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

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Pamela Jones 09/05/07

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.  Charged molecules require ion channels for transport.  Facilitated diffusion of a solute molecule uses a carrier protein by binding and causing a structural change in the protein such that the solute molecule can be discharged on the other side.  Facilitated diffusion occurs down an electrochemical gradient and therefore requires no input of ATP unlike active transport which requires energy input in order to change the carrier protein’s conformation and allow a solute molecule to be transported against a concentration gradient.  

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Beetroot pigments are water soluble anthocyanin derivatives and are found in the cell vacuoles.  

They are large, positively charged molecules which would require a carrier protein to cross the plasma membrane.  Any alteration to protein structure would disrupt this process as protein function depends on precise conformation (1ary to 4ary structures).  Protein structure is temperature and pH dependent.  Protein tertiary and quaternary structures are dependent almost entirely on weak hydrogen bonds, ionic interaction and hydrophilic and hydrophobic interaction.  (The exception is the covalent disulphide bond between cysteine residues.)  Increasing temperature increases ...

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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. Planning 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