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Aim To determine the water potential of a potato tuber cell

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Introduction

Aim To determine the water potential of a potato tuber cell Fig 1- picture of a plant cell Introduction Osmosis is the movement of water through a semi permeable membrane, separating solutions of different concentrations. The water passes from a region of high concentration to a region of low concentration, until the two concentrations are equal in concentrations of water. Many cell membranes behave as semi permeable membranes, and osmosis is a vital mechanism in the transport of fluids in living organisms, for example, in the transport of water from the soil to the roots in plants. If a cell is in contact with a solution of lower water concentration than its own contents, then water leaves the cell by osmosis, through the cell membrane. Water is lost first from the cytoplasm, then the vacuole through the tonoplast (vacuolar membrane). (Fig 1) The living contents of the cell contracts and eventually pulls away from the cell wall and shrinks, this is known as Plasmolysis. If you put a plant cell in water, water enters by Osmosis, and then swells up. However, the cell will not burst. This is due to the fact that the cell walls are made from cellulose, which is extremely strong. Eventually, the cell stops swelling, and when this point is reached, we say the cell is turgid. This is important, because it makes plant stems strong and upright. The formula for water potential is Water potential = osmotic potential + pressure potential Preliminary work For my preliminary work, I cut 6 potato cylinders by using the cork borer. They are all the same length and mass. Then I placed one potato cylinder into each test tube with different concentration. After 24 hours, I re-weight it and record the result. Here is a table of my preliminary result table. Concentration Initial mass of the potato cylinder The mass of the potato cylinder after 24 hours Percentage of mass loss 0.0M 2.8g 3.3g +17.8% 0.2M 2.8g 3.0g +7.1% 0.4M 2.8g 2.7g ...read more.

Middle

I found that the potato cylinders put in high concentrations of water, became fat and firm, while the potato cylinders put in low concentrations of water became soft and thin. In my results I have identified several features. Every sample of potato that I used in my experiment followed the water potential gradient by either loosing or gaining weight. For example, as shown in the table, the most concentrated sucrose solution - which in this case was 1 mole, more weight was lost. Evidently this is due to the fact that the concentration of the external solution was higher than that of the internal solution of the potato cells and therefore the water moved from the region of higher water potential to the region of lower water potential through a partially permeable membrane by osmosis. In the beaker containing the least concentrated sucrose solution osmosis also occurred, however the opposite happened. The potato sample gained weight. The internal solution was of a higher concentration than the external solution and so the water moved down the water potential gradient - hence into the potato cell. The cell became fully plasmolysed over the 24 hour period in which the investigation took place. Plasmolysis occurred in the experiment with the 0.6, 0.8, 1.0 mole concentration of the external solution due to the fact that water was leaving the cell by osmosis because the cell was placed in a solution of a lower water potential. The protoplast of the cell gradually shrunk and began to pull away from the cell wall. This process is called plasmolysis and only occurs in plant cells. When a plant cell is plasmolysed the protoplast shrinks away from the cell wall, the external solution has passes through the cell wall and becomes in direct contact with the shrunken protoplast. As you can see from the graph and the results show a clear negative correlation, a very obvious inversely proportional trend. ...read more.

Conclusion

Nevertheless there were several aspects of my investigation that I would definitely change if I ever were to re-do this investigation in the future. My accuracy of observations and noting down any other significant information down efficiently during the experiments could be improved. I did not record the actual type of potato I used in my investigation, this was important as there are hundreds of types of white potatoes. Due to the fact that there were several sources of error in my investigation there are uncertainties in my results and therefore also uncertainties in the validity of my conclusions. However despite the possible improvements my results justify my prediction, which was based on scientific knowledge. Before carrying out the investigation I predicted that the water potential of the potato tuber cells would be within the range of minus 1280 kPa and minus 260 kPa. My prediction was correct and my results justify this despite the accuracy factors that could be improved. This shows that the uncertainties in the evidence I collected are not a significant problem; in fact they did not affect the validity of my results at all. Reliability: according to my results I was able to get clear results to make statements, which proved my predictions correct. To show how reliable my experiment was, I have drawn a table to display the concentration of sucrose solution and the mass of the potato after 24hours. This table will help me realise how clear this experiment is in reliability. Mass of Potato cylinders after 24 hours (g) Concentration of Sucrose solution (mol dm-3) Set 1 Set 2 Reliability 0.0 2.00 2.00 Very reliable 0.2 1.90 1.90 Very reliable 0.4 1.45 1.45 Very reliable 0.6 1.20 1.40 Not very reliable 0.8 1.00 1.10 Quite reliable 1.0 0.85 0.81 Quite reliable This table show us that there is a result that is not reliable and all the other results are really reliable which tell us this experiment was done well as the two sets of result is quiet reliable. Reference Fig1 - http://www.infovisual.info/01/001_en.html - Fig 2, 3-http://images.google.co.uk/images?q=osmosis&ndsp=20&svnum=10&hl=en&start=0&sa=N Fig 4-- http://books.google.com/books?id=OT-O2DJXrMwC&pg=RA2-PA36&lpg=RA2-PA36&dq=table+of+molarity+and+water+potential&source=web&ots=x7gYCTbDOr&sig=wZL7LEd7HqXp6esGrDeQAfSBTRw - page 36 ...read more.

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