Carbohydrates are essential biomolecules that serve as primary energy sources and play crucial roles in cellular structure and recognition. Understanding their structure and function is fundamental for success in competitive biology exams like NEET.

Carbohydrates, also known as saccharides, are organic compounds composed of carbon (C), hydrogen (H), and oxygen (O) atoms, typically in a ratio of 1:2:1 (CH₂O)n. They are broadly classified based on the number of sugar units they contain.

Monosaccharides are the simplest carbohydrates and cannot be hydrolyzed into simpler sugars. They are typically sweet-tasting and soluble in water. Their general formula is (CH₂O)n, where n is usually between 3 and 7.

Key monosaccharides include:

<strong>Glucose:</strong> The primary source of energy for cells. It's a hexose (6-carbon sugar).

<strong>Fructose:</strong> Found in fruits and honey; also a hexose, often called fruit sugar.

<strong>Galactose:</strong> A component of milk sugar (lactose); also a hexose.

Disaccharides are formed when two monosaccharides are joined together by a glycosidic bond through a dehydration reaction (removal of a water molecule). They are also sweet and soluble.

Common disaccharides include:

<strong>Sucrose (Table Sugar):</strong> Glucose + Fructose. Transported in plants.

<strong>Lactose (Milk Sugar):</strong> Glucose + Galactose. Found in milk.

<strong>Maltose (Malt Sugar):</strong> Glucose + Glucose. Formed during starch digestion.

Polysaccharides are complex carbohydrates made up of long chains of monosaccharide units linked by glycosidic bonds. They can be linear or branched. They are generally not sweet and are often insoluble or sparingly soluble in water.

These serve as energy reserves in organisms.

<strong>Starch:</strong> The primary storage polysaccharide in plants. Composed of amylose (linear) and amylopectin (branched).

<strong>Glycogen:</strong> The storage polysaccharide in animals, primarily stored in the liver and muscles. It is more highly branched than amylopectin.

These provide structural support.

<strong>Cellulose:</strong> A major structural component of plant cell walls. It is a linear polymer of glucose units linked by β-1,4 glycosidic bonds. Humans cannot digest cellulose due to the lack of the enzyme cellulase.

<strong>Chitin:</strong> A structural polysaccharide found in the exoskeletons of arthropods (like insects and crustaceans) and in the cell walls of fungi. It is a polymer of N-acetylglucosamine.

Carbohydrates perform several vital functions in living organisms:

1. <strong>Energy Source:</strong> The primary and most readily available source of energy for cellular respiration. Glucose is the main fuel.

2. <strong>Energy Storage:</strong> Starch in plants and glycogen in animals store excess glucose for later use.

3. <strong>Structural Components:</strong> Cellulose provides rigidity to plant cell walls, and chitin forms exoskeletons and fungal cell walls.

4. <strong>Cell Recognition:</strong> Carbohydrate chains attached to proteins (glycoproteins) and lipids (glycolipids) on the cell surface act as cell surface markers, involved in cell-to-cell recognition, adhesion, and immune responses.

5. <strong>Precursors:</strong> Carbohydrates can be converted into other organic molecules, such as amino acids and lipids.

Focus on the definitions, structures, and functions of monosaccharides, disaccharides, and polysaccharides. Pay special attention to the examples and their biological significance (e.g., starch vs. glycogen, cellulose in plants, chitin in fungi).

Understand the concept of glycosidic bonds and how dehydration synthesis forms them. Also, be aware of the role of carbohydrates in cell recognition and as structural components.