the solution in the boiling tube, this will be noted, because less surface area will be exposed to the solution. The concentration gradient is the variable. The size of potato molecules may vary but this is difficult to control, although the same species of potato will be used throughout the experiment. The same type of sugar (glucose) will be used in the solutions for all the boiling tubes of differing sugar concentrations. The reliability of evidence will be improved and anomalies will be reduced by repeating the results and obtaining the average for each result. Conclusion: The graph shows the general trend that as the concentration of the external sugar solution increases, the percentage change in the masses of the potato chips decreases, so that it becomes negative. This negative correlation shows that the concentration of the external solution is inversely proportional to the percentage change in mass, which is the same pattern as the sketch graph I predicted in the hypothesis. In my prediction I stated that "where the concentration of the external sugar solution is weaker than the internal concentration of the potato chip... the chip will gain in mass" and that "where the concentration of the external solution is stronger than the internal concentration... it will lose mass". My results support this prediction because the chips submerged in 0.2M solution showed an average percentage increase in mass of 2.01%, whereas the chips submerged in 0.4M solution showed an average percentage decrease in mass of 0.68%. ...read more.

Conclusion

If the potatoes used were of different ages, this would affect the results because the aging process may reduce the cells ability to absorb and retain water. This problem could be overcome by making sure the cylinders used were cut from the same potato. The potatoes used in my experiment were all from the same species of potato. The size of potato cells may also vary but this factor is difficult to control. Although the evidence obtained in this experiment is sufficient to support my hypothesis, it is not enough to prove it. To provide additional evidence for the conclusion, it would be necessary to extend the enquiry. A further investigation could be conducted to find out if the internal concentration of a potato cell is indeed 0.34M, and to see if this internal concentration is the same for different types of potato cells. This could be done by carrying out the following experiment: Aim: To find out if all types of potato cells have an internal concentration of 0.34M. Diagram: Method: Cut out potato cylinders of equal circumference, from different species of potato, using the cork borer. Use a guillotine to cut all the potato chips to a length of 4cm, so the surface area is the same for each. Dry all the chips using a blow-dryer and weigh on a balance. Place the chips in test-tubes filled with 0.34M glucose solution and leave them in the solution for 30 minutes. Remove the chips, dry them again using the blow-dryer and reweigh. Calculate the percentage change in mass if any has occurred. If there is no change in mass, the internal concentration of that type of potato cell is 0.34M. ...read more.

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