INTRODUCTION
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Carbohydrates are naturally occurring organic compounds composed of carbon, hydrogen, and oxygen, primarily produced by plants. They constitute up to 80% of the dry weight of plants and serve as the ultimate source of food. In higher animals, the simple sugar glucose is an essential component of blood and exists in a polymeric form as glycogen in the liver and muscles.
- In green plants, carbohydrates are produced through photosynthesis, a process that converts carbon dioxide (CO2) and water (H2O) into glucose (C6H12O6). This conversion is facilitated by the green pigment chlorophyll present in plant leaves, with energy provided by sunlight.
CLASSIFICATION AND NOMENCLATURE
Classification
Carbohydrates are generally classified into two classes:
(i) Sugars
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These are crystalline substances that are sweet and water-soluble, examples include glucose, fructose, and cane sugar.
(ii) Non-sugars
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These are tasteless, insoluble in water, and amorphous, examples include starch and cellulose.
Modern classification categorizes carbohydrates into three major groups:
(a) Monosaccharides
Monosaccharides are the simplest carbohydrates that cannot be hydrolyzed into simpler forms. They are classified based on the presence of an aldehyde or keto group.
- Aldotetroses: Erythrose and Threose (CH2OH(CHOH)2 CHO)
- Ketotetroses: Erythrulose (CH2OHCOCHOHCH2OH)
- Aldopentoses: Ribose, arabinose, Xylose, and Lyxose (CH2OH(CHOH)3 CHO)
- Ketopentoses: Ribulose and Xylulose (CH2OHCO(CHOH)2 CH2OH)
- Aldohexoses: Glucose, mannose, galactose (CH2OH(CHOH)4 CHO)
- Ketohexoses: Fructose, Sorbose (CH2OHCO(CHOH)3 CH2OH)
(b) Oligosaccharides
Oligosaccharides can be hydrolyzed into a definite number of monosaccharide molecules, including disaccharides, trisaccharides, and tetrasaccharides.
- Disaccharides: sucrose, lactose, maltose (C12H22O11)
- Trisaccharides: raffinmose (C18H32O16)
- Tetrasaccharides: stachyose (C24H42O21)
(c) Polysaccharides
Polysaccharides yield a large number of monosaccharide molecules upon hydrolysis. Examples include starch, cellulose, and glycogen.
MONOSACCHARIDES
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Monosaccharides are further classified based on the carbonyl function and the number of carbon atoms in the molecule. They are polyhydric aldehydes and ketones that cannot be hydrolyzed into simpler carbohydrates.
Structures of Monosaccharides
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The common monosaccharides are depicted in the following table:
Conclusion
Carbohydrates are essential for human beings, providing food, clothing, and shelter. They are crucial to the economy of many nations, with sugar being one of the most important commercial commodities.