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Discovering the Lowest Concentration Of Lead Ions Needed to Cause the Loss of Partial Permeability of Cell Membranes.

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Introduction

Discovering the Lowest Concentration Of Lead Ions Needed to Cause the Loss of Partial Permeability of Cell Membranes Aim: To discover the lowest concentration of lead ions needed to cause the loss of partial permeability of cell membranes. The cell membrane I will use to conduct this experiment will be the outer fleshy leaves of red onions. Background information The plasma membrane of living cells is a partially permeable membrane, and osmosis is the principal mechanism by which water enters and leaves plant and animal cells. Whether the net direction of movement is into or out of the cells depends on whether the water potential of the cell solution is more negative or less negative than the water potential of the external solution. When the external water potential is the same as that of the cell, there is no net movement of water in any direction. When the external water potential is less negative than that of the cell (it is surrounded by a more dilute solution), there is a net movement into the cells. The cytoplasm becomes distended by water uptake; if this distension causes pressure against a cell wall the cell is described as turgid.

Middle

supply of 1M glucose solution A 10 ml syringe A stopwatch Microscope Red onion Scalpel Ceramic Tile A beaker Slides and Coverslips Forceps 1. Preparing Lead Nitrate Concentrations: Six concentrations of lead nitrate should be prepared, 0.2M, 0.3M 0.4M 0.5M, 0.6M and 0.7M. To do this, a supply of 1M lead nitrate is needed. Then, using a syringe to measure amounts, the concentrations above should be prepared according to the table below. 20ml of each concentration of lead nitrate should be should be put into a separate, clean, evaporating dish. Clean them if they have already been used once. Only use enough to cover the evaporating dish, so that a piece of membrane can be completely immersed. Concentration 0M 0.2M 0.3M 0.4M 0.5M 0.6M 0.7M Amount of Lead Nitrate 0 ml 4 ml 6 ml 8 ml 10 ml 12 ml 14 ml Amount of Distilled Water 20 ml 16 ml 14 ml 12 ml 10 ml 8 ml 6 ml 2. Preparation of Onion Membrane: Cut the red onion in half on the ceramic tile using the scalpel. Then peel away the dry outer layers of skin, then the dark red layer using forceps if necessary, make sure it is dark red but not too dry.

Conclusion

Care should ensure that this membrane is only 1 cell thick How data will be presented: Lead Concentration (M) After 2 minutes submersion in glucose solution After 2 minutes submersion in distilled water 0 0.2 0.3 0.4 0.5 0.6 0.7 The percentage of plasmolysed cells in each case should be recorded in the table above. Graphs can easily be plotted (see sample below), to analyse data. This graph will enable me to see clearly the critical lead concentration. Safe testing Lead Nitrate is toxic so extra care should be taken so that it does not come into contact with skin. Scalpels are extremely sharp so care should be taken when cutting onion. Reliability Reliable results are extremely important. Make sure that a new and uncontaminated glucose solution is used for after each bathing of the membranes, as lead could get into it and will build up as the experiment progresses. Although it is not critically important, make sure that each piece of membrane is around the same size. Any difference should be negated by the fact it is one cell thick, however. Ensure that all times are well kept to. Do not run over, especially in the glucose solution and distilled water, otherwise the percentage plasmolysed cells would be altered.

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