(Carbohydrates Part 3: Properties, Structure and Examples of Disaccharides)
What are Disaccharides?
Disaccharides are carbohydrates which contain two covalently linked monosaccharide units. Sucrose, Maltose, Lactose, Trehalose and Cellobiose are naturally occurring disaccharides. The individual monosaccharide units in a disaccharide are called ‘residues’. All disaccharides are soluble in water
Glycosidic bonds links monosaccharide units
The monosaccharide units in disaccharides (and also in polysaccharides) are linked through a special type of covalent bond called Glycosidic bond (specifically O-glycosidic bond). O-glycosidic bond is formed by the reaction between the hydroxyl group of one monosaccharide with the anomeric carbon atom of the other. During the glycosidic bond formation, one molecule of water is eliminated as given in the diagram. Glycosidic bonds are strong covalent bonds and they can be hydrolyzed by treating with mild acids. The hydrolysis of the glycosidic bond of a disaccharide releases its corresponding monosaccharide units.
Chemical Structure, Characteristics, Examples & Classification
Monosaccharides are Simplest Sugars
Monosaccharides are the simplest carbohydrates. They are polyhydroxy aldehydes or ketones with a carbon backbone. The carbon backbone in monosaccharides usually consists of 3 – 6 carbon atoms. The simplest monosaccharides are glyceraldehyde and dihydroxyacetone (with 3 carbons). The most abundant monosaccharide in nature is a 6 carbon sugar called glucose. Majority of the monosaccharides follow the empirical formula C(H2O)n. Monosaccharide with five or more carbon can predominantly exist as cyclic structures in the aqueous condition. All monosaccharides are colourless, crystalline solids and that are readily soluble in water but insoluble in nonpolar solvents. Most of the monosaccharides are sweet in taste.
Chemical Structure of Monosaccharides
Ø All monosaccharides are polyhydroxy (contain many hydroxyl groups) aldehydes or ketones.
Ø The hydroxyl groups are attached to the carbon backbone.
Ø The number of carbon atoms in the backbone of monosaccharides varies from 3 to 6.
Ø The carbon backbone of monosaccharides is unbranched and individual carbon atoms are connected by single bonds.
Ø Monosaccharides are broadly classified into Aldoses and Ketoses.
Ø In the open chain conformation of a monosaccharide, one of the carbon atoms of the backbone is double bonded to an oxygen atom to form the carbonyl group (C=O).
Ø If the carbonyl group is at the end of the carbon chain it will be an aldehyde group (R – COH) and thus the sugar formed will be an Aldose sugar.
Ø Similarly, if the carbonyl group is inner to the carbon chain, it will be a keto group (C=O) and the sugar formed will be a Ketose sugar.
(Biochemistry of Carbohydrates: Introduction, Properties, Classification and Biological Significance)
Carbohydrates are polyhydroxy aldehydes or ketones
Carbohydrates are the most abundant bio-macro-molecules on the earth. They are commonly known as sugars because most of them have a sweet taste. Chemically all carbohydrates are polyhydroxy (contain many hydroxyl, – OH, groups) aldehydes or ketones. All carbohydrates are hydrates of carbon and they contain C, H and O. The ratio of hydrogen and oxygen in the majority of carbohydrates will be in 2:1 as in water. Some carbohydrates also contain nitrogen, phosphorous and sulfur. Majority of carbohydrates, not all, have the empirical formula (CH2O)n. In biochemistry, carbohydrates are denoted as saccharides. The term saccharide is derived from a Greek word ‘sakkharon’ meaning sugar.
Green plants fix the energy of sunlight by photosynthesis. In photosynthesis, the light energy is converted into the chemical energy and it is stored in some carbohydrates such as glucose, fructose, sucrose, starch etc. The oxidative breakdown of these carbohydrates by respiration release the energy stored in them and this energy is utilized for the various metabolic activities of the cells.
Collagen vs Keratin
(Similarities and Differences between Collagen and Keratin)
Collagen and Keratin are the main structural proteins. They are two main families of fibrous proteins and are abundantly present in the extracellular matrix and various connective tissues in animals.
Collagen: A linear structural fibrous protein abundantly found in the connective tissue and extracellular space of the cells. They are also found in tendons, ligaments, cornea, cartilage, bone, blood vessels, intervertebral discs and the dentine in teeth.
Keratin: A structural protein abundantly present in epithelial cells. They protect the epithelial cells from damage or stress. Keratin is also present in appendages such as feathers, hairs and nail, horn, claws and hooves of animals. Keratin is the structural material that makes the human skin.
The present post discusses the similarities and Differences between Collagen and Keratin with a Comparison Table.