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Determining the Water Potential of Sweet Potato Tissue

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Introduction

Determining the Water Potential of Sweet Potato Tissue Introduction The aim of this experiment is to determine the water potential of sweet potato tissue using osmosis. This can be achieved by placing the samples inside different molarities of sucrose solution and work out the percentage change in mass and then with the aid of a conversion graph convert molarity to water potential (kPa), without the weight of the sweet potatoes being a factor. Background Knowledge In mature plant cells, the fluid filled vacuole occupies most of the cell volume therefore in order to determine the water potential of the sample I would need to work out the water potential of this fluid inside the cell. Substances can pass in and out of cells by four different processes: * Diffusion * Osmosis * Active transport * Endocytosis & exocytosis All these processes involve substances passing through the cell membrane of the cell. In this investigation, I only need to consider osmosis. OSMOSIS is the movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. This is a colligative property i.e. dependent on the concentration of particles in a solution. The water molecules involved always move down a ? gradient. It happens because of the natural kinetic energy possessed by the particles, which makes them move about at random. As a result, the particles tend to reach an equilibrium situation where they are evenly spread within a given volume i.e. the water potential of both regions is equal. The water molecules despite being very polar can pass rapidly through the phospholipid bilayer because they are small enough. It is a passive process, therefore dose not require ATP. SOLUTE POTENTIAL is the amount by which the solute molecules lower the ? of a solution. This value is therefore always negative and the symbol for solute potential is ?s. ...read more.

Middle

to prepare the 0.6M sucrose solution add 10 cm of distilled water to the correct bottle then using a separate pipette add 15 cm of sucrose solution to the same bottle. 4. Take a clean sweet potato and cut in half using a kitchen knife and wooden board. 5. Using the cork borer prepare twelve sweet potato cylinders. Cut the cylinders to 3 cm in length as accurately as possible, and make sure that there is no skin, using a scalpel and tile. 6. Blot each cylinder (to dry it as much as possible) and weigh each cylinder correct to two decimal places. 7. Place one cylinder in each bottle starting with the most concentrated sucrose solution and put the lid on. Make sure you have recorded which cylinder is in each bottle. 8. Record the time and leave the experiment for about eighteen hours. 9. Using tweezers take out each cylinder and place on paper towel starting with the most concentrated solution. 10. Thoroughly blot each cylinder dry, reweigh, and measure the length of each cylinder. Record the results correspondingly to the initial results. 11. Calculate percentage change in mass, change in mass per gram, and then the average change in mass per gram to allow effective comparisons. Calculate the average change in length in per mm. 12. Plot the average change in mass per gram and the average change per millimetre against the sucrose concentration. Risk Assessment * Sucrose dose not carry any risk therefore safety wear is not essential, however it is advisable to wear goggles. Clean any spillages of sucrose solution thoroughly as it can leave sticky patches on surfaces; it is best to place samples and apparatus on paper towels etc. * Cork borer should be used, by pushing in to the vegetable away from the body on a tile. Also, push out the cylinder at the cutting end of the borer using a dowel. ...read more.

Conclusion

The method I used had many limitations e.g. I could not control changes in temperature, pressure or light, for this sophisticated apparatus is required this would fully assure that these variables are constant. Where I wrote there was no change in length there may have been one but too small to measure. Therefore, I should have used a larger and more accurate scale to measure. I should have also calculated the change in volume by measuring the diameter after and work out the average change in volume. I could have changed my method e.g. used a dilatometer this has a narrow vertical tube which means that small volume changes produce measurable changes in liquid level e.g. if water entered the tissue then there would have been a decrease in volume of sucrose solution. I could have tested the final solution by looking at the colour intensity to work out most or least sucrose content. Alternatively, a colorimeter can be used to measure subtle differences in colour precisely. This passes a beam of light through a sample of solution in a cuvette a detector measures the deflection produced and presents it in the form of percentage transmission of light (however calibration is necessary). This equipment can be interfaced to a data logger or microcomputer, which enables a graph of concentration against time to be drawn quickly and efficiently by illuminating human error. This ensures accuracy and minimises significant sources of error e.g. little handling of the samples is required. I should have worked out the rate of diffusion by taking readings at timed intervals. This is more accurate and reliable. If there is a large concentration gradient then the rate of diffusion will be fast. I will need more readings to do further analysis effectively e.g. histogram. If I did a histogram with the current results, it will probably show nothing significant enough to base a conclusion on. Even though the points I have considered above do affect my results but do not cause doubts in the evidence and the validity of the overall conclusion. ...read more.

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Here's what a teacher thought of this essay

4 star(s)

A very good essay that demonstrates a way to conduct an experiment to determine the water potential of a potatoe. The writer goes through each step and accounts for what they are doing. Calculations are included and results.

There are a few areas that could be enhanced - such as increased use of terminology such as accuracy and precision. The results are a little difficult t interpret and the writer has referred to larger and smaller changes rather than using data.

Overall a good write up with a good structure and layout.

****

Marked by teacher Sam Morran 08/01/2013

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