A carbohydrate stores energy and is a biological molecule which consists of basically three elements, carbon, oxygen, hydrogen. They are also known as saccharides which are further classified as monosaccharides, disaccharides, oligosaccharides, and polysaccharides. These are important biomolecules which play important roles in living organisms. Starch and sugar are the major sources of it and are found in cereals, potatoes, sugarcane, fruits, bread, milk, table sugar etc.
Monosaccharides are the major energy sources and fuel up the metabolism. Polysaccharides are the converted forms of the unwanted monosaccharides which are stored in the form of glycogen in the case of animals. In the case of plants, they are stored as starch. Deoxyribose is a component of DNA. Ribose is a component of RNA. Sucrose and lactose are examples of disaccharides which are composed of two monosaccharide units bound together by a covalent bond.
Every gram for simple sugars contains 3.87 calories of carbohydrates. Refined foods like honey, sweets, bread, jam, cereals etc have a higher level of carbohydrates whereas unrefined foods like rice have comparatively low carbohydrate levels.
They are not much required for humans as we obtain energy from proteins and fats although their complete absence in the body might lead to negative health conditions like higher levels of blood ketone bodies along with high blood sugar, dehydration, and electrolyte imbalance.
Glucose is the universal source of energy. Most of the carbohydrates cannot be metabolized by all organisms. Many organisms can easily break down starches into glucose; most organisms, however, cannot metabolize cellulose.
Carbohydrates are classified as simple and complex. Sugars (monosaccharides and disaccharides) come under simple carbohydrates whereas polysaccharides (or oligosaccharides) are complex carbohydrates as in grape sugar which is the monosaccharide glucose, cane sugar is the disaccharide sucrose. The nutritional value of it is very less and depends on whether it is simple or complex.
However complex carbohydrates raise blood sugar rapidly whereas simple carbohydrates (e.g. fructose) raise blood glucose slowly. The glycemic index theories are developed to describe food behavior during human digestion. Glycemic index is a measure of how quickly food glucose is absorbed, while a glycemic load is a measure of the total absorbable glucose in foods. They rank carbohydrate-rich foods based on the magnitude of their effect on blood glucose levels.
Carbohydrate metabolism is defined as the biochemical mechanism which is liable for the breakdown and formation of carbohydrates in living organisms. Glucose, a simple sugar being the most important carbohydrate, is metabolized by nearly all known living things. In plants, Carbon dioxide and water synthesize carbohydrates by the process of photosynthesis storing the absorbed energy internally in the form of starch or lipids.
Energy obtained from metabolism (e.g., oxidation of glucose) is usually stored temporarily within cells in the form of ATP. Organisms which can respire aerobically metabolize glucose and oxygen to release energy with carbon dioxide and water as byproducts. The cells undergo metabolic reaction known as catabolism to extract energy.