Cells recognize one another because of the saccharides attached to cell surfaces. 

They  are present usually as oligosaccharides associated through covalent links to lipids and/or proteins forming Glycoconjugates. The lipid or protein part is integrated into the cell membrane structure, with the saccharide part towards the external membrane surface.

Membrane carbohydrates  (2-10% of the membranes) are on the extracellular surface bounded to lipids or proteins of the membrane, forming glycoconjugates that serve as docking sites in cell recognition, adhesion and receptor action. These sugars include mainly glucose, galactose, mannose, fucose, N-acetyl galactosamine and  N-acetyl glucosamine.

The different kinds of Glycoconjugates include:

Proteoglycans: In the Proteoglycans, the Glucosaminoglycan moety forms the greater fraction of the molecule (tipically a proteoglycan consists of 95 % of carbohydrates) and is the main site of biological activity, providing multiple binding sites. They are found mainly in the extracellular matrix. They are major components of connective tissue.

Glycoproteins: Membrane bound glycoproteins participate in a wide range of cellular phenomena, including cell recognition, cell surface antigenicity, etc. In the glycoproteins, the majority of the molecule consist of proteins; they have one or more oligosaccharides attached to a protein, and they usually are branched and do not have serial repeats, so they are rich in information, forming highly specific sites for recognition and high affinity binding by other proteins

Glycolipids: are membrane lipids in which the hydrophilic head groups are oligosaccharides.

 As in glycoproteins, glycolipids  act as specific sites for recognition by carbohydrate binding proteins.  The four types of human RBC have different oligosaccharides (antigens) in their cell membranes. Blood groups depends on the gangliosides (a kind of sphingolipid) in the surface of the RBC .

As you know, carbohydrates or glucids are polyhydroxylated aldehydes or ketones, their derivatives and polymers. Glucose is a typical example of carbohydrates.

Observe that glucose has an aldehyde group (drawed in red) and five hydroxyl groups.

Most of carbohydrates are present with a cyclic structure in nature, as a consequence of internal linkages between the carbonyl carbon (of the aldehyde or ketone group) with one of the hydroxyl groups in the same molecule. This graphic represents glucose in a cyclic form:

Considering the polymerization degree (PD) of carbohydrates, they can be classified in Monosaccharides, Oligosaccharides and Polysaccharides.

Monosaccharides:

Monosaccharides are formed by a single molecule. It means that when hydrolyzed they can not release simpler molecules. Examples of this group of carbohydrates are glucose, ribose and fructose, among others.

Monosaccharides can be subclassified according to different criteria, for example:

According to the main (carbonyl) function:

If the carbonyl group belongs to an aldehyde function, the monosaccharide is classified as an aldose. Glucose is a typical aldose. If the main function is a ketone, then the monosaccharide is classified as a ketose. 

Fructose (see structure below) is a ketose, since it is a polyhydroxylated ketone.

According to the number of carbons:

Monosaccharides can be classified in trioses, tetroses, pentoses, hexoses, heptoses and octoses, according to the number of carbons in the molecule.

According to the steric series:

According to the type of esteroisomers, monosaccharides can be classified as L or D (most of the carbohydrates in the animal kingdom belongs to D series)

According to the kind or anomer:

According to the position of the anomeric hydroxyl, monosaccharides can be classified as Alpha or Beta.

Usually, these criteria are combined for describing a monosaccharide, e.g. a compound can be described as a Beta-D-aldohexose.

Oligosaccharides:

 They are formed by 2-9 monomers linked through glycosidic linkages; in other words, when hydrolyzed these compounds release 2 to 9 monosaccharides (some texts say up to 20; in fact, oligosaccharides release “a few” monosaccharides).

According to the number of monosaccharides in the oligosaccharide, oligosaccharides can be dissacharides, trisaccharides, tetrasaccharides, etc. Disaccharides, formed by just 2 monosaccharides, are the most important subgroup of oligosaccharides. Disaccharides that appear in nature are lactose, or milk sugar (formed by galactose and glucose), and sucrose or table sugar (formed by fructose and glucose). Other important disaccharides are produced as result of starch digestion: maltose and isomaltose. These disaccharides are ,both of them, formed by two molecules of glucose, but linked in different ways. Cellobiose is a third dissacharide formed also by two molecules of glucose, but linked in such a way that animals can not break, unless animals have in the digestive system specific microorganisms that hydrolyze these linkages, as herbivors have (Cellobiose is formed as result of the digestion of cellulose).

Polysaccharides:

Polysaccharides are carbohydrates formed by more than 9 monosaccharides (some texts say more than 10 monosaccharides, other texts say more than 20…in fact, they usually are formed by a lot of monosaccharides!). When the polysaccharides are formed by the same type of monosaccharides, they are called homopolysaccharides.

                                                                                          Amylose

The molecules that form starch, glycogen and cellulose are formed by hundreds of molecules of just one type of monosaccharides (glucose, in this cases), linked through glycosidic linkages. These polysaccharides are typical examples of homopolysaccharides.

If the polysaccharide is formed by different types of monosaccharides, then it is called a heteropolysaccharide. Hyaluronic acid, formed by thousands of alternant units of glucuronic acid and N-acetylglucosamine, is an example of heteropolysaccharides.

Hyaluronic acid is an important component of the extracellular matrix in the skin and the conective tissue. This compound has received lately a lot of attention from the media as an antiaging agent.