Pentoses such as ribose an ribulose possess five carbon atoms. Ribose or deoxyribose make up part of nucleotides and as such give structural support to nucleic acids RNA and DNA. Ribose is a constituent of hydrogen carriers such as NAD, NADP and FAD. Further more it is involved in the synthesis of coenzymes and ATP.
The third type of monosaccharides is hexoses like glucose and fructose, which possess six carbon atoms. Glucose has a number of significant functions in both plants and animals. Most importantly it is the major respiratory substrate in plants and animals. In addition to this, it is concerned with the synthesis of disaccharides and polysaccharides, and along with fructose is a constituent of nectar, and therefore essential in upholding particularly flower and insect existence. Galactose, mannose and fructose are three principal respiratory substrates in organisms. Additionally, Galactose is central in the synthesis of lactose. Fructose is also involved in the synthesis of insulin, and it sweetens fruits to attract animals in order to assist in seed dispersal.
When two monosaccharides are joined together they form a disaccharide for instance, sucrose is formed in the joining of glucose and fructose, lactose in the amalgamation of glucose and galactose, and maltose in the bonding of two glucose molecules. These three respiratory substrates are the primary disaccharides in living organisms.
Sucrose is vital in plants as it is the form in which most carbohydrates are transported in the phloem. In a few plants such as the Allium (onion) sucrose is a storage material.
Mammalian milk contains five per cent of the disaccharide lactose. Consequently, it is a chief carbohydrate source for sucklings.
Polysaccharides are formed when large numbers of monosaccharides are joined together in long chains. Generally, they provide structural support in plants or act as food and energy (carbohydrate) stores, e.g. Amylose and Amylopectin (starch) in plants like potato tubers and glycogen in animals and fungi. Inulin is also a major storage polysaccharide for carbohydrates in a select number of plants, such as the Jerusalem artichoke or Dahlia.
The polysaccharide cellulose is the structural component of plant cell walls. It consists of long chains of glucose residues. Hydroxyl groups project outwards from hydrogen bonds with hydroxyl groups of adjacent chains to give the whole structure a high tensile strength. Cellulose is fully permeable to water and solutes and therefore does not effect exchange.
Structurally, chitin, another polysaccharide, is very similar to cellulose. It forms bundles of long parallel chains, which are essential part of the arthropod skeleton.
Monosaccharides, disaccharides, and polysaccharides, are different groups of carbohydrates essential in sustaining life of all plants and animals. By carrying out extremely important functions such as carbohydrate storage, respiratory substrates and support in both plants and animals, roles such as synthesis of nucleic acids and ATP in mammals, and fundamental aids in photosynthesis in plants, carbohydrates are absolutely obligatory in preservation of most life on earth.
