Cell surface membrane
The cell surface membrane regulates the movement of substances across the membrane. It is about 7nm wide and is made up of a phospholipid bilayer which contains proteins, cholesterol, glycoproteins and glycolipids.
This is the fluid mosaic model of the cell surface membrane:
Image supplied by Alila Medical media. Click on the link to see more images of cells.
The fluid mosaic model gets its name from the fluid nature of the model, the proteins can move within the phospholipids. It is called a mosaic due to the pattern. The membrane has several different parts.
Cholesterol regulates the fluidity of the membrane. As more cholesterol is added the membrane becomes more rigid and less fluid.
Glycoproteins are proteins that contain a carbohydrate group. These are often used for cell signalling receptor sites or for cell communication. Glycolipids are lipids with a carbohydrate attached. They have the same function as glycoproteins.
Channel proteins allow water soluble ions to pass through the membrane. They can be opened and closed. They are also specific to a particular ion. The reason channel proteins are needed is because water soluble ions do not exist is isolation. Sodium ions for instance are surrounded by water molecules. This makes the ions much bigger than normal and therefore cannot pass through the insides of the hydrophobic phospholipid bilayer. Instead they pass through the channel proteins. Channel proteins are normally used in facilitated diffusion.
Carrier proteins change their shape to allow molecules to pass through. Carrier proteins can either be used for facilitated diffusion or for active transport. Each carrier protein is normally used to transport a particular molecule through.