Carbohydrates Explained: Classification, Chemical Nature & Biological Roles

What are carbohydrates?

Carbohydrates are essential biomolecules found in almost all forms of life. They are the most abundant organic compounds in nature and play a major role in the structure and function of cells. Chemically, carbohydrates are made up of carbon (C), hydrogen (H), and oxygen (O), usually in the ratio of 1:2:1. Common examples include glucose, starch, and cellulose.

Carbohydrates serve as a primary source of energy in our body and are also involved in various metabolic activities. They are found in foods like rice, wheat, potatoes, fruits, and vegetables. In plants, carbohydrates are produced through photosynthesis, which is the process of converting light energy into chemical energy.

Classification of Carbohydrates

Carbohydrates are broadly classified into three categories based on their chemical structure and complexity:

1. Monosaccharides

Monosaccharides are the simplest form of carbohydrates and cannot be broken down into smaller sugar units. They are the basic building blocks of more complex carbohydrates.

• Examples: Glucose, Fructose, Galactose
• Properties: Sweet in taste, Soluble in water and Reducing sugars

2. Disaccharides

Disaccharides are formed when two monosaccharide molecules are joined together by a glycosidic bond. They can be broken down into monosaccharides through hydrolysis.

• Examples: Sucrose (glucose + fructose), Lactose (glucose + galactose), Maltose (glucose + glucose)
• Properties: Sweet and soluble in water, some are reducing sugars (e.g., maltose), while others are not (e.g., sucrose)

3. Polysaccharides

Polysaccharides are complex carbohydrates made up of many monosaccharide units. They are generally not sweet and are insoluble or partially soluble in water.

• Examples: Starch, Glycogen, Cellulose, Chitin
• Types: Storage Polysaccharides: Starch in plants, glycogen in animals

Structural Polysaccharides: Cellulose in plants, chitin in fungi and exoskeletons of insects

Table: Overview classification of carbohydrates

Chemical Nature of Carbohydrates

Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds upon hydrolysis. The chemical formula of most simple carbohydrates is (CH₂O)_n, where n is the number of carbon atoms.

Basic Structural Features:

1. Functional Group:
   • Aldehyde group (-CHO) in aldoses (e.g., glucose)
   • Ketone group (>C=O) in ketoses (e.g., fructose)
2. Hydroxyl Groups (-OH):
   • Present on most carbon atoms, making carbohydrates polar and soluble in water
3. Stereochemistry:
   • Carbohydrates exhibit isomerism (e.g., D and L forms)
   • Optical isomerism due to asymmetric carbon atoms
4. Glycosidic Bonds:
   • Covalent bonds that link monosaccharide units in disaccharides and polysaccharides
5. Reducing and Non-Reducing Sugars:
   • Reducing sugars have a free aldehyde or ketone group and can reduce Benedict’s or Fehling’s solution
   • Non-reducing sugars do not have free aldehyde or ketone groups

Biological Role of Carbohydrates

Carbohydrates perform several vital roles in living organisms. Here are some of their most important biological functions:

1. Source of Energy

• Carbohydrates like glucose are the body’s main source of energy.
• During cellular respiration, glucose is broken down to produce ATP.

2. Energy Storage

• Animals store carbohydrates in the form of glycogen in liver and muscles.
• Plants store carbohydrates as starch in seeds, roots, and tubers.

3. Structural Role

• Cellulose provides structural support in plant cell walls.
• Chitin serves a similar function in the exoskeletons of insects and crustaceans.

4. Recognition and Signalling

• Glycoproteins and glycolipids on cell surfaces are involved in cell-cell recognition and signalling processes.
• Carbohydrates play a role in immune responses and hormone function.

5. Protective Function

• Mucopolysaccharides like hyaluronic acid provide lubrication and protection in joints and eyes.
• Carbohydrates in mucus help trap pathogens and protect against infections.

6. Role in Metabolism

• Carbohydrates act as metabolic intermediates.
• Ribose and deoxyribose are important sugars in nucleotides (RNA and DNA).

Conclusion

Carbohydrates are fundamental to life. They provide energy and structure and are involved in various biochemical processes. Understanding their classification, chemical properties, and biological functions helps us appreciate their importance in both health and disease. Whether it’s a simple sugar like glucose or a complex molecule like cellulose, each carbohydrate plays a unique and vital role in living organisms.

FAQs

Q1. What is the basic unit of carbohydrates?

Monosaccharides are the basic units or monomers of carbohydrates.

Q2. Are all carbohydrates sugars?

No. While all sugars are carbohydrates, not all carbohydrates are sweet or considered sugars (e.g., cellulose).

Q3. Why are carbohydrates called quick energy sources?

They are easily broken down into glucose, which is rapidly used to produce energy (ATP) in cells.

Q4. What is the difference between starch and cellulose?

Both are polysaccharides made of glucose units, but starch has alpha linkages (digestible by humans), while cellulose has beta linkages (not digestible by humans).

Q5. Do carbohydrates contribute to weight gain?

Excess intake of any macronutrient, including carbohydrates, can contribute to weight gain, especially if not balanced with physical activity.