There are many isomers of glucose, with the same chemical formula (C6H12O6), but different structural formula. These isomers include fructose and galactose. Galactose and fructose both play significant roles in mammalian biology.
Apart for the six carbon molecules, monosaccharides also include five-carbon, sugars (where n = 5, C5H10O5) and theses include ribose, deoxyribose and ribulose. Ribose is such an organic compound that is written in the form of D-ribose. Ribose is very important to RNA molecules as it is the base sugar and is crucial for the process of transcription. Deoxyribose sugar is similar to the sugar ribose but has one less oxygen atom. Dioxyribose is the backbone of DNA which bonds to a nucleotide bases (adenine, cytosine, thymine and guanine) and there is a phosphate group attached to the other side. Deoxyribose and ribose are important in manufacturing of ATP and polypeptides. Ribulose is a five carbon molecule with chemical formula 5105. Ribose reacts to form RuBp which is used in the Calvin cycle during Photosynthesis.
Disaccharides are formed when two monosaccharides are joined together by a glycosidic bond resulting with the loss of water. Disaccharides are formed by condensation reactions and broken down by hydrolysis
Maltose is fromed when two glucose molecules react. It is formed on digestion of starch by amylase, because this enzyme breaks starch down into maltose. Sucrose is formed when glucose reacts with fructose. It is common in plants because it is less reactive than glucose, and it is their main transport sugar. It is known as table sugar which is a component of human nutrition. But if sucrose is consumed too much it can lead to tooth decay and obesity. Lactose is formed when galactose reacts with glucose. It is found only in mammalian milk, and is the main source of energy for infant mammals.
Polysaccharides are long chains of many monosaccharides joined together by glycosidic bonds. Starch is the plant storage polysaccharide. It is insoluble and forms starch granules inside many plant cells. Being insoluble means starch does not change the water potential of cells, so does not cause the cells to take up or lose water by osmosis. It is not a pure substance, but is a mixture of amylose and amylopectin.
Amylose is an unbranched helical chain made up of 6 alpha glucose units in each turn. The monomers are joined together by 1-4 glycosidic bonds. It is a large molecule with energy rich bonds and is a compact structure. It is found in most parts of plants in the form of small granules and is therefore used as an energy reserve when excess glucose is produced. Amylopectin is a branched chain joined by 1-4 glycosidic bonds. Because it has more ends, it can be broken more quickly than amylose by amylase enzymes. Like amylase it is a compact structure andis insoluble. Both amylose and amylopectin are broken down by the enzyme amylase into maltose.
Glycogen is similar in structure to amylopectin. It is so highly branched molecule and can be mobilised very quickly. Glycogen is a polymer of glucose and is formed when insulin converts glucose to glycogen, It provides a quick source of energy to cells that require ATP.
Cellulose is only found in plants, where it is the main component of cell walls. Cellulose contains b-glucose which means that in a chain, alternate glucose molecules are inverted and are joined by 1-4 glycosidic bonds. This tiny difference makes a huge difference in structure and properties. While the alpha glucose polymer in starch coils up to form granules, the beta glucose polymer in cellulose forms straight chains. Hundreds of these chains are linked together by hydrogen bonds to form cellulose microfibrils. These microfibrils are very strong and rigid, and give strength to plant cells, and therefore to young plants and also to materials such as paper, cotton and sellotape.
The glycosidic bond cannot be broken by amylase, but requires a specific cellulase enzyme. The only organisms that possess a cellulase enzyme are bacteria, so herbivorous animals, like cows and termites whose diet is mainly cellulose, and have a mutualistic relationship with the bacteria in their guts so that they can digest cellulose. Other polysaccharides include Chitin which is found in fungal cell walls and the exoskeletons of insects and Lignin found in the walls of xylem cells, is the main component of wood.
Oligosaccharides are another form of carbohydrate and are small polymers of monosachrides and are connected with each other through bonds called glycosidic bonds. They have many functions for example the molecules are found on the plasma membrane of animal cells where they can play a role in cell-cell recognition.
