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Investigation of the Influence of a Variety of Treatments On the Permeability of Plasma Membrane.

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INVESTIGATION OF THE INFLUENCE OF A VARIETY OF TREATMENTS ON THE PERMEABILITY OF PLASMA MEMBRANE SECTIONS A and B Planning Sections A & B would have included an introduction and hypothesis A INTRODUCTION The current model for plasma membrane structure is the Fluid Mosaic Model (JS Singer and G Nicholson,1972, ref 1) This postulates that plasma membrane consists of a phospholipid bilayer with a hydrophobic non-polar core and a hydrophilic polar exterior. Embedded in the phospholipid bilayer is a mosaic of intrinsic proteins, some of which traverse the entire membrane. Extrinsic proteins may also be found attached to the hydrophilic surfaces. The proteins float in the plane of the membrane as the phospholipid bilayer is fluid. Lipids are water-soluble biomolecules that are highly soluble in organic solvents. The plasma membrane acts as a barrier between two aqueous environments because the hydrophobic bonds between the fatty acid tails exclude water. However, small, uncharged molecules such as O2, H2O, CO2 and lipid soluble molecules can diffuse between the phospholipids because fatty acid structure prevents tight packing. Larger molecules require the assistance of carrier proteins which traverse the membrane. Thus a plasma membrane is described as being partially permeable. The plasma membrane's partial permeability depends upon the phospholipid bilayer and protein structural integrity. Transport across a plasma membrane occurs by a number of mechanisms including diffusion, facilitated diffusion, active transport, osmosis and endocytosis and exocytosis. ...read more.


Distilled water at room temperature can diffuse across a plant plasma membrane into the cell by osmosis until the cell is turgid but will not affect transport of large molecules. Similarly a concentrated sucrose solution will result in osmosis out of the cell until plasmolysis if left long enough and if concentrated enough but will not affect transport of large molecules to a great extent. Pigment release into these solutions should be similar and depend on carrier protein activity and therefore they should act as controls. It should be noted, however, that there is evidence (Crowe etal, 1984, 1987,refs 2 & 3) that sucrose and other sugars enable membrane structure to be maintained helping to retain trapped solutes inside dehydrated cells during rehydration. Hydrogen bonding of sugar hydroxyl groups with phosphate on polar phospholipid headgroups is believed to increase spacing between the phospholipids reducing Van der Waals interactions between acyl chains. Therefore, molar sucrose may affect permeability. Ethanol is a small uncharged molecule (C2H5OH) and can diffuse across plasma membrane. It has been shown to affect plasma membrane structure by both altering acyl chain packing of the lipid bilayer and by denaturing protein (Mitchell D and Litman B, ref 4). Ethanol competes with water for hydrogen bonding sites on the surface of both lipids and proteins. Therefore, it can reduce and, at high enough concentrations, destroy the integrity of the plasma membrane. ...read more.


Using a larger diameter cork borer and a finer knife (e.g. scalpel rather than kitchen knife) could reduce the error. b) Cuvettes were not very clean. Since light transmission was being measured, the appropriate faces should have been cleaned just before measurement with a lens tissue. c) Timing was fairly approximate. The pieces of beetroot were added to the treatment tubes and final solutions removed in the same order but the immersion time may have varied. This could be solved by doing a test run for the time taken for each procedure and timing each immersion period exactly. d) No replicates were done. This was the most serious limitation as it has not been possible to interpret most of the anomalous results meaningfully. Between three and five repetitions are desirable. 3. Extensions of the Experiment a) The effect of temperature change Transmission of light would be measured after immersion of standard sized pieces of beetroot in distilled water at temperatures decreasing by 10oC from 100oC to 30oC and including room temperature. The boiling tubes would be immersed in a thermostatically controlled water bath for each temperature. The thermal death point would be identified from a plot of data. b) The effect of pH change at constant temperature ( ~ room temperature) Using different dilutions of HCl and NaOH to identify the point at which proteins are denatured. c) The effect of different concentrations of ethanol Establish where cell death occurs by observing point on plot of results where pigment release accelerates. ...read more.

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**An incomplete piece of work with many of the expected sections missing. The data collected is invalid in some cases and the interpretation of the results difficult due to the large number of variables being altered and the lack of replicates.
Research and Rationale
There is a good description of membrane structure and the writer has used some non standard A level references although they are not particularly current. This section could have been helped by focusing on the variables to be changed in the experiment and predicting the likely outcome more concisely.
No planning section has been included. Planning is the single most important criterion in determining the success of an investigation. Important omissions in this section can often place severe limitations on a student

Marked by teacher Stevie Fleming 22/08/2013

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