Structure & Function of Cell Surface Glycoproteins

The surface of a red blood cell
The surface of a red blood cell (Image: "Blood Cells" is Copyrighted by Flickr user: Andrew Mason (Andrew Mason) under the Creative Commons Attribution license.)

Cell metabolism processes rely on the cell membrane’s ability to filter needed nutrients and supplies into its interior. In animal cells, proteins and lipids play an essential role in maintaining the structure of the membrane, as well as allowing materials to move in and out of the cell. Glycoproteins are a specialized protein-type that assists in this process.

Glycoprotein Structure

Glycoproteins are cell molecules made out of carbohydrates and proteins. Monosaccharides, disaccharides, oligosaccharides, or polysaccharides are the different types of carbohydrates that can combine with a protein. These carbohydrates are also known as glycans. The protein portion is of this molecule is made up of sulfate or phosphate groups. A glycoprotein molecule can vary in its carbohydrate-protein ratio, depending on the type of molecule it is. Carbohydrate amounts can range anywhere from 1 percent to 60 percent of the material contained inside a glycoprotein.

Membrane Structure

The cell membrane is made up of a lipid bilayer where cholesterols, proteins and phospholipids all work together to hold the membrane intact. The interior of the cell is filled with a water-like substance that fills the spaces inbetween the structures that reside inside the cell. The attached carbohydrate portion of glycoproteins assists with stabilizing the position of the protein inside the membrane. Glycoproteins exist as integral membrane proteins that run through both layers of the cell membrane. These molecules are embedded inside the middle layer of the membrane, and so are permanently connected to it.

Membrane Pathways

As integral membrane proteins, glycoproteins can act as chemical pathways that allow ions and molecules to move in and out of the cell. Materials like glucose, amino acids and carbon dioxide are carried across the cell membrane by these channels. Water-soluble cell materials are most reliant on these pathways, as the membrane core is made up of a fatty substance that prevents water-soluble materials from flowing through. These materials bind to the portion of the protein that sits on the outside the cell membrane (“heads.’) This is a selective process in which only certain materials can bind to each glycoprotein type.

Concentration Gradients

Glycoprotein “heads” also play a part in coordinating cellular processes with the cell’s external environment. In cases where there’s an excess amount of glucose in the bloodstream, the pancreas will release a hormone (insulin) into the bloodstream. Glycoproteins head receptors respond to the presence of insulin and begin the process of incorporating glucose molecules into the cell. If the cell already has its needed glucose supply, a process called “active transport” is initiated. Active transport requires the cell to expend energy in order to move materials in and out. Another process called “passive transport” is a natural absorption process that depends on what concentration of chemicals exists on either side of the cell membrane.

Cell Signaling

Some integral membrane pathways require a certain gradient, or pressure, for materials to move through. These pathways play a part in the cell signaling process. As ions move back and forth across the cell membrane, the cell takes on varying electrical charges depending on the amount of ions on either side of the membrane. Inactive cell conditions are said to be in a resting state, or resting potential: ion concentrations are equal on both sides of the membrane. When these concentrations are different, glycoproteins trigger an “action potential” which causes the cell’s internal environment to excrete whatever enzymes are needed to move materials in, or out, of the cell.

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