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Carbohydrates are one of the three macronutrients that are essential to your body’s energy needs. They are vital in every stage of life, and are your brain’s preferred fuel source.
At the molecular level, carbohydrates are composed of carbon, hydrogen and oxygen. They conform to the empirical formula (CH2O)n, where n is typically 3-6.
Monosaccharides
Monosaccharides are the simplest type of carbohydrate. They cannot be broken down into smaller sugars by hydrolysis, but they can combine with other monosaccharides to form more complex types of carbohydrates.
Glucose (C6H12O6) is one of the most common monosaccharides used by most living things. It is also an important energy molecule.
Another example of a monosaccharide is galactose, which occurs in milk and is linked to glucose in the disaccharide lactose.
Fructose, which is found in fruits and other foods, is another common monosaccharide. Although it has the same chemical formula as glucose, the atoms are arranged differently to create molecules with different properties.
These differences are called isomers. They vary in their chirality, which is the way the carbon atoms are arranged.
Disaccharides
Disaccharides are carbohydrate molecules formed when two monosaccharides join together. During this process, an -OH group from one monosaccharide is displaced and an H (hydrogen) atom from another.
A covalent bond between the bonded atoms forms a water molecule, H2O. This bond is known as a glycosidic bond.
Some of the most common examples of disaccharides are sucrose, lactose and maltose. Sucrose, a sugar produced by green plants, consists of one molecule of glucose and one molecule of fructose bonded via an a-,b-linkage; lactose, found in the milk of all mammals, consists of glucose and galactose linked together via a b-1,4-glycosidic bond; maltose is produced by digestion of starch.
In addition, starch is a polysaccharide made of long chains of glucose. Plants and animals use this form of sugar to store energy, and it is a common component in human diets.
Polysaccharides
Polysaccharides (poly- meaning “many”) are long chains of monosaccharides bound together by glycosidic linkages. These chains can be branched or unbranched, and they may contain different types of monosaccharides.
Polysaccharide molecules have a variety of functions in nature, including energy storage, cellular messaging and structural support. The structure and function of a polysaccharide depends on the interactions between its hydroxyl groups (OH), other side groups and the configurations of its molecules.
Animals and plants use glucose, the most abundant sugar in nature, to store energy in the form of starch or glycogen. These branched polysaccharides are usually stored in the liver and muscle for energy when needed, but are also broken down by digestion to release their sugar content.
Plants use cellulose, another polysaccharide, to form strong fibers. Certain animals, like arthropods, make chitin that is used as part of the exoskeleton. Glycosaminoglycans, which are long unbranched polysaccharides made from repeating disaccharide units, are also found in connective tissues such as collagen and elastin.
Glycoproteins
Glycoproteins are proteins that have been modified by attaching sugar (carbohydrate) molecules to specific amino acid side-chains. This modification is known as glycosylation and can be either cotranslational or posttranslational.
Glycosylation takes place within two membrane-bound organelles in the cell: the rough endoplasmic reticulum and the Golgi complex. During the process of glycosylation, carbohydrates are linked to protein through N-linked sugars, which are attached to nitrogen-containing amino acid residues.
O-linked oligosaccharides are also used for bonding to protein and are attached to oxygen-containing amino acid residues, such as serine or threonine. These sugars are attached by enzymes called glycanases and are then further modified in the Golgi complex.
Glycoproteins are important components of a cell and have many functions. These include energy storage, cell-cell and cell-agent recognition, intracellular transport and signaling events.