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

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Discovering the Lowest Concentration Of Lead Ions Needed to Cause the Loss of Partial Permeability of Red Onion Cell Membranes Aim: To discover the lowest concentration of lead ions needed to cause the loss of partial permeability of red onion cell membranes. Prediction: I predict the lowest concentration of lead ions to cause the loss of partial permeability in red onion cell membranes to be 0.4M Background Knowledge Lead ions reduce the partial permeability of cell membranes. The principle mechanism by which lead does this is by reduction in pore size and by reducing the solubility of the phospholipid bilayer, the principle material from which cell membranes made. Pores are proteins which allow uninhibited access to water molecules to and from the cell. These proteins contain thiol groups (-SH), to which lead ions have a particular affinity. A reduction in solubility reduces the permeability of the cell membrane to water; a less polar phosphate group on the head of the phospholipid repels the polar water molecules. ...read more.


This slightly moist red layer should be cut up into 6 small pieces (around 5mm2 should be sufficient) with the scalpel. 4. Immersion in Distilled Water (5 minutes), and Lead Nitrate (8 minutes) Immerse each membrane in water for 5 minutes to ensure each cell is turgid. Put a piece of membrane in the evaporating dishes of lead nitrate for 8 minutes (use the stopclock). The 0M lead nitrate solution will act as the control for the experiment. 5. Immersion in Glucose Solution (2 minutes) Convey the membrane into the glucose solution for 2 minutes (stopclock). 6. Check for % Plasmolysis by Microscopy Put a membrane under the microscope. Align the cells with the graticule, and count how many cells on the graticule line have plasmolysed, and how many have not. Convert to a percentage as below, and record in the table. No. Plasmolysed * 100 = Percent Plasmolysed Total No. Of cells Repeat for each of the other membranes, making sure not to mix them up. ...read more.


Table of Results (to be filled in) 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. Risk Assessment Lead Nitrate is toxic. Care should be taken so that it does not come into contact with skin. Avoid swallowing any. Scalpels are extremely sharp. Take care when cutting the 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. ...read more.

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