Monosaccharides, Disaccharides, and Polysaccharides - SS3 Chemistry Lesson Note
Carbohydrates are one of the three main macronutrients, along with proteins and fats, and they play a crucial role in providing energy and structure to living organisms. Carbohydrates can be classified into three main groups based on their molecular structure: monosaccharides, disaccharides, and polysaccharides.
Monosaccharides: Monosaccharides are the simplest form of carbohydrates, consisting of a single sugar unit. They are often called "simple sugars" and serve as the building blocks for more complex carbohydrates. Monosaccharides have the general chemical formula (CH2O)n, where "n" represents the number of carbon atoms. The most common monosaccharides are:
● Glucose: Also known as blood sugar, glucose is the primary source of energy for cells. It is essential for fueling various cellular processes.
● Fructose: Found in fruits and honey, fructose is another important monosaccharide and is metabolised in the liver.
● Galactose: Present in milk and dairy products, galactose is less commonly encountered than glucose and fructose.
Monosaccharides are water-soluble and are usually colourless, crystalline solids. They can exist in either linear or ring forms, with the ring form being the more stable and predominant configuration in aqueous solutions.
Disaccharides: Disaccharides are formed by the chemical bonding of two monosaccharide units through a process known as dehydration or condensation reaction, resulting in the release of a water molecule. When energy is needed, disaccharides can be hydrolyzed (broken down) into their constituent monosaccharides through the addition of water. The most common disaccharides are:
● Sucrose: Composed of glucose and fructose, sucrose is commonly known as table sugar and is found in sugarcane, sugar beets, and various sweet fruits.
● Lactose: Made up of glucose and galactose, lactose is the sugar found in milk and dairy products.
● Maltose: Consisting of two glucose units, maltose is produced during the digestion of starchy foods and is often found in germinating seeds.
Polysaccharides: Polysaccharides are complex carbohydrates composed of long chains of monosaccharide units linked together by glycosidic bonds. They can be branched or unbranched, and their primary function is to store energy or provide structural support in organisms. Unlike monosaccharides and disaccharides, polysaccharides are generally not sweet-tasting. Some important polysaccharides include:
● Starch: A major energy storage polysaccharide in plants, starch is a glucose polymer. It occurs in two forms: amylose (unbranched) and amylopectin (branched).
● Glycogen: The primary storage polysaccharide in animals, including humans, glycogen is highly branched and serves as a short-term energy reserve in liver and muscle cells.
● Cellulose: The most abundant polysaccharide on Earth, cellulose forms the structural component of plant cell walls. It provides rigidity and strength to the cell walls, aiding in maintaining the overall structure of plants.
● Chitin: Found in the exoskeleton of arthropods (e.g., insects, crustaceans) and the cell walls of fungi, chitin is a structural polysaccharide composed of N-acetylglucosamine units.
Polysaccharides are typically large and insoluble in water, and their complex structures enable them to fulfil diverse functions in living organisms.
In conclusion, carbohydrates are a diverse group of biomolecules, and their various forms – monosaccharides, disaccharides, and polysaccharides – serve critical roles in providing energy and structural support for living organisms. The breakdown and utilisation of these carbohydrates are essential for sustaining life and maintaining proper physiological functions.