Investigating the Movement of Water Into and Out of Plant Cells by Osmosis

Investigating the Movement of Water Into and Out of Plant Cells by Osmosis
The aim of this experiment is to investigate the movement of water into and out of plant cells by osmosis. The cells chosen for study will be taken from potato tubers as they provide a ready supply of uniform material. Background Information Any substance dissolved in water is called a solute; a solvent is a liquid that is able to dissolve another substance, (a solute), to form a solution. The water content of plants varies depending on environmental conditions. In land plants it plays a vital role in structural support and mineral transport and thus a lack of water may lead to wilting or possibly death. Water is mainly absorbed through the roots, which are covered in specially adapted root hair cells, with large surface areas and thin cell walls to aid absorption by osmosis. The evaporation of water through stomata on plant leaves causes a transpiration stream, causing the water to be drawn up through xylem vessels. Osmosis is the flow of water molecules by diffusion through a partially permeable membrane from areas of high water potential (low solute concentrations) to regions of low water potential (high solute concentrations). All plant cell membranes are partially permeable, which means they allow some substances to penetrate them but not others. Whether water enters the cell by osmosis will depend on the balance between external and internal solute and water potentials. If the solutions on each side of the partially permeable membrane are of equal water or solute potential, then there will be no net movement of water molecules across the membrane. This is called an equilibrium state and the solutions are referred to as being isotonic.
A solution that contains more solute particles than another, and hence has a low water potential, is referred to as being hypertonic, whilst the less concentrated solution is hypotonic. If a plant was exposed to a waterlogged environment, with the external solute concentration to the cell being hypotonic to the vacuole contents, the cell will not continue to take in water by osmosis forever. The cellulose wall provides a rigid barrier to uncontrolled expansion. A cell that is full of water is called turgid and cannot expand further as the inward force of the starched wall balances the outward pressure on the cell contents. This wall pressure is called turgor pressure and the internal outward force on the wall is called the osmotic pressure. At the other extreme, a cell placed in a solution that is hypertonic to its contents will lose water molecules by osmosis. The cytoplasm will cease to exert a pressure on the cellulose cell wall and the cell, described as flaccid, will lack support. Water loss can continue to such an extent that the cytoplasm, and attached cell membrane, contracts and detaches from the cell wall. A cell in this condition is said to be plasmolysed and this damage is irreversible. Safety notes ? Use care when working with glassware. ? Wash your hands before and after the lab. ? Use care when using any chemicals in the lab. ? Care will be taken when using the scalpel. ? All laboratory surfaces will be kept as clear and clean as is possible throughout the experiment. Results: Concentration of Sucrose (M) Before weight (g) After weight (g) (1) 0.60 3.79 3.53 (2) 0.60 3.59 3.34 (1) 0.50 3.69 3.74 (2) 0.50 3.91 3.75 (1) 0.40 3.82 3.99 (2) 0.40 3.79 4.02 (1) 0.30 3.89 4.11 (2) 0.30 3.81 4.18 (1) 0.25 3.66 4.01 (2) 0.25 3.74 4.03 (1) 0.20 3.68 4.12 (2) 0.20 3.66 4.02 (1) 0.10 3.89 4.33 (2) 0.10 4.02 4.89 (1) 0.00 2.76 3.64 (2) 0.00 3.68 4.48 The above results are the ?raw? results derived from all the individual experiments in question, i.e. both results from the same concentration. To be able to successfully use the results I have collected, I must find the average of the two in each case. The formulae for achieving this is as follows; Average of ?before weight? = (Before 1 + Before 2) / 2 Average of ?after weight? = (After 1 + After 2) / 2 Average Weights Concentration of Sucrose (M) Average before weight (g) Average after weight (g) 0.6 3.69 3.485 0.5 3.8 3.745 0.4 3.805 4.005 0.3 3.85 4.145 0.25 3.7 4.02 0.2 3.67 4.07 0.1 3.955 4.61 0 3.22 4.06 Percentage change. To be able to formulate an appropriate graph, the percentage change must be calculated; to achieve this I have to subtract the initial weight from the final weight, then divide by the initial weight; After weight ? Before Weight -????????????- Before Weight x100 Concentration of Sucrose (M) Average before weight (g) Average after weight (g) % Change 0.6 3.69 3.485 -5.55 0.5 3.8 3.745 -1.44 0.4 3.805 4.005 5.25 0.3 3.85 4.145 7.66 0.25 3.7 4.02 8.64 0.2 3.67 4.07 10.89 0.1 3.955 4.61 16.56 0 3.22 4.06 26.08 As previously stated, during osmosis water travels across cell membranes from a high solute concentration to a low solute concentration, and this is why the celeriac gained weight when in the higher concentration of sucrose solution. There is a certain point where there is NO net movement of water; this point is indicated on the graph by the point at which the trend curve crosses the concentration axis. Evaluation Generally speaking, the methods employed during this experiment would be considered appropriate & account for the consistency & apparent accuracy of the results obtained. To ensure fair & accurate testing, a primary concern was controlled whilst mixing the solutions. To avoid accidentally making two different concentrations for the same experiment, I made one large, single batch to be used so the concentrations were identical between the two instances of the same experiment. Unavoidable & uncontrollable errors which may have occurred could be the size of the celeriac cylinders; while the length can be controlled without problem the width cannot, and there is no guarantee that the corer I used produced identical widths every time. The ?dab-drying? method (for drying the pieces after the experiment) could also cause inaccuracies, as too much or too little excess liquid could be removed, therefore altering the results given. As I Previously stated, I took precautions to protect against having different concentrations for the same experiment, and although this was a perfectly sound precaution to take in itself, there is always the possibility that my one large batch was not the correct concentration; meaning that results in both experiments would be distorted.

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