Carbohydrate

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Carbohydrates (from 'chitin in animals). Additionally, carbohydrates and their derivatives play major roles in the working process of the immune system, fertilization, pathogenesis, blood clotting, and development.

The basic carbohydrate units are called nitrogen-containing form of glucose. The names of carbohydrates often end in the suffix -ose.

Monosaccharides

Main article: Monosaccharide

Monosaccharides are the simplest carbohydrates in that they cannot be hydrolyzed to smaller carbohydrates. The general chemical formula of an unmodified monosaccharide is (C•H₂O)_n, where n is any number of three or greater.

Classification of monosaccharides

The α and β anomers of glucose. Note the position of the anomeric carbon (red or green) relative to the CH₂OH group bound to carbon 5: they are either on the opposite sides (α), or the same side (β).

Monosaccharides are classified according to three different characteristics: the placement of its ribose is an aldopentose (a five-carbon aldehyde), and fructose is a ketohexose (a six-carbon ketone).

Each carbon atom bearing a hydroxyl group (-OH), with the exception of the first and last carbons, are 1,3-dihydroxyacetone, the ketose corresponding to the aldose glyceraldehye, is a symmetric molecule with no stereocenters). The assignment of D or L is made according to the orientation of the asymmetric carbon furthest from the carbonyl group: in a standard Fischer projection if the hydroxyl group is on the right the molecule is a D sugar, otherwise it is an L sugar. Because D sugars are biologically far more common, the D is often omitted.

Conformation

The aldehyde or ketone group of a straight-chain monosaccharide will react reversibly with a hydroxyl group on a different carbon atom to form a heterocyclic ring with an oxygen bridge between two carbon atoms. Rings with five and six atoms are called furanose and pyranose forms, respectively, and exist in equilibrium with the straight-chain form.

During the conversion from straight-chain form to cyclic form, the carbon atom containing the carbonyl oxygen, called the equilibrium.

Use in living organisms

Monosaccharides are the major source of fuel for metabolism, being used both as an energy source (glucose being the most important in nature) and in biosynthesis. When monosaccharides are not needed by cells they are quickly converted into another form, such as polysaccharides.

Disaccharides

Main article: Disaccharide

Two joined monosaccharides are called disaccharides and represent the simplest polysaccharides. Examples include formula of unmodified disaccharides is C₁₂H₂₂O₁₁. Although there are numerous kinds of disaccharides, a handful of disaccharides are particularly notable.

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.

maltose (two D-glucoses linked α-1,4) and cellobiose (two D-glucoses linked β-1,4).

Oligosaccharides and polysaccharides

Main articles: 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 posttranslational modification. Such posttranslational modifications include the Lewis and ABO oligosaccharides responsible for blood group incompatibilities, the alpha-Gal epitope responsible for hyperacute rejection in xenotransplanation, and O-GlcNAc modifications.

Polysaccharides represent an important class of biological glycogen is used instead. Glycogen's properties allow it to be metabolized more quickly, which suits the active lives of locomotive animals.

nitrogen containing side branches, increasing its strength. It is found in arthropod exoskeletons and in the cell walls of some fungi. It has a variety of uses, for example in surgical threads.

Other polysaccharides include callose or laminarin, xylan, mannan, fucoidan, and galactomannan.

Nutrition

Carbohydrates require less water to digest than proteins or fats and are the most common source of energy. Proteins and fat are vital building components for body tissue and cells and are also a source of energy for the body.

Carbohydrates are not essential nutrients: the body can obtain all its energy from protein and fats ^[3] ^[4]. The brain cannot burn fat and needs glucose for energy, but the body can make this glucose from protein. Carbohydrates and proteins contain 4 kilocalories per gram while fats contain 9 kilocalories and alcohol contains 7 kilocalories per gram.^{[citation needed]}

Foods that are high in carbohydrates include breads, pastas, beans, potatoes, bran, rice and cereals.

Based on evidence for 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.^[5] 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% should be from Free sugars (their definition of simple carbohydrates).^[6]

The distinction between "good carbs" and "bad carbs" is an important attribute of low-carbohydrate diets, which promote a reduction in the consumption of grains and starches in favor of protein. The result is a reduction in ketosis.

Classification

Dieticians and nutritionists commonly classify carbohydrates as simple (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".^[7] Dietary guidelines generally recommend that complex carbohydrates and nutrient-rich simple carbohydrates such as fruit and dairy products make up the bulk of carbohydrate consumption. The USDA's Dietary Guidelines for Americans 2005 dispenses with the simple/complex distinction, instead recommending fiber-rich foods and whole grains.^[8]

The blood insulin levels. This system assumes that high glycemic index foods and low glycemic index foods can be mixed to make the intake of high glycemic foods more acceptable.^{[citation needed]}

The World Health Organisation and Food and Agriculture Organization's joint expert report on Diet, Nutrition and the Prevention of Chronic Diseases (WHO Technical Report Series 916) advises carbohydrate consumption of 55-75% carbohydrate, but restricts "Free sugar" intake to 10%. Its definition is "The term "free sugars" refers to all monosaccharides and disaccharides added to foods by the manufacturer, cook or consumer, plus sugars naturally present in honey, syrups and fruit juices." (page 56 of the report; note to Table 6: Ranges of population nutrient intake goals). This is their effective split between simple and complex carbohydrates.

Metabolism

Main article: Carbohydrate metabolism

^{This short section requires expansion.}

Catabolism

Catabolism is the metabolic reaction cells undergo in order to extract energy. There are two major catabolism:

Oligo/polysaccharides are cleaved first to smaller monosaccharides by enzymes called Glycoside hydrolases. The monosaccharide units can then enter into monosaccharide catabolism.

Carbohydrate chemistry

Carbohydrates are reactants in many organic reactions. For example:

References

^ Matthews, C. E.; K. E. Van Holde; K. G. Ahern (1999) Biochemistry. 3rd edition. Benjamin Cummings. ISBN 0-8053-3066-6
^ N.A.Campbell (1996) Biology (4th edition). Benjamin Cummings NY. p.23 ISBN 0-8053-1957-3
^ *http://www.ajcn.org/cgi/content/full/75/5/951-a
^ *http://jn.nutrition.org/cgi/reprint/136/5/1256?ijkey=ebf0450b5cf21e8d83dd43f62b5559254694f65f
^ Food and Nutrition Board (2002/2005). Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC: The National Academies Press. Page 769. ISBN 0-309-08537-3
^ Joint WHO/FAO expert consultation (2003). Diet, Nutrition and the Prevention of Chronic Diseases (PDF). Geneva: World Health Organization. Pages 55-56. ISBN 92-4-120916-X
^ Joint WHO/FAO expert consultation (1998), Carbohydrates in human nutrition, chapter 1. ISBN 92-5-104114-8.
^ DHHS and USDA, Dietary Guidelines for Americans 2005, Chapter 7 Carbohydrates

Carbohydrates

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