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4. Scientists' models Link to the Medical Research Council web site
 
The cell membrane
All cells are surrounded by a cell membrane. The structure of the cell membrane is shown in Figures 7 and 8, which illustrate the fluid mosaic model.

The cell membrane is formed from a lipid (primarily phospholipid) bilayer. This is interspersed with a variable number of cholesterol molecules. Cholesterol influences the fluidity of the membrane.

Figure 7. The lipid builayer.

Figure 9. Computer model of GnRH-R receptor protein.

What's in the layer?
Floating amongst the phospholipids are protein molecules, some embedded in the inner or the outer lipid bilayer (extrinsic proteins), many spanning the whole membrane (intrinsic proteins).

Most of the protein molecules and many of the lipid molecules have short carbohydrate chains attached to them on the outer surface of the cell membrane, forming glycoproteins and glycolipids.

Figure 8. The lipid bilayer in 3D.
What does it do?
A key function of the cell membrane is to control the passage of substances into and out of the cell, most of the intrinsic proteins acting as channels or carriers. Other proteins are enzymes or receptor molecules. Certain glycolipids and glycoproteins play a role in cell recognition, acting as antigens, whilst others are receptor molecules for neurotransmitters and hormones. Glycolipids and glycoproteins also help stabilise membrane's structure by forming hydrogen bonds with water molecules outside the membrane.
Figure 9. The toilet roll model of the GnRH-R receptor protein.

What sort of models do scientists use?
Figure 9 shows the structure of the GnRH-R receptor protein within the lipid bilayer.

The first model for GnRH-R that was built by Karin Eidne and her team was described as a 'toilet roll' model (see Figure 10). A computer model has also been used. Figure 11 shows a view looking down on the cell membrane of part of this model. The sections of protein which are visible are those within the cell membrane. They form a-helices within the cell membrane. Outside the cell membrane they don't appear to form a-helices.

Figure 6. Click here to enlarge image in a new window.

Figure 10. The positions of the alpha helices in the outer and inner layer.
Click here to enlarge image in a new window.
Build your own model

You can build your own toilet roll model.

You will need:

  • two sheets of card (A4 size)
  • either seven of the cardboard tubes from toilet rolls or several kitchen roll tubes
  • some pipe cleaners to represent the protein - these look better if you can find coloured ones
  • some glue

What you do:

1. Use the two sheets of card for the inner and outer surfaces of the cell membrane.

2. Take measurements from Figures 6 and 11 to mark the position of the a-helices on the A4 card. Mark and carefully cut holes at these points to allow insertion of the toilet rolls.

3. Now glue the toilet rolls between the two sheets of card.

4. Join sufficient pipe cleaners together to represent the protein chain and weave this to and fro through the toilet rolls (use Figures 6 and 11 to guide you). You might try to put helical sections inside the toilet rolls.

Question 6

Explain why the sections of the GnRH-R protein lying within the cell membrane form a-helices.

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