Embedded within the lipid bilayer are different proteins (hydrophobic parts interact with the hydrophobic tails of the phospholipids). The hydrophilic parts of the proteins protrude either in to the cytoplasm or into the ECF. Some of the proteins are anchored in place while other proteins move about within the bilayer.
Various organelles within the cytoplasm are also bounded by membranes similar in structure to the cell membrane except for specific combinations of lipids and proteins depending on the particular functions of the organelle.
Some of the proteins extending into the ECF have carbohydrates attached (glycoprotein)
The cell membrane has various functions.
One of the most important is to isolate the cytoplasm from the external environment. In order to do this, the cells must be able to retain precious nutrients and eliminate wastes. Also, substances that are fat soluble (e.g. vitamins A, D and E) pass through easily but they are rare in the cell’s environment because they are not water soluble. Substances which are water soluble such as salts, amino acids and sugars, cannot dissolve in lipids and so cannot pass through the bilayer.
A second important function of the cell membrane is to regulate the flow of materials into and out of the cell. In order to do this, some proteins form pores that allow small water soluble molecules to penetrate the membrane (hydrophilic amino acids anchor the pore while hydrophilic amino acids form inside the pore)-proteins are specific – some pores are open all the time while others open and close depending on the needs of the cell. It is known that some proteins have specific sites that grab onto specific molecules (e.g. glucose, Na) and may use energy to pump them across.
Last, but not least, cell membranes have the function to communicate with other cells. For example, proteins (i.e. glycoprotein) that stick out of the ECF enable the cell to communicate with other cells.
E.g. Glycoproteins on red blood cells determine blood type and serve as identification markers. Glycoproteins bind hormones such as insulin and trigger a metabolic change in the cell.
Cholesterol is also found on within the cell membranes. Its function is to help keep membrane fluid. At low temperatures cholesterol keeps the phospholipids apart. This keeps the membrane fluid. At high temperatures (around 37C) it attracts the phospholipids and helps stabilize the membrane.
The cell membrane is the only thing between a cell and its outside environment. It has the crucial role to play in the life of a cell: it must control what enters and leave s the cell.