Carbohydrates are macronutrients and are one of the three main ways by which our body obtains its energy. They are called carbohydrates as they comprise of carbon, hydrogen, and oxygen at their chemical level. Carbohydrates are essential nutrients which include sugars, fibers, and starches. They are found in grains, vegetables, fruits, and in milk products. They are the basic food groups which play an important role in a healthy life.

The food containing carbohydrates are converted into glucose/blood sugar during digestion by the digestive system. Our body utilizes this sugar as a source of energy for the cells, organs, and tissues. The extra amount of energy or sugar is stored in our muscles and liver for further requirement. The term ‘carbohydrate’ is derived from a French term ‘hydrate de carbone‘ meaning ‘hydrate of carbon‘. The general formula of this class of organic compounds is Cn(H2O)n.


Physiological Classification of Carbohydrates

The carbohydrates are further classified into simple and complex which is mainly based on their chemical structure and degree of polymerization.

  • Simple Carbohydrates (Monosaccharides, Disaccharides and Oligosaccharides)

Simple carbohydrates have one or two sugar molecules. In simple carbohydrates, molecules are digested and converted quickly resulting in a rise in the blood sugar levels. They are abundantly found in milk products, beer, fruits, refined sugars, candies, etc. These carbohydrates are called as empty calories as they do not possess fiber, vitamins, or minerals.

Plants, being producers, synthesize glucose (C6H12O6) by using raw materials like carbon dioxide and water in the presence of sunlight. This process of photosynthesis converts solar energy to chemical energy. Consumers feed on plants and harvest energy stored in the bonds of the compounds synthesized by plants.

1. Monosaccharides:

Glucose is an example of a carbohydrate monomer or monosaccharide. Other examples of monosaccharides include mannose, galactose, fructose, etc. The structural organization of monosaccharides is as follows:


Monosaccharides may be further classified depending on the number of carbon atoms:

(i)Trioses (C3H6O3): These have three carbon atoms per molecule. Example: Glyceraldehyde

(ii)Tetroses (C4H6O4): These monosaccharides have four carbon atoms per molecule. Example: Erythrose.

Similarly, we have-

(iii) Pentoses,

(iv) Hexoses, and

(v) Heptoses

2. Disaccharides

Two monosaccharides combine to form a disaccharide. Examples of carbohydrates having two monomers include- Sucrose, Lactose, Maltose, etc.


3. Oligosaccharides:

Carbohydrates formed by the condensation of 2-9 monomers are called oligosaccharides. By this convention, trioses, pentoses, hexoses are all oligosaccharides.

  • Complex Carbohydrates (Polysaccharides)

Complex carbohydrates have two or more sugar molecules, hence they are referred as starchy foods. In complex carbohydrates, molecules are digested and converted slowly compared to simple carbohydrates.They are richly found in lentils, beans, peanuts, potatoes, peas, corn, whole-grain bread, cereals, etc.

Polysaccharides are complex carbohydrates formed by the polymerization of a large number of monomers. Examples of polysaccharides include starch, glycogen, cellulose, etc. which exhibit extensive branching and are homopolymers – made up of only glucose units.


  1. Starch is composed of two components- amylose and amylopectin. Amylose forms the linear chain and amylopectin is a much-branched chain.
  2. Glycogen is called animal starch. It has a structure similar to starch but has more extensive branching.
  3. Cellulose is a structural carbohydrate and is the main structural component of plant cell wall. It is a fibrous polysaccharide with a high tensile strength. In contrast to starch and glycogen, cellulose forms a linear polymer.

Functions of Carbohydrates

The main function of carbohydrates is to provide energy and food to the body and to the nervous system.

Carbohydrates are known as one of the basic component of food including sugars, starch, and fiber which are abundantly found in grains, fruits, and milk products.

Carbohydrates are also known as starch, simple sugars, complex carbohydrates and so on.

It is also involved in fat metabolism and prevents ketosis.

Inhibits the breakdown of proteins for energy as they are the primary source of energy.

An enzyme by name amylase assists in the breakdown of starch into glucose, finally to produce energy for metabolism.

Sources of Carbohydrates

  1. Simple sugars are found in the form of fructose in many fruits.
  2. Galactose is present in all dairy products.
  3. Lactose is abundantly found in milk and other dairy products.
  4. Maltose is present in cereal, beer, potatoes, processed cheese, pasta, etc.
  5. Sucrose is naturally obtained from sugar and honey containing small amounts of vitamins and minerals.

These simple sugars that consist of minerals and vitamins exist commonly in milk, fruits, and vegetables. Many refined and processed foods like white flour, white rice, and sugar lack important nutrients and hence, they are labeled “enriched.” It is quite healthy to use vitamins, carbohydrates and all other organic nutrients in their normal forms.

Carbohydrate Foods

Eating too much sugar results in an abnormal increase in calories which finally leads to obesity and in turn low calories leads to malnutrition. Hence, a fine balance needs to be maintained. That is the reason a balanced diet is stressed so much by dietitians.

Good Carbs Bad Carbs
High in Nutrients Low in nutrients
Moderate in calories High in calories
Low in sodium and saturated fats High in sodium and saturated fats
Low in trans-fat and cholesterol High in trans-fat and cholesterol
They are complex carbs. For instance: Legumes, vegetables, whole grains, fruits, and beans. Foods considered bad carbs rarely have any nutritional value. Some of the foods include white flour, rice, pastries, sodas and processed foods.

Stay tuned with Byju’s to learn more about carbohydrates and other biology-related concepts.

Practise This Question

Which one of the following may be lost during washing of vegetables after cutting or peeling it?