are proteins that contain oligosaccharide chains (glycans) covalently attachedto polypeptide side-chains.
A glycoprotein is a branched carbohydrate chain (glycogen) attached to a protein within a cell membrane. This helps in cell to cell recognition.
CD4 is a glycoprotien expressed on the surface of Helper T Cell
The plasma membrane consists of a phospholipid bilayer that contains proteins. There are four types of proteins in the plasma membrane - structural protein, receptor protein, transport protein and glycoprotien.
Try here: http://en.wikipedia.org/wiki/Glycoprotein#Functions
Red blood cells carry oxygen throughout the body, which is an essential component of most cellular activities. When they stop functioning correctly (as happens in diseases like sickle-cell anemia and leukemia or with poisons like carbon monoxide), it reduces the flow of oxygen and can impair or kill cells.