| Carbohydrates are chemical compounds | | | | together by a covalent glycosidic bond. |
| that contain oxygen, hydrogen, and | | | | The binding between the two sugars |
| carbon atoms, and no other elements. | | | | results in the loss of a hydrogen atom |
| They consist of monosaccharide sugars of | | | | (H) from one molecule and a hydroxyl |
| varying chain lengths and that have the | | | | group (OH) from the other. |
| general chemical formula Cn(H2O)n or are | | | | The most common disaccharides are |
| derivatives of such. The smallest value | | | | sucrose (cane or beet sugar - made from |
| for "n" is 3. A 3-carbon sugar is | | | | one glucose and one fructose), lactose |
| referred to as a triose, whereas a | | | | (milk sugar - made from one glucose and |
| 6-carbon sugar is called a hexose (see | | | | one galactose) and maltose (made of two |
| monosaccharides below). | | | | glucoses). The formula of these |
| Certain carbohydrates are an important | | | | disaccharides is C12H22O11. |
| storage and transport form of energy in | | | | Oligosaccharides and polysaccharides |
| most organisms, including plants and | | | | Oligosaccharides and polysaccharides are |
| animals. Carbohydrates are classified by | | | | composed of longer chains of |
| their number of sugar units: | | | | monosaccharide units bound together by |
| monosaccharides (such as glucose and | | | | glycosidic bonds. The distinction |
| fructose), disaccharides (such as | | | | between the two is based upon the number |
| sucrose and lactose), oligosaccharides, | | | | of monosaccharide units present in the |
| and polysaccharides (such as starch, | | | | chain. Oligosaccharides typically |
| glycogen, and cellulose). | | | | contain between three and nine |
| Pure carbohydrates contain carbon, | | | | monosaccharide units, and |
| hydrogen, and oxygen atoms, in a 1:2:1 | | | | polysaccharides contain greater than ten |
| molar ratio, giving the general formula | | | | monosaccharide units. |
| Cn(H2O)n. (This applies only to | | | | Definitions of how large a carbohydrate |
| monosaccharides, see below, although all | | | | must be to fall into each category vary |
| carbohydrates have the more general | | | | however. |
| formula Cn(H2O)m.) However, many | | | | Oligosaccharides are found as a common |
| important "carbohydrates" deviate from | | | | form of protein posttranslational |
| this, such as deoxyribose and glycerol, | | | | modification. Polysaccharides represent |
| although they are not, in the strict | | | | an important class of biological |
| sense, carbohydrates. Sometimes | | | | polymer. |
| compounds containing other elements are | | | | Examples include starch, cellulose, |
| also counted as carbohydrates (e.g. | | | | chitin and glycogen. |
| chitin, which contains nitrogen). | | | | Nutrition Strictly speaking, |
| The simplest carbohydrates are | | | | carbohydrates are not necessary for |
| monosaccharides, which are small | | | | human nutrition because proteins can be |
| straight-chain aldehydes and ketones | | | | converted to carbohydrates. The |
| with many hydroxyl groups added, usually | | | | traditional diet of some cultures |
| one on each carbon except the functional | | | | consists of very little carbohydrate, |
| group. Other carbohydrates are composed | | | | and these people remain relatively |
| of monosaccharide units and break down | | | | healthy.[citation needed] However, |
| under hydrolysis. These may be | | | | carbohydrates require less water to |
| classified as disaccharides, | | | | digest than proteins or fats and are the |
| oligosaccharides, or polysaccharides, | | | | most common source of energy. |
| depending on whether they have two, | | | | Proteins and fat are vital building |
| several, or many monosaccharide units. | | | | components for body tissue and cells, |
| Monosaccharides Monosaccharides may be | | | | and thus it could be considered |
| divided into aldoses, which have an | | | | advisable not to deplete such resources |
| aldehyde group on the first carbon atom, | | | | by necessitating their use in energy |
| and ketoses, which typically have a | | | | production. Carbohydrates, like |
| ketone group on the second. They may | | | | proteins, contain 4 kilocalories per |
| also be divided into trioses, tetroses, | | | | gram while fats contain 9 kilocalories |
| pentoses, hexoses, and so forth, | | | | and alcohol contains 7 kilocalories per |
| depending on how many carbon atoms they | | | | gram. |
| contain. For instance, glucose is an | | | | Based on evidence for risk of heart |
| aldohexose, fructose a ketohexose, and | | | | disease and obesity, the Institute of |
| ribose an aldopentose. | | | | Medicine recommends that American and |
| Further, each carbon atom that supports | | | | Canadian adults get between 40-65% of |
| a hydroxyl group (except for the first | | | | dietary energy from carbohydrates.[1] |
| and last) is optically active, allowing | | | | The Food and Agriculture Organization |
| a number of different carbohydrates with | | | | and World Health Organization jointly |
| the same basic structure. For instance, | | | | recommend that national dietary |
| galactose is an aldohexose but has | | | | guidelines set a goal of 55-75% of total |
| different properties from glucose | | | | energy from carbohydrates.[2] Foods high |
| because the atoms are arranged | | | | in carbohydrates Breads, pastas, |
| differently. | | | | potatoes, bran and cereals are all high |
| The straight-chain structure described | | | | in carbohydrates. |
| here is only one of the forms a | | | | Classification Dietitians and |
| monosaccharide may take. The aldehyde or | | | | nutritionists commonly classify |
| ketone group may react with a hydroxyl | | | | carbohydrates as simple (monosaccharides |
| group on a different carbon atom to form | | | | and disaccharides) or complex |
| a hemiacetal or hemiketal, in which case | | | | (oligosaccharides and polysaccharides), |
| there is an oxygen bridge between the | | | | depending on their chemical structure. |
| two carbon atoms, forming a heterocyclic | | | | The term complex carbohydrate was first |
| ring. Rings with five and six atoms are | | | | used in the Senate Select Committee |
| called furanose and pyranose forms and | | | | publication Dietary Goals for the United |
| exist in equilibrium with the | | | | States (1977), where it denoted "fruit, |
| straight-chain form. | | | | vegetables and whole-grains". Dietary |
| It should be noted that the ring form | | | | guidelines generally recommend that |
| has one more optically active carbon | | | | complex carbohydrates and nutrient-rich |
| than the straight-chain form, and so has | | | | simple carbohydrates such as fruit and |
| both an alpha and a beta form, which | | | | dairy products should make up the bulk |
| interconvert in equilibrium. However, | | | | of carbohydrate consumption. The USDA's |
| the carbohydrate may further react with | | | | Dietary Guidelines for Americans 2005 |
| an alcohol to form an acetal or ketal, | | | | dispenses with the simple/complex |
| in which case the two forms become | | | | distinction, instead recommending |
| distinct. | | | | fiber-rich foods and whole grains. |
| This is the basic type of link between | | | | The glycemic index and glycemic load |
| the monosaccharide units of larger | | | | systems are popular alternative |
| carbohydrates. | | | | classification methods which rank |
| Disaccharides Disaccharides are composed | | | | carbohydrates based on their effect on |
| of two monosaccharide units bound | | | | blood glucose levels. |