Dr Praveen Kumar Agrawal is teaching live on Unacademy Plus
COURSE BIOMOLECULES Lesson Monosaccharides (Simplest sugars) Dr. Praveen Kumar Agrawal
1. Monosaccharides . These are the simplest and smallest carbohydrates. These can not be hydrolysed further to form smaller . carbohydrates. These form the structural units of other carbohydrates. There general formula is CnH2nOn. Examples of monosaccharides include glyceraldehyde (glycerose), ribose, glucose, fructose, galactose and sedo- heptulose . Glyceraldehyde : It is a triose sugar. It is an important intermediary of glycolysis and photosynthesis . Erythrose : It is tetrose and intermediary of photosynthesis. Ribose It is a 5 C sugar and important constituent of nucleotides and RNA. Deoxyribose It is also a 5 C sugar which is an important constituent of DNA. Glucose Biologically, the most important carbohydrate is the glucose. It is the main source of energy for the living organisms. It participates in almost all biological reactions, directly or indirectly. It is also called blood sugar (Note D form of glucose is called dextrose, which is used for clinical purposes) Fructose: Fructose is the sweetest sugar. It is also important in many metabolic processes. It represents the ketose form of hexose sugar. (Isomeric to glucose, which represents aldose form of hexose sugar). It is the commonest form of sugars in fruits. A mixture of glucose and fructose is called invert sugar. . Sedoheptulose : It is a seven carbon atom sugar. It is important in Calvin cycle (photosynthesis).
Monosaccharides are reducing sugars and exist in following two forms - 1. Aldehyde derivatives or aldose sugars These carbohydrates possess the aldehydic group -CHO). These include glucose, ribose etc. . 2. Ketone derivatives or ketose sugars These carbohydrates possess ketonic group (-CO-). These include fructose, ribulose etc. These two forms are usually isomeric to each other, e.g., the formula of glucose and fructose both is C6H1206, but glucose is aldose form while fructose is ketose form. Following table shows aldose and ketose forms of some monosaccharides Ketose form Dihydroxyacetone Erythrulose Ribulose Fructose Monosaccharide Triose Tetrose Pentose Hexose Formula C3H603 C4H804 CsH1005 C6H1206 Aldose form Glycerose Erythrose Ribose Glucose
CH2 OH CHO HCOH CHO HCOH HCOH HCOH OHCH OHCH HCOH HCOH HCOH HCOH CH2 OH CH2 OH CH2 OH Ribose Pentose Glucose Hexose Fructose Hexose
Properties of Monosaccharides Monosaccharides show following physico-chemical properties - 1. Physical properties Crystalline, . Highly soluble in water, .Sweeter in taste. 2. Chemical properties Chemically, monosaccharides are aldehyde or ketone derivatives of polyhydric alcohols, (possessing more than one alcoholic -OH, groups) Free aldehydic and ketonic groups of these sugars can reduce cupric ions (Cu2+) to cuprous ions (Cu+) Therefore these sugars are also called reducing sugars. This property is the basis of Fehling's test and Benedict's test to detect the presence of sugars in the urine. Monosaccharides are optically active and form many optical isomers. Optical activity is due to the presence of asymmetrical carbon atom. Therefore monosaccharides can turn the plane of a polarized light when such light is passed through their solution