Carbohydrates
Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen, typically with a ratio of hydrogen to oxygen similar to water (2:1). They serve as a primary source of energy for the body. Structurally, carbohydrates can be classified into three main categories: monosaccharides, disaccharides and polysaccharides.
Monosaccharides, such as glucose and fructose, are the simplest form of carbohydrates and are the fundamental building blocks for more complex carbohydrates. When two monosaccharides combine, they form a disaccharide through a condensation reaction, which involves the formation of a glycosidic bond.
Glucose exists in two structural forms known as alpha (α) and beta (β) glucose, which differ in the orientation of the hydroxyl group (-OH) attached to the first carbon atom in the glucose molecule. In alpha glucose, the hydroxyl group on the first carbon atom (carbon-1) is oriented downward relative to the plane of the ring formed by the carbon atoms. In beta glucose, the hydroxyl group on the first carbon atom (carbon-1) is oriented upward.
Disaccharides include sucrose and lactose. Sucrose is formed from glucose and fructose, while lactose is composed of glucose and galactose.
Polysaccharides are long chains of monosaccharide units linked together. They play a role in energy storage, structural support and cellular interactions in various organisms.
Types of polysaccharide
Starch is a polysaccharide primarily found in plants and serves as their main energy storage form. It is composed of two types of molecules: amylose and amylopectin. Amylose consists of long, unbranched chains of alpha glucose units connected by 1,4-glycosidic bonds, forming a helical structure. Amylopectin, on the other hand, has a branched structure with alpha glucose units connected by both 1,4-glycosidic bonds and 1,6-glycosidic bonds at the branching points. This branching allows for rapid mobilisation of glucose when energy is needed. Starch is broken down into glucose molecules during digestion through hydrolysis, providing a readily accessible energy source for humans and other animals.
Glycogen is the primary energy storage polysaccharide in animals, analogous to starch in plants. It is primarily found in the liver and muscles. Glycogen’s structure is similar to amylopectin but more extensively branched, which allows for a more rapid release of glucose when needed. It consists of alpha glucose units linked by 1,4-glycosidic bonds with additional 1,6-glycosidic bonds at the branching points. This highly branched structure ensures that glycogen can be quickly mobilised to meet sudden demands for glucose, particularly during intense physical activity or between meals.
Cellulose is a structural polysaccharide found in the cell walls of plants. It provides rigidity and strength to plant cells, contributing to the overall structural integrity of plants. Cellulose is composed of linear chains of beta glucose units connected by 1,4-glycosidic bonds. These chains align parallel to each other and form strong hydrogen bonds, creating a fibrous structure that resists degradation. Unlike starch and glycogen, cellulose cannot be broken down by human digestive enzymes. However, it is an essential dietary fibre that aids in digestion and promotes healthy bowel movements.
Chitin is another important polysaccharide, found in the exoskeletons of arthropods (such as insects and crustaceans) and in the cell walls of fungi. It is similar in structure to cellulose but contains nitrogenous groups attached to the beta glucose units. Chitin provides mechanical support and protection for organisms that possess it, contributing to the strength of their exoskeletons or cell walls.
