The Cell Membrane

The Cell Membrane

Both the cell surface membrane and the membranes surrounding certain organelles have the same basic structure. Much of the membrane is made up of a 'sea' of phospholipids with protein molecules 'floating' in between the phospholipids. Some of these proteins span the whole width of the membrane.

Because the membrane is fluid, and because of the mosaic arrangement of the protein molecules, the structure of the membrane is called the fluid mosaic model.

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The phospholipids are arranged in two layers (a bilayer). The phosphate heads are polar molecules and so are water-soluble. The lipid tails are non-polar and therefore are not water-soluble.

This means that the phospholipids are arranged with the heads in contact with the cytoplasm or extra-cellular fluid, both of which are watery environments. The tails are protected from this, by being as far from the cytoplasm and extra-cellular fluid as possible.

The proteins in the membrane, line pores in the lipid bilayer. The polar groups of the protein molecules mean that substances that would not be able to penetrate the lipid bilayer, (because they are insoluble in lipid), can still move from one side of the membrane to the other.

There are also short polysaccharide chains that are attached to the outer surface of the membrane. Most of these carbohydrates are attached to proteins and are called 'Glycoproteins'. They may help in the recognition of, and interaction with, other cells. They may also play a part in the recognition of hormones and foreign molecules.

Cholesterol is also present in the membrane. It maintains the fluidity and increases the stability of the membrane. Without cholesterol the membrane would easily split apart.

Functions of a membrane it's:

  1. Selectively permeable barrier.

  2. Structural, keeping the cell contents together.

  3. Allows communication with other cells.

  4. Allows recognition of other external substances.

  5. Allows mobility in some organisms, e.g. amoeba.

  6. The site of various chemical reactions.

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