Introduction to Carbohydrates
Carbohydrates are the most abundant organic compounds found in nature and are vital for life. They serve as a primary source of energy for living organisms. Chemically, carbohydrates are polyhydroxy aldehydes or ketones or compounds that yield them on hydrolysis. The general formula is Cn(H2O)n.
Classification of Carbohydrates
1. Monosaccharides
• Simplest form of carbohydrates.
• Cannot be hydrolyzed further.
• General formula: CnH2nOn
• Examples: Glucose, Fructose
2. Oligosaccharides
• Contain 2–10 monosaccharide units.
• Most important oligosaccharide: Disaccharides
• Examples: Sucrose (Glucose + Fructose), Lactose (Glucose + Galactose), Maltose (Glucose + Glucose)
3. Polysaccharides
• Long chains of monosaccharide units.
• Not sweet, insoluble in water.
• Examples: Starch, Cellulose, Glycogen
Structure and Function of Monosaccharides
Monosaccharides are further classified based on:
- Number of carbon atoms: Trioses, Tetroses, Pentoses, Hexoses
- Functional group:
• Aldoses (with –CHO group)
• Ketoses (with –CO group)
Glucose is an aldohexose with the formula C6H12O6. It exists in two cyclic forms: α-glucose and β-glucose.
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Reducing and Non-Reducing Sugars
Reducing Sugars
• Can reduce Fehling’s or Tollen’s reagent.
• Contain free aldehyde or ketone group.
• Examples: Glucose, Maltose, Lactose
Non-Reducing Sugars
• Do not have a free –CHO or –CO group.
• Example: Sucrose
Properties of Glucose
• Sweet, soluble in water
• Undergoes oxidation and reduction
• Does not react with Schiff’s reagent
• Exists as α- and β- anomers in solution
Polysaccharides in Biological Systems
Starch
• Plant storage carbohydrate
• Made of amylose and amylopectin
Glycogen
• Animal storage carbohydrate
• Similar to amylopectin but more branched
Cellulose
• Structural polysaccharide in plants
• Cannot be digested by humans
Tests for Carbohydrates
1. Molisch’s Test
• General test for carbohydrates (violet ring formation)
2. Benedict’s Test
• Detects reducing sugars (brick red precipitate)
3. Fehling’s Test
• Another test for reducing sugars (red ppt. of Cu2O)
Importance of Carbohydrates
• Source of energy: 1 gram = 4 kcal
• Storage: Glycogen in animals, starch in plants
• Structural role: Cellulose in plant cell walls
• Component of biomolecules: DNA, RNA (ribose, deoxyribose)
Conclusion
Carbohydrates are crucial biomolecules that play multiple roles in metabolism and structure. Understanding their classification, structure, and functions provides a strong foundation in organic chemistry and biology. In exams, focus on glucose structure, disaccharide linkages, and polysaccharide functions for scoring high.