Monosaccharide is formed from the Greek word mono meaning one and saccharin meaning sugar. Monosaccharide means a single sugar. Carbohydrates that do not yield any other simple carbohydrate units when broken down or analyzed are called monosaccharides. Their common symbol is CnH2nOn. Monosaccharides have 3-10 carbons. Monosaccharides have a free aldehyde group (-CHO) at carbon 1 or a ketone group (>C=O) at carbon 2, so they are called reducing sugars or reducing sugars. Monosaccharides with aldehyde groups are called aldoses and monosaccharides with ketone groups are called ketoses. Aldehyde and ketone groups are reducing groups and sugars containing reducing groups are called reducing sugars.
Characteristics of monosaccharides
(i) It is a simple sugar.
(ii) It is photoactive.
(iii) It is a small molecule.
(iv) Its molecular weight is less than 10,000 daltons.
(v) It has a sweet taste.
(vi) It is granular and soluble in water.
(vii) It contains aldehyde and ketone groups.
(viii) It is called aldose or ketose sugar.
(ix) It is a repulsive sugar.
Classification of Monosaccharides
Monosaccharides can be divided into two groups based on the aldehyde or ketone group.
1. Aldose: Monosaccharides containing aldehyde groups are called aldoses. Such as glucose, erythrose, ribose, mannose etc. Dr. Siddiq Publications
2. Ketose: Monosaccharides containing ketone group are called ketose. Such as fructose, erythrulose, ribulose, dihydroxyacetone etc.
The different types of monosaccharides based on the number of carbons are-
1. Triose: Monosaccharides with three carbons are called triose. A triose with an aldehyde group is called an aldotriose and a ketone group with a ketotriose. Its molecular symbol is C3H6O3. For example, glyceraldehyde, dihydroxy acetone etc.
2. Tetrose: A four-membered monosaccharide is called a tetrose. Its molecular symbol is C4H8O4. A tetrose with a four-carbon aldehyde group is called an aldotetrose and a ketone group with a ketotetrose. Eg-Erythrose, Thirose, Erythrolose etc. Dr. Siddiq Publications
3. Pentose: Five carbon monosaccharides are called pentoses. Its molecular symbol is C5H10O5. Ribose and deoxyribose are pentose sugars and form nucleotides and nucleic acids. Examples – ribulose, xylulose, arabinose, lyxose, ribose, deoxyribose etc. Dr. Siddiq Publications
4. Hexose: Monosaccharides with six carbons are called hexoses. Its molecular symbol is C6H12O6. Hexoses of aldehyde groups are called aldohexoses and hexoses of ketone groups are called ketohexoses. They are known as blood sugar. There are 16 types of hexoses. The abundant hexose is the glucose that provides energy to living cells. Examples – glucose, fructose, galactose, mannose etc.
5. Heptose: A monosaccharide with seven carbons is called heptose. Its molecular symbol is C7H14O7. Its number is very less. It plays an important role in photosynthesis. Pseudoheptulose, monoheptulose etc.
6. Octose: An eight carbon monosaccharide is called octose. Its molecular symbol is C8H26O8. eg glucooctose. Dr. Siddiq Publications
7. Nenose: A monosaccharide having nine carbons is called nenose. Its molecular symbol is C9H18O9. eg gluconanose.
8. Decose: A ten carbon monosaccharide is called decose. Its molecular symbol is C10H30O10. E.g. Glucodecose.