Monosaccharides (from the Greek monos – one) are colorless crystalline substances that are readily soluble in water and have a sweet taste. Of monosaccharides, ribose, deoxyribose, glucose, fructose, galactose are of the greatest importance for living organisms. Ribose is part of RNA, ATP, B vitamins, a number of enzymes. Deoxyribose is part of DNA. Glucose (grape sugar) is a monomer of polysaccharides (starch, glycogen, cellulose). It is in the cells of all organisms. Fructose is part of oligosaccharides, such as sucrose. It is found in plant cells in free form. Galactose is also part of some oligosaccharides, such as lactose.
Oligosaccharides (from the Greek oligos – a little) are formed by two (then they are called disaccharides) or several monosaccharides, linked covalently to each other via a glycosidic bond. Most oligosaccharides are water soluble and have a sweet taste. Of the oligosaccharides, disaccharides are most widespread: sucrose (cane sugar), maltose (malt sugar), lactose (milk sugar).
Polysaccharides (from Greek poly – many) are polymers and consist of an indefinitely large (up to several hundreds or thousands) of the number of residues of monosaccharide molecules connected by covalent bonds. These include starch, glycogen, cellulose, chitin, etc. It is interesting that starch, glycogen and cellulose, which play an important role in living organisms, are built from glucose monomers, but the bonds in their molecules are different. In addition, in cellulose, the chains do not branch, and in glycogen they branch more strongly than in starch.