Overview
The plasma membrane separates the cell from its surroundings and is important in controlling what enters and leaves the cell. For this reason much attention has been focused on its structure.
The plasma membrane contains lipids. Lipids are fat molecules of which there are many kinds. Cell membranes consist of 2 layers of a type of lipid called phospholipids. These consist of a hydrophobic (water hating) hydocarbon "tail"and a hydrophilic (water loving) glycerol "head". Although in the body, the cell wall is surrounded on both sides by water, the non-polar ends (hydrophobic) are attracted to each other.
These lipids are held together by a protein framework randomly dotted on both sides of the membrane in a mosaic pattern. These proteins are attached to the surface of the membrane while others penetrate into it to varying extents; some of them extend right through it and stick out on the other side.
This membrane is a fluid structure which has led to this model of a cell membrane to be called a fluid-mosaic model.
Membrane Proteins
The proteins are of two types Integral ( passing through the width of the membrane ( act as transport channels) and peripheral ( act as recognition sites ). These peripheral proteins contain carbohydrates to help in cell to cell recognition. These carbohydrates are called oligosaccharides. I f they bind to proteins they become glycoproteins, if they bind to lipids , they become glycolipids. Cholesterol molecules are found in animal membranes to help add support to its structure. The majority of the phospholipids contain unsaturated fatty acids to keep it fluid.
