2.6.1 Classification of Carbohydrates

Carbohydrates are commonly known as sugars or saccharides as most of the familiar carbohydrates are sweet in taste.

• The word saccharide is derived from the Greek word "sakcharon" which means sugars.
• Classification of carbohydrates is based upon the number of saccharide units.
• Carbohydrates are generally classified into three groups:

Comparison of Characteristics of Carbohydrates

Monosaccharides

• Monosaccharides are true carbohydrates which are either polyhydroxy aldehydes or polyhydroxy ketones.
• The general molecular formula for the representation of monosaccharides is (CH₂O)_n where 'n' is the number.
• Number of carbon atoms in monosaccharides ranges from 3 to 7.
• All the carbon atoms in a monosaccharide except one have a hydroxyl group (-OH).
• The remaining carbon atom is either part of an aldehyde or ketone group.

Examples of Monosaccharides:

Oligosaccharides

• Carbohydrates which are derivatives of monosaccharides and upon hydrolysis yield 2 to 10 saccharide units are called oligosaccharides.
• On the basis of number of saccharide units, the oligosaccharides are classified into:
• Disaccharides (2 units)
• Trisaccharides (3 units)
• Tetrasaccharides (4 units)
• And so on.
• The most common among these are disaccharides.

Polysaccharides

• Those carbohydrates which upon hydrolysis yield more than ten monosaccharide units are called polysaccharides.
• This is the largest group of carbohydrates.
• Polysaccharides which are composed by the condensation of only one kind of monosaccharide are called homopolysaccharides.
• Examples: Starch, glycogen, cellulose, chitin
• Polysaccharides which are composed by the condensation of different kinds of monosaccharides are called heteropolysaccharides.
• Examples: Agar, pectin, peptidoglycan

Intermediate Level

Classification of Carbohydrates in Detail

Monosaccharides in Detail

Definition: Monosaccharides are true carbohydrates which are either polyhydroxy aldehydes or polyhydroxy ketones.

General Formula: (CH₂O)_n

Carbon Atoms: 3 to 7

• All carbon atoms except one have a hydroxyl group (-OH).
• The remaining carbon atom is either part of an aldehyde or ketone group.

Types of Monosaccharides:

Important Monosaccharides:

• Also called dextrose or grape sugar.
• Molecular formula: C₆H₁₂O₆
• Aldohexose (contains aldehyde group).
• Primary energy source for cells.

• Also called levulose or fruit sugar.
• Molecular formula: C₆H₁₂O₆
• Ketohexose (contains ketone group).
• Sweetest natural sugar.

• Pentose sugar.
• Molecular formula: C₅H₁₀O₅
• Component of RNA and ATP.

• Pentose sugar.
• Molecular formula: C₅H₁₀O₄
• Component of DNA.

Oligosaccharides in Detail

Definition: Carbohydrates which are derivatives of monosaccharides and upon hydrolysis yield 2 to 10 saccharide units are called oligosaccharides.

Classification:

Disaccharides:

Polysaccharides in Detail

Definition: Those carbohydrates which upon hydrolysis yield more than ten monosaccharide units are called polysaccharides.

Types:

Important Polysaccharides:

• Storage carbohydrate of plants.
• Made of α-glucose.
• Found in roots, stems, seeds.
• Two types: Amylose (unbranched) and Amylopectin (branched).

• Storage carbohydrate of animals.
• Made of α-glucose.
• Found in liver and muscles.
• Also called animal starch.
• More branched than amylopectin.

• Structural carbohydrate of plants.
• Made of β-glucose.
• Major component of plant cell wall.
• Most abundant carbohydrate on earth.

• Structural carbohydrate of fungi and arthropods.
• Made of N-acetylglucosamine.
• Found in fungal cell walls and exoskeleton of arthropods.
• Second most abundant organic molecule on earth.

Important Points

• 1. Carbohydrates are the compounds of carbon, hydrogen, and oxygen.
• 2. Literally, the word carbohydrate means "hydrates of carbon" — a carbon associated with water.
• 3. According to this classic concept, carbohydrates are organic biomolecules or their derivatives in which all the carbon atoms are hydrated, having the generalized formula C_n(H₂O)_n, where n is the number of carbon atoms and the number of water molecules.
• 4. For example, glucose has the molecular formula C₆(H₂O)₆ or C₆H₁₂O₆.
• 5. However, this formula is not exactly applicable in some cases:
• - Lactic acid: C₃H₆O₃
• - Acetic acid: C₂H₄O₂
• 6. 2.6.1 Classification of Carbohydrates:
• - Carbohydrates are commonly known as sugars or saccharides.
• - The word saccharide is derived from the Greek word "sakcharon" which means sugars.
• - Classification of carbohydrates is based upon the number of saccharide units.
• - Carbohydrates are generally classified into three groups:
• 1. Monosaccharides
• 2. Oligosaccharides
• 3. Polysaccharides
• 7. Comparison of characteristics of carbohydrates:
• | Feature | Monosaccharides | Oligosaccharides | Polysaccharides |
• | Saccharide units | Single saccharide unit | 2 to 10 saccharide units | More than 10 saccharide units |
• | Complexity | Simplest carbohydrates | Less complex structure | Highly complex structure |
• | Hydrolysis | Cannot be further hydrolyzed | Yield at least 2 and maximum 10 monosaccharides | Yield at least 11 monosaccharides |
• | Solubility | Highly soluble in water | Less soluble in water | Generally insoluble in water |
• | Taste | Sweetest among all carbohydrates | Less sweet in taste | Tasteless |
• 8. Monosaccharides are true carbohydrates which are either polyhydroxy aldehydes or polyhydroxy ketones.
• 9. The general molecular formula for the representation of monosaccharides is (CH₂O)_n where 'n' is the number.
• 10. Number of carbon atoms in monosaccharides ranges from 3 to 7.
• 11. All the carbon atoms in a monosaccharide except one have a hydroxyl group (-OH), while the remaining carbon atom is either part of an aldehyde or ketone.
• 12. Oligosaccharides are carbohydrates which are derivatives of monosaccharides and upon hydrolysis yield 2 to 10 saccharide units.
• 13. On the basis of number of saccharide units, oligosaccharides are classified into disaccharides, trisaccharides, tetrasaccharides, and so on.
• 14. The most common among these are disaccharides.
• 15. Polysaccharides are those carbohydrates which upon hydrolysis yield more than ten monosaccharide units.
• 16. This is the largest group of carbohydrates.
• 17. Polysaccharides which are composed by the condensation of only one kind of monosaccharide are called homopolysaccharides (e.g., starch, glycogen, cellulose, chitin).
• 18. Polysaccharides which are composed by the condensation of different kinds of monosaccharides are called heteropolysaccharides (e.g., agar, pectin, peptidoglycan).

Remember

• - Carbohydrates = carbon + hydrogen + oxygen
• - General formula = C_n(H₂O)_n
• - Three types:
• 1. Monosaccharides – Single sugar unit
• 2. Oligosaccharides – 2 to 10 sugar units
• 3. Polysaccharides – More than 10 sugar units
• - Monosaccharides = simplest; sweetest; highly soluble
• - Oligosaccharides = less sweet; less soluble
• - Polysaccharides = tasteless; generally insoluble
• - Examples of monosaccharides: Glucose, fructose, ribose
• - Examples of disaccharides: Sucrose, maltose, lactose
• - Examples of polysaccharides: Starch, glycogen, cellulose, chitin
• - Homopolysaccharides = one kind of monosaccharide
• - Heteropolysaccharides = different kinds of monosaccharides

Common Mistakes

• 1. Thinking all carbohydrates are sweet – Only monosaccharides and some oligosaccharides are sweet. Polysaccharides are tasteless.
• 2. Thinking all carbohydrates fit the formula C_n(H₂O)_n – Some carbohydrates (e.g., lactic acid, acetic acid) do not fit this formula exactly.
• 3. Confusing monosaccharides, oligosaccharides, and polysaccharides – Monosaccharides = 1 unit; Oligosaccharides = 2–10 units; Polysaccharides = more than 10 units.
• 4. Thinking all polysaccharides are homopolysaccharides – Some are heteropolysaccharides (e.g., agar, pectin, peptidoglycan).
• 5. Thinking cellulose is a storage carbohydrate – Cellulose is a structural carbohydrate; starch and glycogen are storage carbohydrates.
• 6. Thinking chitin is made of glucose – Chitin is made of N-acetylglucosamine, a modified form of glucose.
• 7. Confusing glycogen and starch – Glycogen is the storage carbohydrate of animals; starch is the storage carbohydrate of plants.
• 8. Thinking humans can digest cellulose – Humans cannot digest cellulose because they lack the enzyme cellulase. It acts as dietary fibre (roughage).
• 9. Forgetting the examples of monosaccharides – Glucose, fructose, ribose, deoxyribose.
• 10. Confusing homopolysaccharides and heteropolysaccharides – Homopolysaccharides = one kind of monosaccharide; Heteropolysaccharides = different kinds of monosaccharides.
• 11. Thinking all carbohydrates are energy sources – Some carbohydrates are structural (cellulose, chitin) and some are components of nucleic acids (ribose, deoxyribose).
• 12. Forgetting the general formula – C_n(H₂O)_n.

Quick Revision

• Carbohydrates are the compounds of carbon, hydrogen, and oxygen.
• Literally, the word carbohydrate means "hydrates of carbon" — a carbon associated with water.
• According to this classic concept, carbohydrates are organic biomolecules or their derivatives in which all the carbon atoms are hydrated, having the generalized formula C_n(H₂O)_n, where n is the number of carbon atoms and the number of water molecules.
• For example, glucose has the molecular formula C₆(H₂O)₆ or C₆H₁₂O₆.
• However, this formula is not exactly applicable in some cases, e.g., lactic acid (C₃H₆O₃) and acetic acid.
• 2.6.1 Classification of Carbohydrates:
• Carbohydrates are commonly known as sugars or saccharides as most of the familiar carbohydrates are sweet in taste.
• The word saccharide is derived from the Greek word "sakcharon" which means sugars.
• Classification of carbohydrates is based upon number of saccharide units.
• Carbohydrates are generally classified into three groups, i.e., monosaccharides, oligosaccharides, and polysaccharides.
• Comparison of characteristics of carbohydrates:

• Monosaccharides are true carbohydrates which are either polyhydroxy aldehydes or polyhydroxy ketones.
• The general molecular formula for the representation of monosaccharides is (CH₂O)_n where 'n' is the number.
• Number of carbon atoms in monosaccharides ranges from 3 to 7.
• All the carbon atoms in a monosaccharide except one have a hydroxyl group (-OH) while the remaining carbon atom is either the part of aldehyde or ketone.
• Oligosaccharides are carbohydrates which are derivatives of monosaccharides and upon hydrolysis yield 2 to 10 saccharide units.
• On the basis of number of saccharide units, the oligosaccharides are classified into disaccharides, trisaccharides, tetrasaccharides, and so on.
• The most common among these are disaccharides.
• Polysaccharides are those carbohydrates which upon hydrolysis yield more than ten monosaccharide units.
• This is the largest group of carbohydrates.
• Polysaccharides which are composed by the condensation of only one kind of monosaccharides are called homopolysaccharides, e.g., starch, glycogen, cellulose, chitin.
• Polysaccharides which are composed by the condensation of different kind of monosaccharides are called heteropolysaccharides, e.g., agar, pectin, peptidoglycan.

60-Second Revision

• Carbohydrates = carbon + hydrogen + oxygen
• General formula = C_n(H₂O)_n
• Three types:
• Monosaccharides = simplest; sweetest; highly soluble
• Oligosaccharides = less sweet; less soluble
• Polysaccharides = tasteless; generally insoluble
• Examples of monosaccharides: Glucose, fructose, ribose
• Examples of disaccharides: Sucrose, maltose, lactose
• Examples of polysaccharides: Starch, glycogen, cellulose, chitin
• Homopolysaccharides = one kind of monosaccharide
• Heteropolysaccharides = different kinds of monosaccharides
• Warning: Not all carbohydrates are sweet. Not all fit the formula C_n(H₂O)_n. Polysaccharides are tasteless and insoluble. Humans cannot digest cellulose. Chitin is made of N-acetylglucosamine, not glucose.