An organic compound is any member of a large class of chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of compounds such as carbonates, simple oxides of carbon and cyanides, as well as the allotropes of carbon, are considered inorganic. The division between "organic" and "inorganic" carbon compounds while "useful in organizing the vast subject of chemistry...is somewhat arbitrary".
Organic chemistry is the science concerned with all aspects of organic compounds. Organic synthesis is the methodology of their preparation.
Classification
See Organic chemistry#Classification of organic substances
Organic compounds may be classified in a variety of ways. One major distinction is between natural and synthetic compounds. They may also be distinguished by the presence of additional atoms of further elements, so-called heteroatoms. Organometallic compounds constitute a further subsection, characterized by covalent bonds between organic carbon and a metal.
There is also a large number of inorganic carbon compounds to distinguish from organic compounds.
Another distinction, based upon the size of organic compounds, distinguishes between small molecules and polymers.
Natural compounds
Natural compounds refer to those that are produced by plants or animals. Many of these are still extracted from natural sources because they would be far too expensive to be produced artificially. Examples include most sugars, some alkaloids and terpenoids, certain nutrients such as vitamin B12, and in general, those natural products with large or stereoisometrically complicated molecules which are present in reasonable concentrations in living organisms.
Further compounds of prime importance in biochemistry are antigens, carbohydrates, enzymes, hormones, lipids and fatty acids, neurotransmitters, nucleic acids, proteins, peptides and amino acids, vitamins and fats and oils.
Synthetic compounds
Compounds that are prepared by reaction of other compounds are referred to as "synthetic". They may be either compounds that already are found in plants or animals, or those that do not occur naturally.
Many polymers, including all plastics, are organic compounds.
Carbohydrates
[α] or saccharides[β] are the most abundant of the four major classes of biomolecules. They fill numerous roles in living things, such as the storage and transport of energy (e.g., starch, glycogen) and structural components (e.g., cellulose in plants and chitin in animals). In addition, carbohydrates and their derivatives play major roles in the working process of the immune system, fertilization, pathogenesis, blood clotting, and development.[1]
Carbohydrates make up most of the organic matter on Earth because of their extensive roles in all forms of life. First, carbohydrates serve as energy stores, fuels, and metabolic intermediates. Second, ribose and deoxyribose sugars form part of the structural framework of RNA and DNA. Third, polysaccharides are structural elements in the cell walls of bacteria and plants. In fact, cellulose, the main constituent of plant cell walls, is one of the most abundant organic compounds in the biosphere. Fourth, carbohydrates are linked to many proteins and lipids, where they play key roles in mediating interactions between cells and interactions between cells and other elements in the cellular environment.
Carbohydrates are simple organic compounds that are aldehydes or ketones with many hydroxyl groups added, usually one on each carbon atom that is not part of the aldehyde or ketone functional group. The basic carbohydrate units are called monosaccharides; examples are glucose, galactose, and fructose. The general stoichiometric formula of an unmodified monosaccharide is (C·H2O)n, where n is any number of three or greater; however, not all carbohydrates conform to this precise stoichiometric definition (e.g., uronic acids, deoxy-sugars such as fucose), nor are all chemicals that do conform to this definition automatically classified as carbohydrates.
1. Monosaccharides
This can be linked together into what are called polysaccharides (or oligosaccharides) in a large variety of ways. Many carbohydrates contain one or more modified monosaccharide units that have had one or more groups replaced or removed. For example, deoxyribose, a component of DNA, is a modified version of ribose; chitin is composed of repeating units of N-acetylglucosamine, a nitrogen-containing form of glucose.
While the scientific nomenclature of carbohydrates is complex, the names of carbohydrates very often end in the suffix -ose. Glycoinformatics is the specialised field of study that deals with the specific and unique bioinformatics of carbohydrates.
2. Dissacharide
Two joined monosaccharides are called a disaccharide and these are the simplest polysaccharides. Examples include sucrose and lactose. They are composed of two monosaccharide units bound together by a covalent bond known as a glycosidic linkage formed via a dehydration reaction, resulting in the loss of a hydrogen atom from one monosaccharide and a hydroxyl group from the other. The formula of unmodified disaccharides is C12H22O11. Although there are numerous kinds of disaccharides, a handful of disaccharides are particularly notable.
Sucrose, pictured to the right, is the most abundant disaccharide, and the main form in which carbohydrates are transported in plants. It is composed of one D-glucose molecule and one D-fructose molecule. The systematic name for sucrose, O-α-D-glucopyranosyl-(1→2)-D-fructofuranoside, indicates four things:
Its monosaccharides: glucose and fructose
Their ring types: glucose is a pyranose, and fructose is a furanose
How they are linked together: the oxygen on carbon number 1 (C1) of α-D-glucose is linked to the C2 of D-fructose.
The -oside suffix indicates that the anomeric carbon of both monosaccharides participates in the glycosidic bond.
Lactose, a disaccharide composed of one D-galactose molecule and one D-glucose molecule, occurs naturally in mammalian milk. The systematic name for lactose is O-β-D-galactopyranosyl-(1→4)-D-glucopyranose. Other notable disaccharides include maltose (two D-glucoses linked α-1,4) and cellulobiose (two D-glucoses linked β-1,4).
3. Polysaccharide
Oligosaccharides and polysaccharides are composed of longer chains of monosaccharide units bound together by glycosidic bonds. The distinction between the two is based upon the number of monosaccharide units present in the chain. Oligosaccharides typically contain between two and nine monosaccharide units, and polysaccharides contain greater than ten monosaccharide units. Definitions of how large a carbohydrate must be to fall into each category vary according to personal opinion. Examples of oligosaccharides include the disaccharides mentioned above, the trisaccharide raffinose and the tetrasaccharide stachyose.
Oligosaccharides are found as a common form of protein posttranslational modification. Such posttranslational modifications include the Lewis and ABO oligosaccharides responsible for blood group classifications and so of tissue incompatibilities, the alpha-Gal epitope responsible for hyperacute rejection in xenotransplanation, and O-GlcNAc modifications.
Polysaccharides represent an important class of biological polymers. Their function in living organisms is usually either structure- or storage-related. Starch (a polymer of glucose) is used as a storage polysaccharide in plants, being found in the form of both amylose and the branched amylopectin. In animals, the structurally-similar glucose polymer is the more densely-branched glycogen, sometimes called 'animal starch'. Glycogen's properties allow it to be metabolized more quickly, which suits the active lives of moving animals.
Cellulose and chitin are examples of structural polysaccharides. Cellulose is used in the cell walls of plants and other organisms, and is claimed to be the most abundant organic molecule on earth. It has many uses such as a significant role in the paper and textile industries, and is used as a feedstock for the production of rayon (via the viscose process), cellulose acetate, celluloid, and nitrocellulose. Chitin has a similar structure, but has nitrogen-containing side branches, increasing its strength. It is found in arthropod exoskeletons and in the cell walls of some fungi. It also has multiple uses, including surgical threads.
Other polysaccharides include callose or
laminarin, chrysolaminarin, xylan, mannan, fucoidan,starch and galactomannan.
Nutrition
Grain products: rich sources of complex and simple carbohydratesFoods high in carbohydrates include breads, pastas, beans, potatoes, bran, rice, and cereals. Most such foods are high in starch. Carbohydrates require less water to digest than proteins or fats and are the most common source of energy in living things. Proteins and fat are necessary building components for body tissue and cells, and are also a source of energy for most organisms.
Carbohydrates are not essential nutrients in humans: the body can obtain all its energy from protein and fats. However, the brain and neurons generally cannot burn fat and need glucose for energy; the body can make some glucose from a few of the amino acids in protein and also from the glycerol backbone in triglycerides. Carbohydrate contains 15.8 kilojoules (3.75 calories) and proteins 16.8 kilojoules (4 calories) per gram, respectively, while fats contain 37.8 kilojoules (9 calories) per gram. In the case of protein, this is somewhat misleading as only some amino acids are usable for fuel. Likewise, in humans, only some carbohydrates are usable for fuel, as in many monosaccharides and some disaccharides. Other carbohydrate types can be used, but only with the assistance of gut bacteria. Ruminants and termites can even process cellulose, which is indigestible to other organisms.
Based on the effects on risk of heart disease and obesity, the Institute of Medicine recommends that American and Canadian adults get between 40-65% of dietary energy from carbohydrates.The Food and Agriculture Organization and World Health Organization jointly recommend that national dietary guidelines set a goal of 55-75% of total energy from carbohydrates, but only 10% directly from sugars (their term for simple carbohydrates).
Classification
Carbohydrates can be classified as simple (monosaccharides and disaccharides) or complex (oligosaccharides and polysaccharides). The term complex carbohydrate was first used in the Senate Select Committee publication Dietary Goals for the United States (1977), where it denoted "fruit, vegetables and whole-grains".[9] Dietary guidelines generally recommend that complex carbohydrates, and such nutrient-rich simple carbohydrate sources such as fruit (glucose or fructose) and dairy products (lactose) make up the bulk of carbohydrate consumption. This excludes such sources of simple sugars as candy and sugary drinks.
The USDA's Dietary Guidelines for Americans 2005 dispensed with the simple/complex distinction, instead recommending fiber-rich foods and whole grains.[10]
The glycemic index and glycemic load concepts have been developed to characterize food behavior during human digestion. They rank carbohydrate-rich foods based on the rapidity of their effect on blood glucose levels. The insulin index is a similar, more recent classification method that ranks foods based on their effects on blood insulin levels, which are caused by glucose (or starch) and some amino acids in food. Glycemic index is a measure of how quickly food glucose is absorbed, while glycemic load is a measure of the total absorbable glucose in foods.
