Beetroot pigments

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Assessed Practical Plan

     I plan to investigate the effects of varying temperature on the movement of pigment molecules across a membrane, travelling out of beetroot cells.

Aim:

     The aim of this investigation is to determine the effect that temperature has on the rate of diffusion of beetroot pigments when slices of beetroot are placed in various temperatures of water.  

Background Knowledge:

     The pigments found in beetroot are known as betalain pigments. Some areas of a beetroot may contain more pigment molecules than others. These molecules appear to be polar molecules as they are soluble in water. Molecules which have groups with dipoles are said to be polar. They are usually attached to water molecules. They also have dipoles, and these molecules are hydrophilic, and tend to be soluble in water.

     The process by which the betalain molecules leave the cell, can be explored by considering the following:

  • Simple Diffusion
  • Facilitated Diffusion
  • Osmosis
  • Bulk Transport
  • Active Uptake

     We can eliminate the possibility that betalain pigments travel by osmosis, since osmosis only involves the movement of water molecules, whereas in this experiment we are researching the movement of beetroot pigment molecules. Also, the possibility of the betalain pigments travelling through the membrane by active uptake or bulk transport can be discarded, as both of these processes require energy and ATP in moving molecules against their concentration gradient. Furthermore, simple diffusion is the movement of molecules through phospholipids. Therefore, this means the molecules must be lipid soluble and non-polar, and the beetroot molecules are not. However, facilitated diffusion is the movement of molecules through a membrane by protein channels, and since transport proteins have hydrophilic interior, hydrophilic molecules are able to pass.    

     Facilitated diffusion is the movement of molecules or ions from a region of high concentration to a region low concentration. This results in the molecules travelling down the concentration gradient until equilibrium is reached; where the molecules or ions are evenly distributed.  Diffusion is a passive process and it does not require metabolic energy.  Diffusion can occur from one part of a fluid to another or across a membrane separating two fluid regions.  It is this that is relevant to the experiment; exploring diffusion across the phospholipid bilayer of the cell’s plasma membrane.  

     The cell of a membrane is a double layer, a bilayer, of phospholipid molecules. The phospholipid tails point inwards, facing each other and forming a non-polar hydrophobic interior. The heads face the aqueous medium that surrounds the medium. Transport proteins have hydrophilic interior for ions and hydrophilic molecules to be able to pass. They control what enters or leaves the cell.

  The structure of a membrane is shown clearly in this picture taken from the website

   

  The cell of a membrane is a bilayer of phospholipid molecules. The phospholipid tails point inwards, facing each other and forming a non-polar hydrophobic interior. The heads face the aqueous medium that surrounds the medium. Transport proteins have hydrophilic interior for ions and hydrophilic molecules to be able to pass. They control what enters or leaves the cell.

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     Facilitated diffusion occurs when large molecules and charged ions cannot pass freely through the cell’s lipid-based plasma membrane and result in carrier proteins binding to a diffusing molecule. This binding, produces a change in the shape of the carrier that allows the molecules to pass through the protein and across the plasma membrane, being released at the other side.  

         The effect of temperature on a membrane is similar to that of an enzyme, in that they both denature at high temperatures. The protein pumps allow any molecule through as they lose their specific ...

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*** A good description of the experimental plan including results from a preliminary experiment. However, the author does not appear to have a clear grasp of variables and the background information about phospholipid membrane structure could be more detailed.