Glycosid formation

Glycosid formation
Molecules having only one actual or potential sugar group are called monosaccharides (Greek, mono = one; saccharide = sugar). They cannot be further hydrolyzed into smaller units. When two monosaccharides are combined together with elimination of a water molecule, it is called a disaccharide (e.g. C12H22O11). Trisaccharides contain three sugar groups. Further addition of sugar groups will correspondingly produce tetrasaccharides, pentasaccharides and so on, commonly known as oligosaccharides (Greek, oligo = a few). When more than 10 sugar units are combined, they are generally named as polysaccharides (Greek, poly = many). Polysaccharides having only one type of monosaccharide units are called homopolysaccharides and those having different monosaccharide units are heteropolysaccharides. Sugars having aldehyde group are called aldoses and sugars with keto group are ketoses. Depending on the number of carbon atoms, the monosaccharides are named as triose (C3), tetrose (C4), pentose (C5), hexose) C6), heptose (C7) and so on.
Functions of Carbohydrates
1- Carbohydrates are the main sources of energy in the body. Brain cells and RBCs are almost wholly dependent on carbohydrates as the energy source. Energy production from carbohydrates will be 4 kcal/g.
2- Storage form of energy (starch and glycogen) .
3- Excess carbohydrate is converted to fat.
4- Glycoproteins and glycolipids are components of cell membranes and receptors.
5- Structural basis of many organisms: Cellulose of plants; exoskeleton of insects, cell wall of micro- organisms, mucopolysaccharides as ground substance in higher organisms. The general molecular formula of carbohydrate is Cn(H2O)n For example, glucose has the molecular formula C6H12O6. Carbohydrates are polyhydroxy aldehydes or ketones or compounds which yield these on hydrolysis .
Optical Activity
The presence of asymmetrical carbon atom causes optical activity. When a beam of plane-polarized light is passed through a solution of carbohydrates, it will rotate the light either to right or to left. Please note that the D- and L-notation has no bearing with the optical activity. Depending on the rotation, molecules are called dextrorotatory (+) (d) or levorotatory (-) (l). Thus D-glucose is dextrorotatory but D-fructose is levorotatory. Equimolecular mixture of optical isomers has no net rotation (racemic mixture) .
STEREOISOMERS
Compounds having same structural formula, but differing in spatial configuration are known as stereoisomers. While writing the molecular formula of monosaccharides, the spatial arrangements of H and OH groups are important, since they contain asymmetric carbon atoms. Asymmetric carbon means that four different groups are attached to the same carbon. The reference molecule is glyceraldehyde (glycerose) which has a single asymmetric carbon atom .