Carbohydrates
The Most Abundant Biomolecule
Carbohydrates, also known as saccharides (coming from the word for "sugar"), are a biomolecule that consists of carbon, hydrogen, and oxygen.
The monomer for a carbohydrate is a monosaccharide, which exists in the ratio of CnH2nOn. Monosaccharides are classified based on their carbonyl functional group - if it is an aldehyde, the carb is an aldose; if it is a ketone, the carb is a ketose. The ending "-ose" refers to a sugar. The most well-known monosaccharide is the aldohexose (an aldose with six carbons, with "hex" referring to six), glucose, which is depicted in the image to the right and has the formula C6H12O6. While carbohydrates can be found as linear molecules, they tend to form rings in aqueous solutions.
As a polymer, polysaccharides exist as strings of monosaccharides held together by a type of covalent bond known as a glycosidic bond. Because of the propensity of the monosaccharides to cyclize in different forms, to vary in configuration, to be modified by the replacing of hydrogens with different functional groups, and to polymerize in a huge variety of ways (such forming branched or unbranched chains that can consist of all the same or of different monosaccharides), carbohydrates tend to vary more than other biomolecules in size and composition.
Energy
The most well-known function of carbohydrates is that of energy storage. Monosaccharides, such as glucose, can be used as a quick source of energy for cells/organisms.
Polysaccharides, such as glycogen and starch, are used to store energy for later use. While this energy is stored, it is still used for more immediate energy needs, as opposed to lipids, which will function as long-term energy storage.
Structure
Carbohydrates are also highly involved in structure. Polysaccharides form rigid structures that are used for structure and support, such as cellulose, which makes up the cell wall of a plant cell, and chitin, which is what the exoskeleton of an insect is composed of.
Information Carrying
Carbohydrates are not always found on their own - glycoconjugates are when carbohydrates are covalently bound to proteins or lipids and form a new type of biologically active molecule. These glycoconjugates usually involve short chain polymers of carbohydrates known as oligosaccharides.
Proteoglycans and glycoproteins are proteins with carbohydrates bound to them that are found in the extracellular matrix and on cell surfaces. Glycosphingolipids are lipids found in the plasma membrane that have carbohydrates as the hydrophilic head.
Glycoconjugates are highly involved in information carrying and cell communication. Some work for cell-to-cell communication, some label proteins so the cell knows what to do with them or where to send them, some serve as recognition sites for signaling molecules, and some serve as "identification" of types of organisms (such as specific bacteria) or for specific cell types (such as glycolipids that ID red blood cells as A, B, or AB blood type).