Introduction
Carbohydrates are major organic compounds found in fruits, vegetables, grains and legumes. They are made of carbon, hydrogen and oxygen, usually in a ratio of 2:1 (H:O), and are often referred to as hydrates of carbon. Chemically, carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones or compounds that form them on hydrolysis. They are produced in green plants by photosynthesis and act as an important energy source in animals. Carbohydrates also form structural materials, such as cellulose in plants and chitin in certain animals.
Classification of Carbohydrates
Carbohydrates are mainly classified into monosaccharides, oligosaccharides and polysaccharides.
Monosaccharides
Monosaccharides are the simplest sugars and cannot be hydrolysed further. They are classified based on the number of carbon atoms:
- Trioses: 3-carbon sugars, e.g., glyceraldehyde.
- Tetroses: 4-carbon sugars, e.g., erythrose, threose.
- Pentoses: 5-carbon sugars found in plants, e.g., ribose, arabinose, xylose.
- Hexoses: 6-carbon sugars; most abundant, e.g., glucose, fructose.
Monosaccharides are also classified based on the functional group:
- Aldoses: Contain aldehyde group (e.g., glucose, galactose, ribose).
- Ketoses: Contain ketone group (e.g., fructose).
Reducing and Non-Reducing Sugars
- Reducing sugars: Contain free hemiacetal or hemiketal groups. Examples: glucose, galactose, fructose, maltose, lactose.
- Non-reducing sugars: Do not contain hemiacetal groups. Example: sucrose.
Disaccharides
Two monosaccharides joined by a glycosidic bond form disaccharides. Examples:
- Sucrose = glucose + fructose
- Lactose = galactose + glucose
- Maltose = glucose + glucose
Trisaccharides
These are oligosaccharides made of three monosaccharides. Example:
- Raffinose = glucose + fructose + galactose
Polysaccharides
Polysaccharides are long chains of monosaccharide units linked by glycosidic bonds.
Types
- Digestible polysaccharides: Starch (main energy storage in plants).
- Structural polysaccharides: Cellulose, lignin, mannans, inulin, pectic substances, alginic acid, agar, chitin.
Oligosaccharides
These contain 2–10 monosaccharide units. A common example is Fructo-Oligosaccharides (FOS), naturally present in several vegetables.
Functions of Carbohydrates
- Provide energy for body functions (starch in plants, glycogen in animals).
- Protein-saving action by supplying energy.
- Essential for normal fat metabolism.
- Glucose is necessary for brain and red blood cell function.
- Ribose and deoxyribose are part of RNA and DNA structures.
- Help in detoxification; e.g., glucuronic acid binds bilirubin and hormones.
- Carbohydrates bind to proteins and lipids forming glycoproteins and glycolipids.
- Structural role in cellulose (plants) and chitin (insects, crustaceans).
- Used in making paper, textiles and wood products.
Chemical Tests for Carbohydrates
1. Molisch Test
- General test for all carbohydrates.
- Carbohydrates form furfural or hydroxymethylfurfural when treated with acid.
- These condense with α-naphthol to give a purple ring.
2. Fehling’s Test
- Detects reducing sugars.
- Cu2+ in Fehling’s solution is reduced to red Cu2O precipitate.
3. Benedict’s Test
- Used for reducing sugars.
- Blue Benedict’s reagent turns reddish-brown after heating.
4. Barfoed’s Test
- Differentiates monosaccharides and disaccharides.
- Monosaccharides give a quick red precipitate (2–3 minutes).
- Disaccharides react slowly (around 10 minutes).
5. Iodine Test
- Starch forms a blue-violet complex with iodine.
- Cellulose does not react.
6. Seliwanoff’s Test
- Differentiates aldohexoses and ketohexoses.
- Ketohexoses like fructose give a deep red colour rapidly.
- Aldohexoses give a slow pink colour.
7. Bial’s Test
- Used for pentoses.
- Pentoses form a blue-green colour with Bial’s reagent.
- Hexoses may give brown or green shades.
Detailed Notes:
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