Carbohydrates: Definition classification, types

Carbohydrates are the most abundant biomolecules on earth. Oxidation of carbohydrates is the central energy-yielding pathway in most non-photosynthetic cells. Originally the term carbohydrate was used to describe compounds that were literally “carbohydrates,” because they had the empirical formula CH2O. Carbohydrates have been classified in recent years on the basis of carbohydrate structures, not their formulae. Such aldehydes and ketones are now known as polyhydroxy. Cellulose, starch, and glycogen are among the compounds that belong to this family.

Definition: Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis. carbohydrates have the empirical formula (CH2O)n.

Sources of Carbohydrates

  1. Potatoes
  2. Maze
  3. Milk
  4. Popcorn
  5. Bread

Types of Carbohydrates

The different types of carbohydrates can be classified on the basis of their behaviour in hydrolysis. They are mainly classified into three groups:

  1. Monosaccharides
  2. Disaccharides
  3. Polysaccharides

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1. Monosaccharides

Monosaccharides, or simple sugars, consist of a single polyhydroxy aldehyde or ketone unit. The most abundant monosaccharide in nature is the six-carbon sugar Dglucose, sometimes referred to as dextrose.

2. Oligosaccharides

Oligosaccharides consist of short chains of monosaccharide units, or residues, joined by characteristic linkages called glycosidic bonds. The most abundant are the disaccharides, with two monosaccharide units. Example: sucrose (cane sugar).

3. Polysaccharides

The polysaccharides are sugar polymers containing more than 20 or so monosaccharide units, and some have hundreds or thousands of units. Example: starch.
Polysaccharides are of two types based on their function and composition. Based on function, polysaccharides of two types storage and structural.
A. Storage polysaccharide – starch.
B. Structural polysaccharide – cellulose.

General properties of carbohydrates

• Carbohydrates act as energy reserves, also stores fuels, and metabolic intermediates.
• Ribose and deoxyribose sugars forms the structural frame of the genetic material, RNA and DNA.
• Polysaccharides like cellulose are the structural elements in the cell walls of bacteria and plants.
• Carbohydrates are linked to proteins and lipids that play important roles in cell interactions.
• Carbohydrates are organic compounds, they are aldehydes or ketones with many hydroxyl groups.

Physical Properties of Carbohydrates

• Steroisomerism – Compound shaving same structural formula but they differ in spatial configuration. Example: Glucose has two isomers with respect to penultimate carbon atom. They are D-glucose and L glucose.
• Optical Activity – It is the rotation of plane polarized light forming (+) glucose and (-) glucose.
• Diastereoisomeers – It the configurational changes with regard to C2, C3, or C4 in glucose. Example: Mannose, galactose.
• Annomerism – It is the spatial configuration with respect to the first carbon atom in aldoses and second carbon atom in ketoses.

Biological Importance

• Carbohydrates are chief energy source, in many animals, they are instant source of energy. Glucose is broken down by glycolysis/ kreb’s cycle to yield ATP.
• Glucose is the source of storage of energy. It is stored as glycogen in animals and starch in plants.
• Stored carbohydrates acts as energy source instead of proteins.
• Carbohydrates are intermediates in biosynthesis of fats and proteins.
• Carbohydrates aid in regulation of nerve tissue and is the energy source for
• Carbohydrates gets associated with lipids and proteins to form surface antigens, receptor molecules, vitamins and antibiotics.
• They form structural and protective components, like in cell wall of plants and microorganisms.
• In animals they are important constituent of connective tissues.
• They participate in biological transport, cell-cell communication and activation of growth factors.
• Carbohydrates that are rich in fiber content help to prevent constipation.
• Also they help in modulation of immune system.


Read Also: Proteins: Definition structure and functions


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