The empirical formula of carbohydrates is CH2O. The formula explains the name ‘Carbohydrate’, which put means watered carbon. It contains carbon, hydrogen and oxygen in a similar proportion that is found in water. Carbohydrates are naturally occurring ketones or aldehydes which contain multiple hydroxyl groups.
Carbohydrate is one of the primary dietary sources of energy for humans and animals. They are found as macronutrients in different food and beverages. For example, fruits, vegetables, grains are natural sources of carbohydrates. Carbohydrates help regulate the nerve tissues and are the primary source of energy for the brain and the human body.
They are present in plant cells in cellulose and starch and glucose (simple sugar) and glycogen in animal cells. Carbohydrates structure and classification is explained below in detail.
Classification of Carbohydrates
Carbohydrates can be classified into three types based on the number of sugar molecules in the compound- Monosaccharides, Disaccharides and Polysaccharides.
Monosaccharides:
These are single-molecule and simple sugars. These cannot be hydrolysed further into simple compounds. These are further categorised based on the presence of Ketones and Aldehydes in them, known as Ketoses and Aldoses.
The suffix ‘ose’ in the end denotes the presence of sugar molecules. Monosaccharides are also classified based on the number of atoms in them, for example, Triose, Tetrose, Pentose, Hexose, heptoses etc. The most common monosaccharides are Glucose, Fructose and Galactose.
- Glucose:
Glucose is the most abundant monosaccharide. The molecular formula of glucose is C6H12O6. It comes in the form of colourless or white solids and is very soluble in water. As it has 6 carbon atoms, it is also called Aldohexose. It is a significant source of energy found in all organisms.
It circulates in the blood as blood sugar. Glucose is considered the building block of disaccharides and polysaccharides. The below figure shows the depiction of glucose molecule both in 6 membered ring and open-chain form.
- Fructose:
Fructose is fruit sugar typically found in fruits, honey, and even vegetables. It is another dietary monosaccharide similar to glucose, and the formula is the same too, C6H12O6. Fructose is ketonic and is a 5-member ring structure, which makes it different from glucose, hence called Ketohexose. It is directly absorbed into the blood during digestion.
Fructose is bonded with glucose to form the disaccharide sucrose.
- Galactose:
This is also known as milk sugar. It also exists in both cycling and open-chain form. At the same time, the chemical formula is C6H12O6. It has 4 cyclic isomers, of which two are 6-membered rings, and the other two are five-membered rings. Galactose is found in milk and other dairy products, sugar beets, avocado.
- Triose (3C), Tetrose (4C), Pentose (5C) and Hexose (6C):
Based on the number of carbon atoms, Monosaccharide carbohydrates can be classified as Triose (3C), Tetrose (4C), Pentose (5C) and Hexose (6C). The prefixes indicate the number of carbons in a sugar. These are further categorised into Ketose and Aldose.
| Sugar Name | Aldose | Ketose |
| Triose (C₃H₆O₃) | Glyceraldehyde | Dihydroxyacetone |
| Tetrose (C₄H₈O₄) | Erythrose | Erythrulose |
| Pentose (C₅H₁₀O₅) | Ribose | Ribulose |
| Hexose (C₆H₁₂O₆) | Glucose | Fructose |
Disaccharides:
As the name suggests, these are two molecule sugars. These are formed when two same or different monosaccharides are merged through a covalent bond. They have a glycosidic linkage which is formed through a dehydration reaction.
This results in the loss of a hydrogen atom and hydroxyl group. The generic formula of disaccharides is C12H22O11. The common disaccharides are Sucrose, Lactose and Maltose.
- Sucrose:
Sucrose is formed by the combination of one molecule from glucose and one molecule from fructose. It is connected by an acetal oxygen bridge from the hemiacetal of glucose and fructose. It is naturally and abundantly found in plants and helps in refining table sugar. It is highly soluble in water.
Sucrose is classified as non-reducing sugar as it does not contain any anomeric hydroxyl group.
- Lactose:
Lactose is composed of galactose and glucose subunits. It results from condensing galactose and glucose together by β-(1→4) glycosidic linkage. It is found in the milk and mammary glands of mammals. It does have a free anomeric carbon in the glucose residue.
- Maltose:
Maltose, also known as Malt Sugar. It is made from malt and starch by dehydration reaction of two glucose molecules joined α(1→4) glycosidic linkage. It is a two-unit member amylose homologous series that results from the enzymatic hydrolysis.
It gets converted into two molecules by enzyme maltase that hydrolyses the glycosidic bond. It is found in germinating seeds and tissues where the starch gets broken down.
Polysaccharides:
Polysaccharides are known as Glycan. These are polymers and are composed of 10 or more monosaccharide units. They are combined through glycosidic linkages. Hence, they are known to be complex carbohydrates formed through the process of polymerisation.
The generic formula of polysaccharides is (C6H10O5)n. Here n is a number larger than 40.
Polysaccharides are of two types: The first type is known as Homopolysaccharides made up of the same type of monosaccharide. The second type is known as Heteropolysaccharides made up of two or more types of monosaccharides. Polysaccharides are found in starch, glycogen, cellulose. They are all homopolysaccharides.
- Starch: Glucose monomers make starch when combined by α 1-4 or α 1-6 glycosidic linkages. It is a mixture of amylose and amylopectin. When plants synthesise excess glucose, it is stored in different parts of the plant. Enzymes break down the starch consumed by humans into smaller molecules of glucose and maltose. The cells then absorb the glucose.
Conclusion:
To summarise, Carbohydrates are classified into three types Monosaccharides, Disaccharides and Polysaccharides. They have different monomers in their molecules. The glycosidic bonds and the dehydration reactions turn Monosaccharides into disaccharides and polysaccharides by eliminating the water molecule in each bond formed.
Carbohydrates are the primary source of energy in animals and humans and act as food reserves for plants. Examples of carbohydrates are grains like wheat, rice, vegetables like corn, potatoes, dairy products, fruits, legumes etc.
