Monosaccharides
Monosaccharides are the simplest form of carbohydrates and serve as the fundamental building blocks of more complex sugars. They have a chemical formula generally based on the unit Cn(H2O)n, where 'n' typically ranges from 3 to 7. The most common monosaccharides, such as glucose, fructose, and galactose, play critical roles in energy metabolism.
The molecular structure of monosaccharides can be straight chained or take on a ring form, called a cyclic hemiacetal. For instance, glucose can exist as either alpha or beta glucose depending on the orientation of the -OH group on the anomeric carbon in the ring.
This versatile chemical structure of monosaccharides leads to their involvement in various biological processes. Apart from their role in energy provision, they also contribute to the structural integrity of cell walls and the formation of nucleic acids.
Disaccharides
When two monosaccharides undergo a dehydration reaction, they form a disaccharide. This process forms a covalent bond known as a glycosidic bond. Disaccharides like sucrose, lactose, and maltose are common in our diet and provide an immediate source of energy once broken down into their constituent monosaccharides by digestive enzymes.
Where monosaccharides are akin to single beads, disaccharides are two beads joined together, and this formation impacts their physical and chemical properties. For instance, lactose is a disaccharide found in milk, consisting of glucose and galactose, and its digestion is vital for the metabolism of milk sugars in mammals.
Polysaccharides
Polysaccharides are composed of long chains of monosaccharides linked by glycosidic bonds and are involved in storage and structural functions in organisms. Starch, a storage polysaccharide in plants, and glycogen, found in animals, are both used to store glucose for energy.
Structural polysaccharides like cellulose give plants their rigidity, and chitin provides strength to the exoskeletons of insects. Due to their large size and complex structure, the molecular mass of polysaccharides is much greater than that of monosaccharides and disaccharides. In terms of solubility, polysaccharides are less soluble in water compared to mono- and disaccharides because of their larger and more complex structures.
Glycosidic Bond
The glycosidic bond is a type of covalent bond that links a carbohydrate molecule to another molecule, which can be another carbohydrate or a different type of organic molecule. It forms when a hydroxyl group of one sugar molecule reacts with the anomeric carbon of another, releasing a molecule of water – this is why it is often called a dehydration reaction.
The specific type of glycosidic bond, whether it is alpha or beta, affects the properties and digestibility of the carbohydrate. For example, the beta-glycosidic bonds in cellulose are difficult for humans to digest, whereas the alpha-glycosidic bonds in starch are readily broken down by enzymes in our digestive tract.
Molecular Mass Comparison
The molecular mass of a saccharide reflects its size and complexity. Monosaccharides, with only one sugar unit, have the lowest molecular mass. As we create more complex structures like disaccharides and polysaccharides, the molecular mass increases accordingly.
Polysaccharides have the highest molecular mass due to their long chains of monosaccharide units, allowing them to fulfill roles that smaller molecules cannot, such as storing energy or providing structural support in biological systems. Disaccharides, being intermediate, have a molecular mass greater than monosaccharides but less than polysaccharides.
