Biofelsefe — KARBOHİDRAT
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Biofelsefe — KARBOHİDRAT


SİTE İÇİ ARAMA       
 
 
   
 

🛑 KARBOHİDRAT

  • Karbohidratlar (sulu karbonlar) Yeryüzünde en bol bulunan biomoleküllerdir.
  • Her yıl fotosentez 100 milyar metrik ton CO2 ve H2O kütlesini selüloza ve başka bitki ürünlerine dönüştürür.
  • Karbohidratların oksidasyonu fotosentez yapmayan hücrelerin çoğunda birincil enerji kaynağıdır.
  • Karbohidrat polimerler (glikanlar) bakteri ve bitkilerin hücre duvarlarında ve hayvanların bağ dokularında yapısal ve koruyucu öğeler olarak hizmet eder.
  • Başka karbohidrat polimerler iskelet eklemlerine kayganlık verir.

 

  • Birçok karbohidrat (CH2O)n yapısındadır; başkaları azot, fosfat ya da kükürt de kapsar.
  • Karbohidratların başlıca üç sınıfı monosakkaridler, oligosakkaridler ve polisakkaridlerdir.
  • Sakkarid sözcüğü Yunanca ‘şeker’ anlamına gelen ‘sakkharon’ sözcüğünden türemiştir.
  • Monosakkaridler (yalın şekerler) tek bir polihidroksi aldehid ya da keton birimden oluşur.
  • Doğada en bol bulunan monosakkarid altı-karbonlu şeker D-glukozdur (dekstrot).
  • Dört ya da daha fazla karbonlu monosakkaridler döngülü yapıda olmaya yatkındır.
 
   

Gliseraldehid molekülünün iki enantiomerini temsil etmenin iki yolu. Enantiomerler birbirinin ayna imgesidir. Top-ve-çubuk modelleri moleküllerin edimsel betilenimini gösterir.

 

 



 
  Carbohydrate (B)

Carbohydrate (B)

Carbohydrate (B)

Introduction (B)

Introduction

Introduction (B)

Carbohydrate, class of naturally occurring compounds and derivatives formed from them. In the early part of the 19th century, substances such as wood, starch, and linen were found to be composed mainly of molecules containing atoms of carbon (C), hydrogen (H), and oxygen (O) and to have the general formula C6H12O6 other organic molecules with similar formulas were found to have a similar ratio of hydrogen to oxygen. The general formula Cx(H2O)y is commonly used to represent many carbohydrates, which means “watered carbon.”
 
 
Pathways for the utilization of carbohydrates.
 
Carbohydrates are probably the most abundant and widespread organic substances in nature, and they are essential constituents of all living things. Carbohydrates are formed by green plants from carbon dioxide and water during the process of photosynthesis. Carbohydrates serve as energy sources and as essential structural components in organisms; in addition, part of the structure of nucleic acids, which contain genetic information, consists of carbohydrate.
 

 



 
General Features (B)

Classification and nomenclature

Classification and nomenclature (B)


Although a number of classification schemes have been devised for carbohydrates, the division into four major groups—monosaccharides, disaccharides, oligosaccharides, and polysaccharides—used here is among the most common. Most monosaccharides, or simple sugars, are found in grapes, other fruits, and honey. Although they can contain from three to nine carbon atoms, the most common representatives consist of five or six joined together to form a chainlike molecule. Three of the most important simple sugars—glucose (also known as dextrose, grape sugar, and corn sugar), fructose ( fruit sugar), and galactose—have the same molecular formula, (C6H12O6), but, because their atoms have different structural arrangements, the sugars have different characteristics; i.e., they are isomers.

 
   

 

Slight changes in structural arrangements are detectable by living things and influence the biological significance of isomeric compounds. It is known, for example, that the degree of sweetness of various sugars differs according to the arrangement of the hydroxyl groups (―OH) that compose part of the molecular structure. A direct correlation that may exist between taste and any specific structural arrangement, however, has not yet been established; that is, it is not yet possible to predict the taste of a sugar by knowing its specific structural arrangement. The energy in the chemical bonds of glucose indirectly supplies most living things with a major part of the energy that is necessary for them to carry on their activities. Galactose, which is rarely found as a simple sugar, is usually combined with other simple sugars in order to form larger molecules.

Two molecules of a simple sugar that are linked to each other form a disaccharide, or double sugar. The disaccharide sucrose, or table sugar, consists of one molecule of glucose and one molecule of fructose; the most familiar sources of sucrose are sugar beets and cane sugar. Milk sugar, or lactose, and maltose are also disaccharides. Before the energy in disaccharides can be utilized by living things, the molecules must be broken down into their respective monosaccharides. Oligosaccharides, which consist of three to six monosaccharide units, are rather infrequently found in natural sources, although a few plant derivatives have been identified.

Polysaccharides (the term means many sugars) represent most of the structural and energy-reserve carbohydrates found in nature. Large molecules that may consist of as many as 10,000 monosaccharide units linked together, polysaccharides vary considerably in size, in structural complexity, and in sugar content; several hundred distinct types have thus far been identified. Cellulose, the principal structural component of plants, is a complex polysaccharide comprising many glucose units linked together; it is the most common polysaccharide. The starch found in plants and the glycogen found in animals also are complex glucose polysaccharides. Starch (from the Old English word stercan, meaning “to stiffen”) is found mostly in seeds, roots, and stems, where it is stored as an available energy source for plants. Plant starch may be processed into foods such as bread, or it may be consumed directly—as in potatoes, for instance. Glycogen, which consists of branching chains of glucose molecules, is formed in the liver and muscles of higher animals and is stored as an energy source.

 


Composition of cellulose and glucose
Cellulose and glucose are examples of carbohydrates.
 
The generic nomenclature ending for the monosaccharides is -ose; thus, the term pentose (pent = five) is used for monosaccharides containing five carbon atoms, and hexose (hex = six) is used for those containing six. In addition, because the monosaccharides contain a chemically reactive group that is either an aldehyde group or a keto group, they are frequently referred to as aldopentoses or ketopentoses or aldohexoses or ketohexoses. The aldehyde group can occur at position 1 of an aldopentose, and the keto group can occur at a further position (e.g., 2) within a ketohexose. Glucose is an aldohexose—i.e., it contains six carbon atoms, and the chemically reactive group is an aldehyde group.
 
 
   

 



Biological significance

Biological significance (B)

The importance of carbohydrates to living things can hardly be overemphasized. The energy stores of most animals and plants are both carbohydrate and lipid in nature; carbohydrates are generally available as an immediate energy source, whereas lipids act as a long-term energy resource and tend to be utilized at a slower rate. Glucose, the prevalent uncombined, or free, sugar circulating in the blood of higher animals, is essential to cell function. The proper regulation of glucose metabolism is of paramount importance to survival.

The ability of ruminants, such as cattle, sheep, and goats, to convert the polysaccharides present in grass and similar feeds into protein provides a major source of protein for humans. A number of medically important antibiotics, such as streptomycin, are carbohydrate derivatives. The cellulose in plants is used to manufacture paper, wood for construction, and fabrics.

 



Role in the biosphere

Role in the biosphere (B)

The essential process in the biosphere, the portion of Earth in which life can occur, that has permitted the evolution of life as it now exists is the conversion by green plants of carbon dioxide from the atmosphere into carbohydrates, using light energy from the Sun. This process, called photosynthesis, results in both the release of oxygen gas into the atmosphere and the transformation of light energy into the chemical energy of carbohydrates. The energy stored by plants during the formation of carbohydrates is used by animals to carry out mechanical work and to perform biosynthetic activities.

During photosynthesis, an immediate phosphorous-containing product known as 3-phosphoglyceric acid is formed.

 
 
   
 
This compound then is transformed into cell wall components such as cellulose, varying amounts of sucrose, and starch—depending on the plant type—and a wide variety of polysaccharides, other than cellulose and starch, that function as essential structural components. For a detailed discussion of the process of photosynthesis, see photosynthesis.

 



Role in human nutrition

Role in human nutrition (B)

The total caloric, or energy, requirement for an individual depends on age, occupation, and other factors but generally ranges between 2,000 and 4,000 calories per 24-hour period (one calorie, as this term is used in nutrition, is the amount of heat necessary to raise the temperature of 1,000 grams of water from 15 to 16 °C [59 to 61 °F]; in other contexts this amount of heat is called the kilocalorie). Carbohydrate that can be used by humans produces four calories per gram as opposed to nine calories per gram of fat and four per gram of protein. In areas of the world where nutrition is marginal, a high proportion (approximately one to two pounds) of an individual’s daily energy requirement may be supplied by carbohydrate, with most of the remainder coming from a variety of fat sources.

Although carbohydrates may compose as much as 80 percent of the total caloric intake in the human diet, for a given diet, the proportion of starch to total carbohydrate is quite variable, depending upon the prevailing customs. In East Asia and in areas of Africa, for example, where rice or tubers such as manioc provide a major food source, starch may account for as much as 80 percent of the total carbohydrate intake. In a typical Western diet, 33 to 50 percent of the caloric intake is in the form of carbohydrate. Approximately half (i.e., 17 to 25 percent) is represented by starch; another third by table sugar (sucrose) and milk sugar (lactose); and smaller percentages by monosaccharides such as glucose and fructose, which are common in fruits, honey, syrups, and certain vegetables such as artichokes, onions, and sugar beets. The small remainder consists of bulk, or indigestible carbohydrate, which comprises primarily the cellulosic outer covering of seeds and the stalks and leaves of vegetables. (See also nutrition.)

 



Role in energy storage

Role in energy storage (B)

Starches, the major plant-energy-reserve polysaccharides used by humans, are stored in plants in the form of nearly spherical granules that vary in diameter from about three to 100 micrometres (about 0.0001 to 0.004 inch). Most plant starches consist of a mixture of two components: amylose and amylopectin. The glucose molecules composing amylose have a straight-chain, or linear, structure. Amylopectin has a branched-chain structure and is a somewhat more compact molecule. Several thousand glucose units may be present in a single starch molecule. (In the diagram, each small circle represents one glucose molecule.)

 
 
   

In addition to granules, many plants have large numbers of specialized cells, called parenchymatous cells, the principal function of which is the storage of starch; examples of plants with these cells include root vegetables and tubers. The starch content of plants varies considerably; the highest concentrations are found in seeds and in cereal grains, which contain up to 80 percent of their total carbohydrate as starch. The amylose and amylopectin components of starch occur in variable proportions; most plant species store approximately 25 percent of their starch as amylose and 75 percent as amylopectin. This proportion can be altered, however, by selective-breeding techniques, and some varieties of corn have been developed that produce up to 70 percent of their starch as amylose, which is more easily digested by humans than is amylopectin.

In addition to the starches, some plants (e.g., the Jerusalem artichoke and the leaves of certain grasses, particularly rye grass) form storage polysaccharides composed of fructose units rather than glucose. Although the fructose polysaccharides can be broken down and used to prepare syrups, they cannot be digested by higher animals.

Starches are not formed by animals; instead, they form a closely related polysaccharide, glycogen. Virtually all vertebrate and invertebrate animal cells, as well as those of numerous fungi and protozoans, contain some glycogen; particularly high concentrations of this substance are found in the liver and muscle cells of higher animals. The overall structure of glycogen, which is a highly branched molecule consisting of glucose units, has a superficial resemblance to that of the amylopectin component of starch, although the structural details of glycogen are significantly different. Under conditions of stress or muscular activity in animals, glycogen is rapidly broken down to glucose, which is subsequently used as an energy source. In this manner, glycogen acts as an immediate carbohydrate reserve. Furthermore, the amount of glycogen present at any given time, especially in the liver, directly reflects an animal’s nutritional state. When adequate food supplies are available, both glycogen and fat reserves of the body increase, but when food supplies decrease or when the food intake falls below the minimum energy requirements, the glycogen reserves are depleted quite rapidly, while those of fat are used at a slower rate.

 



Role in plant and animal structure

Role in plant and animal structure (B)

Whereas starches and glycogen represent the major reserve polysaccharides of living things, most of the carbohydrate found in nature occurs as structural components in the cell walls of plants. Carbohydrates in plant cell walls generally consist of several distinct layers, one of which contains a higher concentration of cellulose than the others. The physical and chemical properties of cellulose are strikingly different from those of the amylose component of starch.

In most plants, the cell wall is about 0.5 micrometre thick and contains a mixture of cellulose, pentose-containing polysaccharides (pentosans), and an inert (chemically unreactive) plastic-like material called lignin. The amounts of cellulose and pentosan may vary; most plants contain between 40 and 60 percent cellulose, although higher amounts are present in the cotton fibre.

Polysaccharides also function as major structural components in animals. Chitin, which is similar to cellulose, is found in insects and other arthropods. Other complex polysaccharides predominate in the structural tissues of higher animals.

 



 
Structural Arrangements And Properties

Stereoisomerism

Stereoisomerism (B)

Studies by German chemist Emil Fischer in the late 19th century showed that carbohydrates, such as fructose and glucose, with the same molecular formulas but with different structural arrangements and properties (i.e., isomers) can be formed by relatively simple variations of their spatial, or geometric, arrangements. This type of isomerism, which is called stereoisomerism, exists in all biological systems. Among carbohydrates, the simplest example is provided by the three-carbon aldose sugar glyceraldehyde. There is no way by which the structures of the two isomers of glyceraldehyde, which can be distinguished by the so-called Fischer projection formulas, can be made identical, excluding breaking and reforming the linkages, or bonds, of the hydrogen (―H) and hydroxyl (―OH) groups attached to the carbon at position 2. The isomers are, in fact, mirror images akin to right and left hands; the term enantiomorphism is frequently employed for such isomerism. The chemical and physical properties of enantiomers are identical except for the property of optical rotation.

Optical rotation is the rotation of the plane of polarized light. Polarized light is light that has been separated into two beams that vibrate at right angles to each other; solutions of substances that rotate the plane of polarization are said to be optically active, and the degree of rotation is called the optical rotation of the solution. In the case of the isomers of glyceraldehyde, the magnitudes of the optical rotation are the same, but the direction in which the light is rotated—generally designated as plus, or d for dextrorotatory (to the right), or as minus, or l for levorotatory (to the left)—is opposite; i.e., a solution of D-(d)-glyceraldehyde causes the plane of polarized light to rotate to the right, and a solution of L-(l)-glyceraldehyde rotates the plane of polarized light to the left. Fischer projection formulas for the two isomers of glyceraldehyde are given below.

 

 



Configuration

Configuration (B)

Molecules, such as the isomers of glyceraldehyde—the atoms of which can have different structural arrangements—are known as asymmetrical molecules. The number of possible structural arrangements for an asymmetrical molecule depends on the number of centres of asymmetry; i.e., for n (any given number of) centres of asymmetry, 2n different isomers of a molecule are possible. An asymmetrical centre in the case of carbon is defined as a carbon atom to which four different groups are attached. In the three-carbon aldose sugar, glyceraldehyde, the asymmetrical centre is located at the central carbon atom.

 
 

The position of the hydroxyl group (―OH) attached to the central carbon atom—i.e., whether ―OH projects from the left or the right—determines whether the molecule rotates the plane of polarized light to the left or to the right. Since glyceraldehyde has one asymmetrical centre, n is one in the relationship 2n, and there thus are two possible glyceraldehyde isomers. Sugars containing four carbon atoms have two asymmetrical centres; hence, there are four possible isomers (22). Similarly, sugars with five carbon atoms have three asymmetrical centres and thus have eight possible isomers (23). Keto sugars have one less asymmetrical centre for a given number of carbon atoms than do aldehyde sugars.

A convention of nomenclature, devised in 1906, states that the form of glyceraldehyde whose asymmetrical carbon atom has a hydroxyl group projecting to the right is designated as of the D-configuration; that form, whose asymmetrical carbon atom has a hydroxyl group projecting to the left, is designated as L. All sugars that can be derived from D-glyceraldehyde—i.e., hydroxyl group attached to the asymmetrical carbon atom most remote from the aldehyde or keto end of the molecule projects to the right—are said to be of the D-configuration; those sugars derived from L-glyceraldehyde are said to be of the L-configuration.

 
 
Representative disaccharides and oligosaccharides

common name component sugars linkages sources
*The linkage joins carbon atom 1 (in the β configuration) of one glucose molecule and carbon atom 4 of the second glucose molecule; the linkage may also be abbreviated β-1, 4.
**Note that raffinose and stachyose are galactosyl sucroses.
cellobiose glucose, glucose β1 → 4* hydrolysis of cellulose
gentiobiose glucose, glucose β1 → 6 plant glycosides, amygdalin
isomaltose glucose, glucose α1 → 6 hydrolysis of glycogen, amylopectin
raffinose** galactose, glucose, fructose α1 → 6, α1 → 2 sugarcane, beets, seeds
stachyose** galactose, galactose, glucose, fructose α1 → 6, α1 → 6, α1 → 2 soybeans, jasmine, twigs, lentils
 

The configurational notation D or L is independent of the sign of the optical rotation of a sugar in solution. It is common, therefore, to designate both, as, for example, D-(l)-fructose or D-(d)-glucose; i.e., both have a D-configuration at the centre of asymmetry most remote from the aldehyde end (in glucose) or keto end (in fructose) of the molecule, but fructose is levorotatory and glucose is dextrorotatory—hence the latter has been given the alternative name dextrose. Although the initial assignments of configuration for the glyceraldehydes were made on purely arbitrary grounds, studies that were carried out nearly half a century later established them as correct in an absolute spatial sense. In biological systems, only the D or L form may be utilized.

When more than one asymmetrical centre is present in a molecule, as is the case with sugars having four or more carbon atoms, a series of DL pairs exists, and they are functionally, physically, and chemically distinct. Thus, although D- xylose and D- lyxose both have five carbon atoms and are of the D-configuration, the spatial arrangement of the asymmetrical centres (at carbon atoms 2, 3, and 4) is such that they are not mirror images.

 



Hemiacetal and hemiketal forms

Hemiacetal and hemiketal forms (B)

Although optical rotation has been one of the most frequently determined characteristics of carbohydrates since its recognition in the late 19th century, the rotational behaviour of freshly prepared solutions of many sugars differs from that of solutions that have been allowed to stand. This phenomenon, known as mutarotation, is demonstrable even with apparently identical sugars and is caused by a type of stereoisomerism involving formation of an asymmetrical centre at the first carbon atom (aldehyde carbon) in aldoses and the second one (keto carbon) in ketoses.

 
 
 

Most pentose and hexose sugars, therefore, do not exist as linear, or open-chain, structures in solution but form cyclic, or ring, structures in hemiacetal or hemiketal forms, respectively. As illustrated for glucose and fructose, the cyclic structures are formed by the addition of the hydroxyl group (―OH) from either the fourth, fifth, or sixth carbon atom to the carbonyl group

 

at position 1 in glucose or 2 in fructose. In the case of five-membered cyclic ketohexose or six-membered cyclic aldohexose, the cyclic forms are in equilibrium with (i.e., the rate of conversion from one form to another is stable) the open-chain structure—a free aldehyde if the solution contains glucose, a free ketone if it contains fructose; each form has a different optical rotation value. Since the forms are in equilibrium with each other, a constant value of optical rotation is measurable; the two cyclic forms represent more than 99.9 percent of the sugar in the case of a glucose solution.

By definition, the carbon atom containing the aldehyde or keto group is called the anomeric carbon atom; similarly, carbohydrate stereoisomers that differ in configuration only at this carbon atom are called anomers. When a cyclic hemiacetal or hemiketal structure forms, the structure with the new hydroxyl group projecting on the same side as that of the oxygen involved in forming the ring is called the alpha anomer; that with the hydroxyl group projecting on the opposite side from that of the oxygen ring is called the beta anomer.

 
The spatial arrangements of the atoms in these cyclic structures are better shown (glucose is used as an example) in the representation devised by British organic chemist Sir Norman Haworth about 1930; they are still in widespread use. In the formulation the asterisk indicates the position of the anomeric carbon atom; the carbon atoms, except at position 6, usually are not labelled.
 
The large number of asymmetrical carbon atoms and the consequent number of possible isomers considerably complicates the structural chemistry of carbohydrates.

 



 
Classes Of Carbohydrates

Monosaccharides

Monosaccharides (B)

Sources

The most common naturally occurring monosaccharides are D-glucose, D-mannose, D-fructose, and D-galactose among the hexoses and D-xylose and L-arabinose among the pentoses. In a special sense, D-ribose and 2-deoxy-D-ribose are ubiquitous because they form the carbohydrate component of ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), respectively; these sugars are present in all cells as components of nucleic acids.

 
Some naturally occurring monosaccharides
sugar sources
L-arabinose mesquite gum, wheat bran
D-ribose all living cells; as component of ribonucleic acid
D-xylose corncobs, seed hulls, straw
D-ribulose as an intermediate in photosynthesis
2-deoxy-D-ribose as constituent of deoxyribonucleic acid
D-galactose lactose, agar, gum arabic, brain glycolipids
D-glucose sucrose, cellulose, starch, glycogen
D-mannose seeds, ivory nut
D-fructose sucrose, artichokes, honey
L-fucose marine algae, seaweed
L-rhamnose poison-ivy blossom, oak bark
D-mannoheptulose avocado
D-altroheptulose numerous plants
 

D-Xylose, found in most plants in the form of a polysaccharide called xylan, is prepared from corncobs, cottonseed hulls, or straw by chemical breakdown of xylan. D-Galactose, a common constituent of both oligosaccharides and polysaccharides, also occurs in carbohydrate-containing lipids, called glycolipids, which are found in the brain and other nervous tissues of most animals. Galactose is generally prepared by acid hydrolysis (breakdown involving water) of lactose, which is composed of galactose and glucose. Since the biosynthesis of galactose in animals occurs through intermediate compounds derived directly from glucose, animals do not require galactose in the diet. In fact, in most human populations the majority of people do not retain the ability to manufacture the enzyme necessary to metabolize galactose after they reach the age of four, and many individuals possess a hereditary defect known as galactosemia and never have the ability to metabolize galactose.

D-Glucose (from the Greek word glykys, meaning “sweet”), the naturally occurring form, is found in fruits, honey, blood, and, under abnormal conditions, in urine. It is also a constituent of the two most common naturally found disaccharides, sucrose and lactose, as well as the exclusive structural unit of the polysaccharides cellulose, starch, and glycogen. Generally, D-glucose is prepared from either potato starch or cornstarch.

D-Fructose, a ketohexose, is one of the constituents of the disaccharide sucrose and is also found in uncombined form in honey, apples, and tomatoes. Fructose, generally considered the sweetest monosaccharide, is prepared by sucrose hydrolysis and is metabolized by humans.

 
Chemical reactions

The reactions of the monosaccharides can be conveniently subdivided into those associated with the aldehyde or keto group and those associated with the hydroxyl groups.

The relative ease with which sugars containing a free or potentially free aldehyde or keto group can be oxidized to form products has been known for a considerable time and once was the basis for the detection of these so-called reducing sugars in a variety of sources. For many years, analyses of blood glucose and urinary glucose were carried out by a procedure involving the use of an alkaline copper compound. Because the reaction has undesirable features—extensive destruction of carbohydrate structure occurs, and the reaction is not very specific (i.e., sugars other than glucose give similar results) and does not result in the formation of readily identifiable products—blood and urinary glucose now are analyzed by using the enzyme glucose oxidase, which catalyzes the oxidation of glucose to products that include hydrogen peroxide. The hydrogen peroxide then is used to oxidize a dye present in the reaction mixture; the intensity of the colour is directly proportional to the amount of glucose initially present. The enzyme, glucose oxidase, is highly specific for β-D-glucose.

In another reaction, the aldehyde group of glucose

 
reacts with alkaline iodine to form a class of compounds called aldonic acids. One important aldonic acid is ascorbic acid (vitamin C), an essential dietary component for humans and guinea pigs. The formation of similar acid derivatives does not occur with the keto sugars.
 

Either the aldehyde or the keto group of a sugar may be reduced (i.e., hydrogen added) to form an alcohol; compounds formed in this way are called alditols, or sugar alcohols. The product formed as a result of the reduction of the aldehyde carbon of D-glucose is called sorbitol (D-glucitol). D-Glucitol also is formed when L-sorbose is reduced. The reduction of mannose results in mannitol, that of galactose in dulcitol.

Sugar alcohols that are of commercial importance include sorbitol (D-glucitol), which is commonly used as a sweetening agent, and D-mannitol, which is also used as a sweetener, particularly in chewing gums, because it has a limited water solubility and remains powdery and granular on long storage.

 
Formation of glycosides

The hydroxyl group that is attached to the anomeric carbon atom (i.e., the carbon containing the aldehyde or keto group) of carbohydrates in solution has unusual reactivity, and derivatives, called glycosides, can be formed; glycosides formed from glucose are called glucosides. It is not possible for equilibration between the α- and β-anomers of a glycoside in solution (i.e., mutarotation) to occur. The reaction by which a glycoside is formed involves the hydroxyl group (―OH) of the anomeric carbon atom (numbered 1) of both α and β forms of D-glucose—α and β forms of D-glucose are shown in equilibrium in the reaction sequence—and the hydroxyl group of an alcohol (methyl alcohol in the reaction sequence); methyl α-D-glucosides and β-D-glucosides are formed as products, as is water.

 
 

Among the wide variety of naturally occurring glycosides are a number of plant pigments, particularly those red, violet, and blue in colour; these pigments are found in flowers and consist of a pigment molecule attached to a sugar molecule, frequently glucose. Plant indican (from Indigofera species), composed of glucose and the pigment indoxyl, was important in the preparation of indigo dye before synthetic dyes became prevalent. Of a number of heart muscle stimulants that occur as glycosides, digitalis is still used. Other naturally occurring glycosides include vanillin, which is found in the vanilla bean, and amygdalin (oil of bitter almonds); a variety of glycosides found in mustard have a sulfur atom at position 1 rather than oxygen.

A number of important antibiotics are glycosides; among the best known are streptomycin and erythromycin. Glucosides—i.e., glycosides formed from glucose—in which the anomeric carbon atom (at position 1) has phosphoric acid linked to it, are extremely important biological compounds. For example, α-D-glucose-1-phosphate is an intermediate product in the biosynthesis of cellulose, starch, and glycogen; similar glycosidic phosphate derivatives of other monosaccharides participate in the formation of naturally occurring glycosides and polysaccharides.

 
 
The hydroxyl groups other than the one at the anomeric carbon atom can undergo a variety of reactions. Esterification, which consists of reacting the hydroxyl groups with an appropriate acidic compound, results in the formation of a class of compounds called sugar esters. Among the common ones are the sugar acetates, in which the acid is acetic acid. Esters of phosphoric acid and sulfuric acid are important biological compounds; glucose-6-phosphate, for example, plays a central role in the energy metabolism of most living cells, and D-ribulose 1,5-diphosphate is important in photosynthesis.
 
Formation of methyl ethers

Treatment of a carbohydrate with methyl iodide or similar agents under appropriate conditions results in the formation of compounds in which the hydroxyl groups are converted to methyl groups (―CH3). Called methyl ethers, these compounds are employed in structural studies of oligosaccharides and polysaccharides because their formation does not break the bonds, called glycosidic bonds, that link adjacent monosaccharide units. An example is the etherification of a starch molecule carried out using methyl iodide, in which methyl groups become attached to the glucose molecules, forming a methylated segment in the starch molecule; note that the glycosidic bonds are not broken by the reaction with methyl iodide. When the methylated starch molecule then is broken down (hydrolyzed), hydroxyl groups are located at the positions in the molecule previously involved in linking one sugar molecule to another, and a methylated glucose, in this case named 2,3,6 tri-O-methyl-D-glucose, forms. The linkage positions (which are not methylated) in a complex carbohydrate can be established by analyzing the locations of the methyl groups in the monosaccharides. This technique is useful in determining the structural details of polysaccharides, particularly since the various methylated sugars are easily separated by techniques involving gas chromatography, in which a moving gas stream carries a mixture through a column of a stationary liquid or solid, the components thus being resolved.

 
 

When the terminal group (CH2OH) of a monosaccharide is oxidized chemically or biologically, a product called a uronic acid is formed. Glycosides that are derived from D-glucuronic acid (the uronic acid formed from D-glucose) and fatty substances called steroids appear in the urine of animals as normal metabolic products; in addition, foreign toxic substances are frequently converted in the liver to glucuronides before excretion in the urine. D-Glucuronic acid also is a major component of connective tissue polysaccharides, and D-galacturonic acid and D-mannuronic acid, formed from D-galactose and D-mannose, respectively, are found in several plant sources.

Other compounds formed from monosaccharides include those in which one hydroxyl group, usually at the carbon at position 2, is replaced by an amino group (―NH2); these compounds, called amino sugars, are widely distributed in nature. The two most important ones are glucosamine (2-amino-2-deoxy-D-glucose) and galactosamine (2-amino-2-deoxy-D-galactose).

 

Neither amino sugar is found in the uncombined form. Both occur in animals as components of glycolipids or polysaccharides; e.g., the primary structural polysaccharide (chitin) of insect outer skeletons and various blood group substances.

In a number of naturally occurring sugars, known as deoxy sugars, the hydroxyl group at a particular position is replaced by a hydrogen atom. By far the most important representative is 2-deoxy-D-ribose, the pentose sugar found in deoxyribonucleic acid (DNA); the hydroxyl group at the carbon atom at position 2 has been replaced by a hydrogen atom.

 
Other naturally occurring deoxy sugars are hexoses, of which L-rhamnose (6-deoxy-L-mannose) and L-fucose (6-deoxy-L-galactose) are the most common; the latter, for example, is present in the carbohydrate portion of blood group substances and on the outer surface of red blood cells.

 



Disaccharides and oligosaccharides

Disaccharides and oligosaccharides (B)

Disaccharides are a specialized type of glycoside in which the anomeric hydroxyl group of one sugar has combined with the hydroxyl group of a second sugar with the elimination of the elements of water. Although an enormous number of disaccharide structures are possible, only a limited number are of commercial or biological significance.

 



Sucrose and trehalose

Sucrose and trehalose (B)

Sucrose, or common table sugar, is a major commodity worldwide. By the second decade of the 21st century, its world production had amounted to more than 170 million tons annually. The unusual type of linkage between the two anomeric hydroxyl groups of glucose and fructose means that neither a free aldehyde group (on the glucose moiety) nor a free keto group (on the fructose moiety) is available to react unless the linkage between the monosaccharides is destroyed; for this reason, sucrose is known as a nonreducing sugar. Sucrose solutions do not exhibit mutarotation, which involves formation of an asymmetrical centre at the aldehyde or keto group. If the linkage between the monosaccharides composing sucrose is broken, the optical rotation value of sucrose changes from positive to negative; the new value reflects the composite rotation values for D-glucose, which is dextrorotatory (+52°), and D-fructose, which is levorotatory (−92°). The change in the sign of optical rotation from positive to negative is the reason sucrose is sometimes called invert sugar.

 
 
 

The commercial preparation of sucrose takes advantage of the alkaline stability of the sugar, and a variety of impurities are removed from crude sugarcane extracts by treatment with alkali. After this step, syrup preparations are crystallized to form table sugar. Successive “crops” of sucrose crystals are “harvested,” and the later ones are known as brown sugar. The residual syrupy material is called either cane final molasses or blackstrap molasses; both are used in the preparation of antibiotics, as sweetening agents, and in the production of alcohol by yeast fermentation. Sucrose is formed following photosynthesis in plants by a reaction in which sucrose phosphate first is formed.

The disaccharide trehalose is similar in many respects to sucrose but is much less widely distributed. It is composed of two molecules of α-D-glucose and is also a nonreducing sugar. Trehalose is present in young mushrooms and in the resurrection plant (Selaginella); it is of considerable biological interest because it is also found in the circulating fluid (hemolymph) of many insects. Since trehalose can be converted to a glucose phosphate compound by an enzyme-catalyzed reaction that does not require energy, its function in hemolymph may be to provide an immediate energy source, a role similar to that of the carbohydrate storage forms (i.e., glycogen) found in higher animals.

 

 



Lactose and maltose

Lactose and maltose (B)

Lactose is one of the sugars (sucrose is another) found most commonly in human diets throughout the world; it constitutes about 7 percent of human milk and about 4–5 percent of the milk of mammals such as cows, goats, and sheep. Lactose consists of two aldohexoses—β-D-galactose and glucose—linked so that the aldehyde group at the anomeric carbon of glucose is free to react; i.e., lactose is a reducing sugar.
 
 
 

A variety of metabolic disorders related to lactose may occur in infants; in some cases, they are the result of a failure to metabolize properly the galactose portion of the molecule.

Although not found in uncombined form in nature, the disaccharide maltose is biologically important because it is a product of the enzymatic breakdown of starches during digestion. Maltose consists of α-D-glucose linked to a second glucose unit in such a way that maltose is a reducing sugar. Maltose, which is readily hydrolyzed to glucose and can be metabolized by animals, is employed as a sweetening agent and as a food for infants whose tolerance for lactose is limited.

 



Polysaccharides

Polysaccharides (B)

Polysaccharides, or glycans, may be classified in a number of ways; the following scheme is frequently used. Homopolysaccharides are defined as polysaccharides formed from only one type of monosaccharide. Homopolysaccharides may be further subdivided into straight-chain and branched-chain representatives, depending upon the arrangement of the monosaccharide units. Heteropolysaccharides are defined as polysaccharides containing two or more different types of monosaccharides; they may also occur in both straight-chain and branched-chain forms. In general, extensive variation of linkage types does not occur within a polysaccharide structure, nor are there many polysaccharides composed of more than three or four different monosaccharides; most contain one or two.
 
Representative homopolysaccharides
*May contain sulfate groups.
homopolysaccharide sugar component linkage function sources
cellulose glucose β, 1 → 4 structural throughout plant kingdom
amylose glucose α, 1 → 4 food storage starches, especially corn, potatoes, rice
chitin N-acetylglucosamine β, 1 → 4 structural insect and crustacean skeleton
inulin fructose β, 2 → 1 food storage artichokes, chicory
xylan xylose β, 1 → 4 structural all land plants
glycogen glucose α, 1 → 4,
6 ← 1, α
food storage liver and muscle cells of all animals
amylopectin glucose α, 1 → 4,
6 ← 1, α
food storage starches, especially corn, potatoes, rice
dextran glucose α, 1 → 6,
4 ← 1, α
unknown primarily bacterial
agar* galactose α, 1 → 3 structural seaweeds

 



Homopolysaccharides

Homopolysaccharides (B)

In general, homopolysaccharides have a well-defined chemical structure, although the molecular weight of an individual amylose or xylan molecule may vary within a particular range, depending on the source; molecules from a single source also may vary in size, because most polysaccharides are formed biologically by an enzyme-catalyzed process lacking genetic information regarding size.

The basic structural component of most plants, cellulose, is widely distributed in nature. It has been estimated that 50 billion to 100 billion tons of cellulose are synthesized yearly as a result of photosynthesis by higher plants. The proportion of cellulose to total carbohydrate found in plants may vary in various types of woods from 30 to 40 percent, and to more than 98 percent in the seed hair of the cotton plant. Cellulose, a large, linear molecule composed of 3,000 or more β-D-glucose molecules, is insoluble in water.

The chains of glucose units composing cellulose molecules are frequently aligned within the cell-wall structure of a plant to form fibre-like or crystalline arrangements. This alignment permits very tight packing of the chains and promotes their structural stability but also makes structural analysis difficult. The relationships between cellulose and other polysaccharides present in the cell wall are not well established; in addition, the presence of unusual chemical linkages or nonglucose units within the cellulose structure has not yet been established with certainty.

During the preparation of cellulose, raw plant material is treated with hot alkali; this treatment removes most of the lignin, the hemicelluloses, and the mucilaginous components. The cellulose then is processed to produce papers and fibres. The high resistance of cellulose to chemical or enzymatic breakdown is important in the manufacture of paper and cloth. Cellulose also is modified chemically for other purposes; e.g., compounds such as cellulose acetate are used in the plastics industry, in the production of photographic film, and in the rayon-fibre industry. Cellulose nitrate (nitrocellulose) is employed in the lacquer and explosives industries.

The noteworthy biological stability of cellulose is dramatically illustrated by trees, the life-span of which may be several thousand years. Enzymes capable of breaking down cellulose are generally found only among several species of bacteria and molds. The apparent ability of termites to utilize cellulose as an energy source depends on the presence in their intestinal tracts of protozoans that can break it down. Similarly, the single-celled organisms present in the rumina of sheep and cattle are responsible for the ability of these animals to utilize the cellulose present in typical grasses and other feeds.

Xylans are almost as ubiquitous as cellulose in plant cell walls and contain predominantly β-D-xylose units linked as in cellulose. Some xylans contain other sugars, such as L-arabinose, but they form branches and are not part of the main chain. Xylans are of little commercial importance.

The term starch refers to a group of plant reserve polysaccharides consisting almost exclusively of a linear component (amylose) and a branched component ( amylopectin). The use of starch as an energy source by humans depends on the ability to convert it completely to individual glucose units; the process is initiated by the action of enzymes called amylases, synthesized by the salivary glands in the mouth, and continues in the intestinal tract. The primary product of amylase action is maltose, which is hydrolyzed to two component glucose units as it is absorbed through the walls of the intestine.

A characteristic reaction of the amylose component of starch is the formation with iodine of a complex compound with a characteristic blue colour. About one iodine molecule is bound for each seven or eight glucose units, and at least five times that many glucose units are needed in an amylose chain to permit the effective development of the colour.

The amylopectin component of starch is structurally similar to glycogen in that both are composed of glucose units linked together in the same way, but the distance between branch points is greater in amylopectin than in glycogen, and the former may be thought of as occupying more space per unit weight.

 
 
 

The applications of starches other than as foods are limited. Starches are employed in adhesive manufacture, and starch nitrate has some utility as an explosive.

Glycogen, which is found in all animal tissues, is the primary animal storage form of carbohydrate and, indirectly, of rapidly available energy. The distance between branch points in a glycogen molecule is only five or six units, which results in a compact treelike structure. The ability of higher animals to form and break down this extensively branched structure is essential to their well-being; in conditions known as glycogen storage diseases, these activities are abnormal, and the asymmetrical glycogen molecules that are formed have severe, often fatal, consequences. Glycogen synthesis and breakdown are controlled by substances called hormones.

Large molecules—e.g., pectins and agars—composed of galactose or its uronic-acid derivative (galacturonic acid) are important because they can form gels. Pectins, which are predominantly galacturonans, are produced from citrus fruit rinds; they are used commercially in the preparation of jellies and jams. Agar is widely employed in biological laboratories as a solidifying agent for growth media for microorganisms and in the bakery industry as a gelling agent; it forms a part of the diet of people in several areas of East Asia.

Dextrans, a group of polysaccharides composed of glucose, are secreted by certain strains of bacteria as slimes. The structure of an individual dextran varies with the strain of microorganism. Dextrans can be used as plasma expanders (substitutes for whole blood) in cases of severe shock. In addition, a dextran derivative compound is employed medically as an anticoagulant for blood.

Chitin is structurally similar to cellulose, but the repeating sugar is 2-deoxy-2-acetamido-D-glucose (N-acetyl-D-glucosamine) rather than glucose.

 
 
 
Sometimes referred to as animal cellulose, chitin is the major component of the outer skeletons of insects, crustaceans, and other arthropods, as well as annelid and nematode worms, mollusks, and coelenterates. The cell walls of most fungi also are predominantly chitin, which comprises nearly 50 percent of the dry weight of some species. Since chitin is nearly as chemically inactive as cellulose and easily obtained, numerous attempts, none of which has thus far been successful, have been made to develop it commercially. The nitrogen content of the biosphere, however, is stabilized by the ability of soil microorganisms to degrade nitrogen-containing compounds such as those found in insect skeletons; these microorganisms convert the nitrogen in complex molecules to a form usable by plants. If such microorganisms did not exist, much of the organic nitrogen present in natural materials would be unavailable to plants.

 



Heteropolysaccharides

Heteropolysaccharides (B)

In general, heteropolysaccharides (heteroglycans) contain two or more different monosaccharide units. Although a few representatives contain three or more different monosaccharides, most naturally occurring heteroglycans contain only two different ones and are closely associated with lipid or protein. The complex nature of these substances has made detailed structural studies extremely difficult. The major heteropolysaccharides include the connective-tissue polysaccharides, the blood group substances, glycoproteins (combinations of carbohydrates and proteins) such as gamma globulin, and glycolipids (combinations of carbohydrates and lipids), particularly those found in the central nervous system of animals and in a wide variety of plant gums.
 
Representative heteropolysaccharides
*Covalently linked to protein; the proportion of protein to carbohydrate in such complex molecules varies from about 10% protein in the case of chondroitin-4-sulfate to better than 95% for gamma globulin.
heteropolysaccharide component sugars functions distribution
hyaluronic acid D-glucuronic acid and N-acetyl-D-glucosamine lubricant, shock absorber, water binding connective tissue, skin
chondroitin-4-sulfate* D-glucuronic acid and N-acetyl-D-galactosamine-4-O-sulfate calcium accumulation, cartilage and bone formation cartilage
heparin* D-glucuronic acid, L-iduronic acid, N-sulfo-D-glucosamine anticoagulant mast cells, blood
gamma globulin* N-acetyl-hexosamine, D-mannose, D-galactose antibody blood
blood group substance* D-glucosamine, D-galactosamine, L-fucose, D-galactose blood group specificity cell surfaces, especially red blood cells
 

The most important heteropolysaccharides are found in the connective tissues of all animals and include a group of large molecules that vary in size, shape, and interaction with other body substances. They have a structural role, and the structures of individual connective-tissue polysaccharides are related to specific animal functions; hyaluronic acid, for example, the major component of joint fluid in animals, functions as a lubricating agent and shock absorber.

The connective-tissue heteropolysaccharides contain acidic groups (uronic acids or sulfate groups) and can bind both water and inorganic metal ions. They can also play a role in other physiological functions; e.g., in the accumulation of calcium before bone formation. Ion-binding ability also appears to be related to the anticoagulant activity of the heteropolysaccharide heparin.

The size of the carbohydrate portion of glycoproteins such as gamma globulin or hen-egg albumin is usually between five and 10 monosaccharide units; several such units occur in some glycoprotein molecules. The function of the carbohydrate component has not yet been established except for glycoproteins associated with cell surfaces; in this case, they appear to act as antigenic determinants—i.e., they are capable of inducing the formation of specific antibodies.

 



Preparation and analysis

Preparation and analysis (B)

In general, monosaccharides are prepared by breakdown with acids of the polysaccharides in which they occur. Sugars usually are difficult to obtain in crystalline form, and the crystallization process usually is begun by “seeding” a concentrated solution of the sugar with crystals. The techniques employed for separation of monosaccharides depend to some extent on their physical and chemical properties; chromatographic procedures are often used.

Oligosaccharides and polysaccharides are prepared from natural sources by techniques that take advantage of size, alkaline stability, or some combination of these and other properties of the molecule of interest. It should be noted that preparation of an oligosaccharide or polysaccharide usually results in a range of molecular sizes of the desired molecule. The purity of a carbohydrate preparation, which is frequently based on an analysis of its composition, is more easily established for monosaccharides and disaccharides than for large, insoluble molecules such as cellulose.

 



Analytical techniques

Analytical techniques (B)

A variety of organic chemical analytical techniques are generally applicable to studies involving carbohydrates. Optical rotation, for example, once was frequently used to characterize carbohydrates. The ability to measure the rotation of the plane of polarized light transmitted through a solution containing a carbohydrate depends on finding a suitable solvent; water usually is used, with light at a wavelength of 589 mμ (millimicrons). Optical rotation is no longer widely used to characterize monosaccharides. The magnitude and sign of the optical rotation of glycosides, however, is useful in assigning configuration (α or β) to the hydroxyl group at the anomeric centre; glycosides of the α-configuration generally have rotations of higher magnitude than do the same glycosides of the β-configuration. Optical rotation is not a completely additive property; a trisaccharide composed of three glucose residues, for example, does not have a rotation three times that of one glucose molecule. Sugar alcohols cannot form ring structures; their rotation values are extremely small, suggesting a relationship between ring structure and the ability of a carbohydrate to rotate the plane of polarized light. Certain types of reactions (e.g., glycoside hydrolysis) can be monitored by measuring the change in optical rotation as a function of time. This technique is frequently used to examine the breakdown of disaccharides or oligosaccharides to monosaccharide units, especially if a large change in the net optical rotation may be expected, as occurs in the hydrolysis of sucrose.

 



Spectroscopic techniques

Spectroscopic techniques (B)

Several other optical techniques used in chemistry have been applied to the analysis of carbohydrates. Infrared spectroscopy, used to measure vibrational and rotational excitation of molecules, and nuclear magnetic-resonance spectroscopy, which measures the excitation of certain components of molecules in a magnetic field induced by radio-frequency radiation, are valuable, although the similarity of the functional groups (i.e., the hydroxyl groups) limits use of the former technique for most sugars. Proton magnetic-resonance spectroscopy, nuclear magnetic resonance applied to protons (H atoms), is employed to identify the relative spatial arrangements of individual hydrogen atoms in a molecule. When they are precisely placed, the corresponding positions of the hydroxyl groups attached to the same carbon atom can be deduced. An extension of this technique utilizes the resonance spectroscopy of carbon-13, a nonradioactive isotope of carbon, so that ring structures can be established with great accuracy. Both the proton and carbon magnetic resonance methods are best applied to monosaccharides; they are less valuable in studying polysaccharides because an individual hydrogen atom in a large molecule is too small for accurate detection.

 



Identification of subunits

Identification of subunits (B)

The study of polysaccharide structure usually focuses on the chemical composition, the linkage between the monosaccharide units, and the size and shape of the molecule. The size and shape of a polysaccharide can be ascertained by techniques that are usually applied to large molecules; e.g., the most accurate molecular weight determination measures the sedimentation properties of the molecule in an applied gravitational field (e.g., the rate at which a solid material is deposited from a state of suspension or solution in a liquid). Indications of the shape of polysaccharide molecules in solution are obtained from viscosity measurements, in which the resistance of the molecules to flow (viscosity) is equated with the end-to-end length of the molecule; the viscosity of hyaluronic acid, for example, shows a marked dependence on both concentration of the acid and the salt content of the solution, and, under conditions approximating those found in biological systems, a hyaluronic acid molecule may be thought of as occupying a great deal of space. Alternatively, the compact nature of a glycogen molecule of molecular weight equal to that of a molecule of hyaluronic acid results in its accommodation to a much smaller space than the latter molecule.

The identification of sugars in a mixture resulting from the hydrolytic breakdown of a heteropolysaccharide is most often carried out by chromatography of the mixture on paper, silica gel, or cellulose. Ready separations can be achieved between pentoses, hexoses and, for example, deoxy sugars; closely related compounds such as D-glucose and D-galactose also can be separated using chromatographic techniques. The linkage positions in polysaccharides are usually determined using the methylation procedure described previously. The various monosaccharide methyl ethers produced by the methylation are separated by gas–liquid chromatography.

Detailed statements about polysaccharide structure and function are limited by the statistical nature of some measurements (e.g., branching frequency), the biological variability of parameters such as size and molecular weight, and incomplete information about associative interactions in living things.

 



 







 
  Carbohydrate (W)

Carbohydrate

Carbohydrate (W)

 
   
Lactose is a disaccharide found in animal milk. It consists of a molecule of D-galactose and a molecule of D-glucose bonded by beta-1-4 glycosidic linkage.  
   

A carbohydrate is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula Cm(H2O)n (where m may be different from n). This formula holds true for monosaccharides. Some exceptions exist; for example, deoxyribose, a sugar component of DNA, has the empirical formula C5H10O4. The carbohydrates are technically hydrates of carbon; structurally it is more accurate to view them as aldoses and ketoses.

The term is most common in biochemistry, where it is a synonym of saccharide, a group that includes sugars, starch, and cellulose. The saccharides are divided into four chemical groups: monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Monosaccharides and disaccharides, the smallest (lower molecular weight) carbohydrates, are commonly referred to as sugars. The word saccharide comes from the Greek word σάκχαρον (sákkharon), meaning "sugar". While the scientific nomenclature of carbohydrates is complex, the names of the monosaccharides and disaccharides very often end in the suffix -ose, as in the monosaccharides fructose (fruit sugar) and glucose (starch sugar) and the disaccharides sucrose (cane or beet sugar) and lactose (milk sugar).

Carbohydrates perform numerous roles in living organisms. Polysaccharides serve for the storage of energy (e.g. starch and glycogen) and as structural components (e.g. cellulose in plants and chitin in arthropods). The 5-carbon monosaccharide ribose is an important component of coenzymes (e.g. ATP, FAD and NAD) and the backbone of the genetic molecule known as RNA. The related deoxyribose is a component of DNA. Saccharides and their derivatives include many other important biomolecules that play key roles in the immune system, fertilization, preventing pathogenesis, blood clotting, and development.

They are found in a wide variety of natural and processed foods. Starch is a polysaccharide. It is abundant in cereals (wheat, maize, rice), potatoes, and processed food based on cereal flour, such as bread, pizza or pasta. Sugars appear in human diet mainly as table sugar (sucrose, extracted from sugarcane or sugar beets), lactose (abundant in milk), glucose and fructose, both of which occur naturally in honey, many fruits, and some vegetables. Table sugar, milk, or honey are often added to drinks and many prepared foods such as jam, biscuits and cakes.

Cellulose, a polysaccharide found in the cell walls of all plants, is one of the main components of insoluble dietary fiber. Although it is not digestible, insoluble dietary fiber helps to maintain a healthy digestive system by easing defecation. Other polysaccharides contained in dietary fiber include resistant starch and inulin, which feed some bacteria in the microbiota of the large intestine, and are metabolized by these bacteria to yield short-chain fatty acids.

 
Terminology

Terminology

Terminology (W)

In scientific literature, the term "carbohydrate" has many synonyms, like "sugar" (in the broad sense), "saccharide", "ose", "glucide", "hydrate of carbon" or "polyhydroxy compounds with aldehyde or ketone". Some of these terms, specially "carbohydrate" and "sugar", are also used with other meanings.

In food science and in many informal contexts, the term "carbohydrate" often means any food that is particularly rich in the complex carbohydrate starch (such as cereals, bread and pasta) or simple carbohydrates, such as sugar (found in candy, jams, and desserts).

Often in lists of nutritional information, such as the USDA National Nutrient Database, the term "carbohydrate" (or "carbohydrate by difference") is used for everything other than water, protein, fat, ash, and ethanol. This includes chemical compounds such as acetic or lactic acid, which are not normally considered carbohydrates. It also includes dietary fiber which is a carbohydrate but which does not contribute much in the way of food energy (kilocalories), even though it is often included in the calculation of total food energy just as though it were a sugar.

In the strict sense, "sugar" is applied for sweet, soluble carbohydrates, many of which are used in food.

 



 
Structure

Structure

Structure (W)

Formerly the name "carbohydrate" was used in chemistry for any compound with the formula Cm (H2O)n. Following this definition, some chemists considered formaldehyde (CH2O) to be the simplest carbohydrate, while others claimed that title for glycolaldehyde. Today, the term is generally understood in the biochemistry sense, which excludes compounds with only one or two carbons and includes many biological carbohydrates which deviate from this formula. For example, while the above representative formulas would seem to capture the commonly known carbohydrates, ubiquitous and abundant carbohydrates often deviate from this. For example, carbohydrates often display chemical groups such as: N-acetyl (e.g. chitin), sulphate (e.g. glycosaminoglycans), carboxylic acid (e.g. sialic acid) and deoxy modifications (e.g. fucose and sialic acid).

Natural saccharides are generally built of simple carbohydrates called monosaccharides with general formula (CH2O)n where n is three or more. A typical monosaccharide has the structure H–(CHOH)x(C=O)–(CHOH)y–H, that is, an aldehyde or ketone with many hydroxyl groups added, usually one on each carbon atom that is not part of the aldehyde or ketone functional group. Examples of monosaccharides are glucose, fructose, and glyceraldehydes. However, some biological substances commonly called "monosaccharides" do not conform to this formula (e.g. uronic acids and deoxy-sugars such as fucose) and there are many chemicals that do conform to this formula but are not considered to be monosaccharides (e.g. formaldehyde CH2O and inositol (CH2O)6).

The open-chain form of a monosaccharide often coexists with a closed ring form where the aldehyde/ketone carbonyl group carbon (C=O) and hydroxyl group (–OH) react forming a hemiacetal with a new C–O–C bridge.

Monosaccharides 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-acetyl glucosamine, a nitrogen-containing form of glucose.

 



 
Division

Division

Division (W)

Carbohydrates are polyhydroxy aldehydes, ketones, alcohols, acids, their simple derivatives and their polymers having linkages of the acetal type. They may be classified according to their degree of polymerization, and may be divided initially into three principal groups, namely sugars, oligosaccharides and polysaccharides.

 
The major dietary carbohydrates

Class (DP)* Subgroup Components
Sugars (1–2) Monosaccharides Glucose, galactose, fructose, xylose
Disaccharides Sucrose, lactose, maltose, trehalose
Polyols Sorbitol, mannitol
Oligosaccharides (3–9) Malto-oligosaccharides Maltodextrins
Other oligosaccharides Raffinose, stachyose, fructo-oligosaccharides
Polysaccharides (>9) Starch Amylose, amylopectin, modified starches
Non-starch polysaccharides Glycogen, Cellulose, Hemicellulose, Pectins, Hydrocolloids
* DP = Degree of polymerization

 



 
Monosaccharides

Monosaccharides

Monosaccharides (W)

Main article: Monosaccharide
 
   
D-glucose is an aldohexose with the formula (C·H2O)6. The red atoms highlight the aldehyde group and the blue atoms highlight the asymmetric center furthest from the aldehyde; because this -OH is on the right of the Fischer projection, this is a D sugar.  
   

Monosaccharides are the simplest carbohydrates in that they cannot be hydrolyzed to smaller carbohydrates. They are aldehydes or ketones with two or more hydroxyl groups. The general chemical formula of an unmodified monosaccharide is (C•H2O)n, literally a "carbon hydrate". Monosaccharides are important fuel molecules as well as building blocks for nucleic acids. The smallest monosaccharides, for which n=3, are dihydroxyacetone and D- and L-glyceraldehydes.

 



Classification of monosaccharides

Classification of monosaccharides (W)

 
   
The α and β anomers of glucose. Note the position of the hydroxyl group (red or green) on the anomeric carbon relative to the CH2OH group bound to carbon 5: they either have identical absolute configurations (R,R or S,S) (α), or opposite absolute configurations (R,S or S,R) (β).  
   

Monosaccharides are classified according to three different characteristics: the placement of its carbonyl group, the number of carbon atoms it contains, and its chiral handedness. If the carbonyl group is an aldehyde, the monosaccharide is an aldose; if the carbonyl group is a ketone, the monosaccharide is a ketose. Monosaccharides with three carbon atoms are called trioses, those with four are called tetroses, five are called pentoses, six are hexoses, and so on. These two systems of classification are often combined. For example, glucose is an aldohexose (a six-carbon aldehyde), 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 asymmetric, making them stereo centers with two possible configurations each (R or S). Because of this asymmetry, a number of isomers may exist for any given monosaccharide formula. Using Le Bel-van't Hoff rule, the aldohexose D-glucose, for example, has the formula (C·H2O)6, of which four of its six carbons atoms are stereogenic, making D-glucose one of 24=16 possible stereoisomers. In the case of glyceraldehydes, an aldotriose, there is one pair of possible stereoisomers, which are enantiomers and epimers. 1, 3-dihydroxyacetone, the ketose corresponding to the aldose glyceraldehydes, is a symmetric molecule with no stereo centers. 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. The "D-" and "L-" prefixes should not be confused with "d-" or "l-", which indicate the direction that the sugar rotates plane polarized light. This usage of "d-" and "l-" is no longer followed in carbohydrate chemistry.

 



Ring-straight chain isomerism

Ring-straight chain isomerism (W)

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 hemiacetal or hemiketal, forming 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 the cyclic form, the carbon atom containing the carbonyl oxygen, called the anomeric carbon, becomes a stereogenic center with two possible configurations: The oxygen atom may take a position either above or below the plane of the ring. The resulting possible pair of stereoisomers is called anomers. In the α anomer, the -OH substituent on the anomeric carbon rests on the opposite side (trans) of the ring from the CH2OH side branch. The alternative form, in which the CH2OH substituent and the anomeric hydroxyl are on the same side (cis) of the plane of the ring, is called the β anomer.

 



Use in living organisms

Use in living organisms (W)

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 immediately needed by many cells, they are often converted to more space-efficient forms, often polysaccharides. In many animals, including humans, this storage form is glycogen, especially in liver and muscle cells. In plants, starch is used for the same purpose. The most abundant carbohydrate, cellulose, is a structural component of the cell wall of plants and many forms of algae. Ribose is a component of RNA. Deoxyribose is a component of DNA. Lyxose is a component of lyxoflavin found in the human heart. Ribulose and xylulose occur in the pentose phosphate pathway. Galactose, a component of milk sugar lactose, is found in galactolipids in plant cell membranes and in glycoproteins in many tissues. Mannose occurs in human metabolism, especially in the glycosylation of certain proteins. Fructose, or fruit sugar, is found in many plants and in humans, it is metabolized in the liver, absorbed directly into the intestines during digestion, and found in semen. Trehalose, a major sugar of insects, is rapidly hydrolyzed into two glucose molecules to support continuous flight.

 



 
Disaccharides

Disaccharides

Disaccharides (W)

Main article: Disaccharide

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). Disaccharides can be classified into two types: reducing and non-reducing disaccharides. If the functional group is present in bonding with another sugar unit, it is called a reducing disaccharide or biose.

 



 
Metabolism

Metabolism

Metabolism (W)

Carbohydrate metabolism denotes the various biochemical processes responsible for the formation, breakdown and interconversion of carbohydrates in living organisms.

The most important carbohydrate is glucose, a simple sugar (monosaccharide) that is metabolized by nearly all known organisms. Glucose and other carbohydrates are part of a wide variety of metabolic pathways across species: plants synthesize carbohydrates from carbon dioxide and water by photosynthesis storing the absorbed energy internally, often in the form of starch or lipids. Plant components are consumed by animals and fungi, and used as fuel for cellular respiration. Oxidation of one gram of carbohydrate yields approximately 16 kJ (4 kcal) of energy, while the oxidation of one gram of lipids yields about 38 kJ (9 kcal). The human body stores between 300 and 500 g of carbohydrates depending on body weight, with the skeletal muscle contributing to a large portion of the storage. Energy obtained from metabolism (e.g., oxidation of glucose) is usually stored temporarily within cells in the form of ATP. Organisms capable of anaerobic and aerobic respiration metabolize glucose and oxygen (aerobic) to release energy, with carbon dioxide and water as byproducts.

 



Catabolism

Catabolism (W)

Catabolism is the metabolic reaction which cells undergo to break down larger molecules, extracting energy. There are two major metabolic pathways of monosaccharide catabolism: glycolysis and the citric acid cycle.

In glycolysis, oligo- and polysaccharides are cleaved first to smaller monosaccharides by enzymes called glycoside hydrolases. The monosaccharide units can then enter into monosaccharide catabolism. A 2 ATP investment is required in the early steps of glycolysis to phosphorylate Glucose to Glucose 6-Phosphate (G6P) and Fructose 6-Phosphate (F6P) to Fructose 1,6-biphosphate (FBP), thereby pushing the reaction forward irreversibly. In some cases, as with humans, not all carbohydrate types are usable as the digestive and metabolic enzymes necessary are not present.

 



 







 

 


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