Biomolecules:
Carbohydrates

Variations on a Theme

The ABO Blood Type System

One large role carbohydrates play in cells is in cell-cell recognition. Because of all their possible structural variation discussed in the previous page, carbohydrates allow specific recognition between cells using relatively small molecules.

One well-known example of cell-cell recognition mediated by carbohydrates is the ABO blood type system. Anybody who has donated or received blood probably knows the following facts about blood compatibility. People with blood type O can receive only O type blood but can donate to any blood type. People with blood type A can receive only A and O type blood but can donate to A and AB recipients. Similarly, people with blood type B can receive only B and O type blood but can donate to B and AB recipients. People with blood type AB can receive any type of blood but can only donate to other AB types. All people synthesize a precursor carbohydrate, called the H antigen, which is attached to lipids or proteins on the outer surface of red blood cells. Specific enzymes synthesized by the ABO genes attach additional monosaccharides to the H antigen, and the completed carbohydrate determines that person's blood type.

In the radio boxes at left, click on a blood type to see the structure of the ABO blood type antigens.

	A
	B
	AB
	O
The A gene codes for a protein that adds N-acetyl galactosamine (yellow) in an α1-->3 glycosidic bond to the galactose residue at the end of the H antigen. The B gene codes for a protein that adds galactose (green) in an α1-->3 glycosidic bond to the galactose residue at the end of the H antigen. Since both A and B genes are present in AB individuals, some H antigens get an N-acetyl galactosamine residue (yellow) while others get a galactose residue (green). O individuals have neither the A nor the B gene, so no additional carbohydrate is added to the H antigen.

Monomers for Nucleotides

One specific carbohydrate, the 5-carbon sugar ribose, serves as the building block for another major class of biomolecules, the nucleic acids. The monomers of nucleic acids consist of modified ribose molecules.

Click on the step numbers below to see the modifications that are made to ribose to form nucleotides, the monomers of DNA. Click on the mouse at left to clear the text.

	A phosphate group is attached to the C5 position of ribose (called the 5' position in a nucleotide).
1.
	A nitrogen-containing base is attached to the C1 position of ribose (called the 1' position in a nucleotide).
2.
	Instead of a hydroxyl group at the C2 position of ribose (called the 2' position in a nucleotide) there is a hydrogen atom.
3.

You can learn more about ribose and nucleic acids in the DNA 1 and DNA 2 modules.

Other Functions