The carbohydrates may be defined as optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis.
Carbohydrates are primarily produced by plants and form a very large group of naturally ccurring organic compounds. Some common examples of carbohydrates are cane sugar, glucose, starch, etc. Most of them have a general formula, Cx(H2O)y , and were considered as hydrates of carbon from where the name carbohydrate was derived. For example, the molecular formula of glucose (C6H12O6) fits into this general formula, C6(H2O)6. But all the compounds which fit into this formula may not be classified as carbohydrates. For example acetic acid (CH3COOH) fits into this general formula, C2(H2O)2but is not a carbohydrate. Similarly, rhamnose, C6H12O5 is a carbohydrate but does not fit in this definition. A large number of their reactions have shown that they contain specific functional groups.
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.
Classification of Carbohydrates
Carbohydrates are classified on the basis of their behaviour on hydrolysis. They have been broadly divided into following three groups.
(i) Monosaccharides: A carbohydrate that cannot be hydrolysed further to give simpler unit of polyhydroxy aldehyde or ketone is called a monosaccharide. About 20 monosaccharides are known to occur in nature. Some common examples are glucose, fructose, ribose, etc.
(ii) Oligosaccharides: Carbohydrates that yield two to ten monosaccharide units, on hydrolysis, are called oligosaccharides. They are further classified as disaccharides, trisaccharides, tetrasaccharides, etc., depending upon the number of monosaccharides, they provide on hydrolysis. Amongst these the most common are disaccharides.
The two monosaccharide units obtained on hydrolysis of a disaccharide may be same or different. For example, one molecule of sucrose on hydrolysis gives one molecule of glucose and one molecule of fructose whereas maltose gives two molecules of only glucose.
(iii) Polysaccharides: Carbohydrates which yield a large number of monosaccharide units on hydrolysis are called polysaccharides. Some common examples are starch, cellulose, glycogen, gums, etc. Polysaccharides are not sweet in taste, hence they are also
The carbohydrates may also be classified as either reducing or nonreducing sugars. All those carbohydrates which reduce Fehling’s solution and Tollens’ reagent are referred to as reducing sugars. All monosaccharides whether aldose or ketose are reducing sugars.
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.
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.
Preparation of Glucose
1.From sucrose (Cane sugar): If sucrose is boiled with dilute HCl or H2SO4 in alcoholic solution, glucose and fructose are obtained in equal amounts.
2.From starch: Commercially glucose is obtained by hydrolysis of starch by boiling it with dilute H2SO4 at 393 K under pressure.
Structure of Glucose
Glucose is an aldohexose and is also known as dextrose. It is the monomer of many of the larger carbohydrates, namely starch, cellulose. It is probably the most abundant organic compound on earth. It was assigned the structure given below on the basis of the following evidences:
1.Its molecular formula was found to be C6H12O6
2.On prolonged heating with HI, it forms n-hexane, suggesting that all the six carbon atoms are linked in a straight chain.
3. Glucose reacts with hydroxylamine to form an oxime and adds a molecule of hydrogen cyanide to give cyanohydrin. These reactions confirm the presence of a carbonyl group (>C = O) in glucose.
4. Glucose gets oxidised to six carbon carboxylic acid (gluconic acid) on reaction with a mild oxidising agent like bromine water. This indicates that the carbonyl group is present as an aldehydic group.
5. Acetylation of glucose with acetic anhydride gives glucose pentaacetate which confirms the presence of five –OH groups. Since it exists as a stable compound, five –OH groups should be attached to different carbon atoms.
6. On oxidation with nitric acid, glucose as well as gluconic acid both yield a dicarboxylic acid, saccharic acid. This indicates the presence of a primary alcoholic (–OH) group in glucose.
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
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.
Role in human nutrition
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
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
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
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.
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.
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
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.
The structure (I) of glucose explained most of its properties but the
following reactions and facts could not be explained by this structure.
- Despite having the aldehyde group, glucose does not give Schiff’s test and it does not form the hydrogensulphite addition product with NaHSO3
- The pentaacetate of glucose does not react with hydroxylamine indicating the absence of free —CHO group.
- Glucose is found to exist in two different crystalline forms which are named as α and β. The α-form of glucose (m.p. 419 K) is obtained by crystallisation from concentrated solution of glucose at 303 K while the β-form (m.p. 423 K) is obtained by crystallisation from hot and
saturated aqueous solution at 371 K. This behaviour could not be explained by the open chain structure (I) for glucose. It was proposed that one of the —OH groups may add to the —CHO group and form a cyclic hemiacetal structure. It was found that glucose forms a six-membered ring in which —OH at C-5 is involved in ring formation. This explains the absence of —CHO group and also existence of glucose in two forms as shown below. These two cyclic forms exist in equilibrium with open chain structure.
The two cyclic hemiacetal forms of glucose differ only in the configuration of the hydroxyl group at C1, called anomeric carbon . Such isomers, i.e., α-form and β-form, are called anomers. The six membered cyclic structure of glucose is called pyranose structure (α– or β–), in analogy with pyran. Pyran is a cyclic organic compound with one oxygen atom
and five carbon atoms in the ring. The cyclic structure of glucose is more correctly represented by Haworth structure as given below
Classes of carbohydrates
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.
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.
Fructose is an important ketohexose. It is obtained along with glucose by the hydrolysis of disaccharide, sucrose. It is a natural monosaccharide found in fruits, honey and vegetables. In its pure form it is used as a sweetner. It is also an important ketohexose.
Fructose also has the molecular formula C6H12O6 and on the basis of its reactions it was found to contain a ketonic functional group at carbon number 2 and six carbons in straight chain as in the case of glucose. It belongs to D-series and is a laevorotatory compound.It is appropriately written as D-(–)-fructose. Its open
chain structure is as shown.
It also exists in two cyclic forms which are obtained by the addition of —OH at C5 to the group. The ring, thus formed is a five membered ring and is named as furanose with analogy to the compound furan. Furan is a five membered cyclic compound with one
oxygen and four carbon atoms. The cyclic structures of two anomers of fructose are represented by Haworth structures as given
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 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 is made up of one molecule of glucose and one molecule of fructose joined together. It is a disaccharide, a molecule composed of two monosaccharides: glucose and fructose. Sucrose is produced naturally in plants, from which table sugar is refined. It has the molecular formula C12H22O11.
For human consumption, sucrose is extracted and refined from either sugarcane or sugar beet. Sugar mills – typically located in tropical regions near where sugarcane is grown – crush the cane and produce raw sugar which is shipped to other factories for refining into pure sucrose. Sugar beet factories are located in temperate climates where the beet is grown, and process the beets directly into refined sugar. The sugar refining process involves washing the raw sugar crystals before dissolving them into a sugar syrup which is filtered and then passed over carbon to remove any residual colour. The sugar syrup is then concentrated by boiling under a vacuum and crystallized as the final purification process to produce crystals of pure sucrose that are clear, odorless, and sweet.
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.
Maltose also known as maltobiose or malt sugar, is a disaccharide formed from two units of glucose joined with an α(1→4) bond. In the isomerisomaltose, the two glucose molecules are joined with an α(1→6) bond. Maltose is the two-unit member of the amylosehomologous series, the key structural motif of starch. When alpha-amylase breaks down starch, it removes two glucose units at a time, producing maltose. An example of this reaction is found in germinating seeds, which is why it was named after malt. Unlike sucrose, it is a reducing sugar.
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 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.
People with lactose intolerance are unable to fully digest the sugar (lactose) in milk. As a result, they have diarrhea, gas and bloating after eating or drinking dairy products. The condition, which is also called lactose malabsorption, is usually harmless, but its symptoms can be uncomfortable.Too little of an enzyme produced in your small intestine (lactase) is usually responsible for lactose intolerance. You can have low levels of lactase and still be able to digest milk products. But if your levels are too low you become lactose intolerant, leading to symptoms after you eat or drink dairy.Most people with lactose intolerance can manage the condition without having to give up all dairy foods.
Lactase is found in the microvilli (small fingerlike projections) of the cells lining the small intestine. The environment in the small intestine where the enzyme is made ranges from pH 6 to 8, with the pH being 6 to 6.5 in the part of the intestine closest to the stomach. The optimal pH for human lactase is 6.0. At pH 8 to 9, it exhibits 50% of its maximum activity and has little activity below pH 3.
In most people, lactase expression diminishes between the ages of 2 and 12. As most people mature to adulthood, the cells that produce lactase are programmed to stop making significant amounts of the enzyme. The lactase gene is still intact, but is no longer expressed or is expressed only at very low levels. Once people stop producing lactase, they become lactose intolerant. They cannot comfortably digest dairy products that contain lactose, or are limited in the amount of dairy they can comfortably consume.
People can also become lactose intolerant because they have a condition that damages the lactase-producing cells in their intestines. For example, people with celiac disease have an immune reaction to the gluten protein found in some grain. Over time, this immune reaction damages the intestinal lining to the point that it no longer produces lactase.
Lactose intolerance should not be confused with lactase deficiency, which is relatively rare. People who are lactase deficient are said to have a “congenital lactase deficiency”; throughout their lives they are unable to make any functional lactase enzyme. The presence of a mutation that creates a stop codon in the lactase gene is one example of how this type of lactase deficiency occurs.Lactose intolerance is marked by the production of excessive gas, bloating, and abdominal pain. In some lactose intolerant people, ingesting lactose causes diarrhea, as well as a variety of other symptoms throughout their bodies. Most of the intestinal symptoms that mark lactose intolerance occur as a result of bacteria in the large intestine digesting lactose.
As the bacteria in the large intestine ferment lactose, they produce hydrogen, methane, and carbon dioxide gas. This excess gas causes bloating and abdominal pain. The hydrogen produced by the bacteria acidifies the intestine and the lactose present increases osmotic pressure in the intestine. The increased acid and osmotic pressure lead to the flow of fluid and ions into the intestine, causing diarrhea. In contrast, people who make lactase are able to digest the lactose in their small intestines before it has a chance to reach their large intestines and cause discomfort.As mentioned, there are people with lactose intolerance who experience symptoms that are more systemic, meaning that other parts of their bodies (besides their intestines) are affected. Some scientists hypothesize that these symptoms may be linked to compounds produced by lactose-fermenting bacteria in these individuals’ large intestines.
It is estimated that approximately 75% of the adult world population is not able to digest lactose, the main carbohydrate in milk.The ability to make lactase as an adult varies between populations. Populations of northern Europeans or of northern European descent and those from some African tribes are among the groups with the greatest number of people who continue to make lactase even after they are adults. The ability to express lactase into adulthood has been most commonly linked with 2 single base pair changes that are most commonly found in northern Europeans or in people of northern European descent.
These 2 mutations are found in the same region of the human genome. One of these 2 base pair changes, the one most closely associated with adult lactase expression in Europeans, changes a cytosine 13,910 base pairs upstream of the lactase gene to a thymine. Although this cytosine-to-thymine change is in a different gene from the lactase gene, evidence suggests that the mutation effects the regulation of lactase gene transcription.In addition to the 2 commonly found single base pair mutations mentioned above, multiple different single base pair changes linked to the ability of adults to express lactase have been found throughout the world. Examination of the sequence of the genomic DNA surrounding these different single base pair changes suggests that at least some of the changes arose independently in different populations.
From studying the DNA sequence of the human genome, scientists have been able to determine where many of these mutations originally occurred. Some of these mutations are found not only in populations located where the mutation originally arose, but in other populations as well. For example, the most common mutation conferring the ability to express the lactase enzyme into adulthood first arose in northern Europe, most likely in the region that is now Sweden, but it is also found in populations in other parts of the world. Studying the frequency of each of these mutations in different populations around the world reveals interesting information about the migration of people.
For instance, the most common mutation referred to above is found at fairly high frequency in Swedish populations. However, in general, as you move further east and south in Europe, the frequency of the mutation in populations decreases, suggesting that the population in which the mutation arose has not spread itself evenly across Europe.Archeologic evidence suggests that humans began to collect and drink milk from other animals in the Middle East around 9,000 years ago. Evidence also suggests that early dairying practices may have involved making cheese. Some cheeses contain little or no lactose and thus could have been eaten by people producing low levels of lactase.
Current evidence suggests that the ability of humans to express the lactase enzyme into adulthood did not arise until after dairying practices had begun. In the presence of cheeses with no or only a little lactose, even those not making lactase could benefit from dairying practices. However, those that continued to express lactase into adulthood would have also had access to the milk as well as a broader variety of cheeses. This access could have given them and their offspring a selective advantage, which would have selected for the spread of the allele in the population.
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.
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.
It is the storage polysaccharide of most plants. Human beings obtain it from cereal grains (e.g., rice, wheat), legumes (pea, gram, beans), potato, tapioca, banana etc. It is polyglucan homosaccharide and is formed as an end product of photosynthesis. Starch is stored either inside chloroplasts or special leucoplasts called amyloplasts. Starch occurs in the form of microscopic granules called starch grains.
Starch grains may occur singly or in groups. The two types are known as simple and compound starch grains. Starch grains may be rounded, oval, polygonal or rod shaped in outline (Fig. 9.6). Each grain has a number of shells or layers arranged in concentric or eccentric fashion around a proteinaceous point called hilum.Starch consists of two components, amylose and amylopectin (Fig. 9.7). Amylose is more soluble in water than amylopectin. In general, 20-30% of starch consists of amylose and the rest as amylopectin. Waxy starch of some varieties of Maize and other cereals consists entirely of amylopectin. On the other hand, the starch of some varieties of Pea having wrinkled surface may have as much as 98% of amylose
Both amylose and amylopectin are formed by the condensation of α -D-glucose (pyranose forms). Amylose is in the form of a continuous straight but helically arranged chain where each turn contains about six glucose units.
The successive glucose units are linked together by 1-4 α-linkages, that is, the link is between carbon atom 1 of one and carbon atom 4 of the other (Fig. 9.8). A molecule of water is lost during the formation of the linkage. A straight chain of amylose consists of 200-1000 glucose units.
Amylopectin contains a large number of glucose units (2000-200,000). Besides a straight chain it bears several side chains which may be branched further.
Branching is usually at intervals of about 25 residues. At the place of origin of a side chain, the carbon atom 6 of a glucose residue of a straight chain is linked to the carbon atom 1 of the first glucose unit of side chain (1-6 α-link- age). Wolform and Thompson (1956) have also reported 1 → 3 linkages in case of amylopectin.
Amylose fraction gives blue-black colour with iodine solution (Iodine-Potassium iodide solution) while amylopectin fraction gives red-violet colour.
It is the polysaccharide food reserve of animals, bacteria and fungi. Glycogen is popularly called animal starch. Glycogen is mainly stored inside liver (up to 0.1 kg) and muscles. In shape the complex carbohydrate appears as ellipsoid flattened granules that lie freely inside the cells. The polysaccharide gives reddish colour with iodine. Chemically, it is similar to starch.
It has about 30,000 glucose residues and a molecular weight of about 4.8 million. Glucose residues are arranged in a highly branched bush like chains. There are two types of linkages 1-4 α -linkages in the straight part and 1-6 linkages in the area of branching. The straight part is helically twisted with each turn having six glucose units. The distance between two branching points is 10-14 glucose residues.
It is a fructan storage polysaccharide of roots and tubers of Dahlia and related plants. Inulin is not metabolized in human body and is readily filtered through the kidney. It is, therefore, used in testing of kidney function, especially glomerular filtration.
They are polysaccharides that take part in forming the structural frame work of the cell walls in plants and skeleton of animals. Structural polysaccharides are of two main types: chitin and cellulose.
It is the second most abundant organic substance. Chitin is a complex carbohydrate of heteropolysaccharide type which is found as the structural component of fungal walls and exoskeleton of arthropods. In fungal walls, chitin is often known as fungus cellulose. Chitin is soft and leathery. Therefore, it provides both strength and elasticity. It becomes hard when impregnated with certain proteins and calcium carbonate.
In chitin, basic unit is not glucose but a nitrogen containing glucose derivative known as N-acetyl glucosamine. Chitin has an un-branched configuration. Monomers are joined together by 1- 4 β-linkages (Fig. 9.9). Adjacent residues lie at 180°. Molecules occur in parallel and are held together by hydrogen bonds.
It is fibrous homopolysaccharide of high tensile strength which forms a structural element of cell wall in all plants, some fungi and protists. Tunicin of tunicates (=ascidians) is related to cellulose (also called animal cellulose).
In absolute terms, cellulose is the most abundant organic substance of the biosphere forming 50% of carbon found in plants. Cotton fibres have about 90% of cellulose while wood contains 25-50% cellulose. The other materials of the cell wall include lignin, hemicellulose, pectins, wax, etc.
Cellulose molecules have un-branched and linear chains unlike the branched and helical chains of starch and glycogen. A chain of cellulose molecule contains 6000 or more glucose residues.
The successive glucose residues are joined together by 1-4 β-linkages (Fig. 9.10). Consequently alternate glucose molecules lie at 180° to each other. Hydroxyl groups of glucose residues, therefore, project in all directions. The molecular weight of cellulose ranges between 0.5 to 2.5 millions.
Cellulose molecules do not occur singly. Instead a number of chains are arranged in close antiparallel fashion. The molecules are held together by intermolecular hydrogen bonds between hydroxyl group at position 6 of glucose residues of one molecule and glycosidic oxygen between two glucose residues of the adjacent molecule.
There is also intermolecular strengthening of the chain by the formation of hydrogen bonds between hydroxyl group at position three and oxygen atom of the next residue. About 2000 cellulose chains or molecules are packed together to form a micro fibril visible under the electron microscope.
(1) Cellulose constitutes the bulk of human food. However, due to being polymer of β-glucose, cellulose is not acted upon by amylases present in human digestive juices. In humans, cellulose has a roughage value which keeps the digestive tract in functional fitness.
(2) Cellulose is an important constituent of diet for ruminants like cows and buffaloes. The stomach of ruminants contain micro-organisms capable of digesting or breaking down cellulose. Termites and snails also possess micro-organisms in their gut for this purpose.
(3) Microbes are used in producing soluble sugars from cellulose. The sugars are then allowed to undergo fermentation for obtaining ethanol, butanol, acetone, methane, etc.
(4) Cellulose rich wood is employed in building furniture, tools, sports articles, paper etc.
(5) Depending upon the percentage of cellulose present in the fibres, the latter are used in textiles (e.g., Cotton, Linen), preparation of sacs (e.g., Jute) or ropes (e.g., Hemp, China Jute, Deccan Hemp).
(6) Rayon and cellophane are formed of cellulose xanthate.
(6) Rayon and cellophane are formed of cellulose xanthate.
(7) Cellulose acetate is obtained by treating wood pulp with acetic acid, acetic anhydride and a catalyst. Cellulose acetates are used in preparing fibres for double knits, tericot, wrinkle proof, and moth proof clothing. Cigarette filters are also prepared from these fibres. Other uses of cellulose acetates include preparation of plastics and shatter proof glass.
8) Cellulose nitrate is used in propellent explosives.
(9) Carboxymethyl cellulose is used as emulsifier and smoothening reagent of ice creams, cosmetics and medicines.
Mucilage, mucus or slime forming substances are called mucosubstances. They are of two types, mucopolysaccharides and mucoproteins (= glycoproteins).
They are slimy substances or mucilages which possess acidic or aminated polysaccharides formed from galactose, mannose, sugar derivatives and uronic acids. Mucopolysaccharides or mucilages are quite common in both plants and animals.
They can be observed by cutting the unripe fruits of Okra (Lady’s finger, vern Bhindi) or soaking the husks or seeds of Plantago ovata (Plantain, vern. Isabgol). Mucopolysaccharides occur inside the plant cell walls, outside the cells or bodies of bacteria, blue-green algae and many aquatic plants, cementing layer between cells, inside body fluids, connective tissues and cartilages.
The important functions of mucopolysaccharides are as follows:
(1) They bind proteins in the cell walls and connective tissue.
(2) Water is held in the interstitial spaces due to mucopolysaccharides.
(3) Mucopolysaccharides occur in the cell walls of bacteria and blue-green algae. They form a layer of mucilage around them as well as other aquatic plants and protects the organisms against rotting effect of water, prevents desiccation, growth of epiphytes and attack of pathogens.
(4) They provide lubrication in ligaments and tendons.
(5) As keratan sulphate (acetyglucosamine + galactose + sulphuric acid) they occur inside skin and cornea providing both strength and flexibility.
(6) Chondroitin sulphate (glucuronic or iduronic acid + acetyl aminogalactose) is the mucopolysaccharide found in the matrix of cartilage and connective tissue for support and elasticity.
(7) Hyaluronic acid (glucuronic acid +acetyl glucosamine) is the mucopolysaccharide met in extracellular fluid of animal tissues, vitreous humor of eye, synovial fluid, cerebrospinal fluid, etc. It also occurs in cementing material between animal cells as well as inside cell coat.
(8) Husk of Plantago ovata contains mucilage which is used medicinally in treating intestinal problems. It relieves irritation. Mucilage present in Aloe barbadensis reduces inflammation. It is also purgative.
(9) Mucopolysaccharide heparin (α-1, 4 glucosamine + glucuronic acid) is blood anticoagulant.
(10) Agar. Marine brown and red algae, called sea weeds, yield mucopolysaccharides of commercial value, e.g., agar, alginic acid, carragenin, etc.
Agar (agar-agar) is polymer of D-galactose, 3-6 anhydro L-galactose having sulphate esterification after every tenth galactose unit. It is used as culture medium in the laboratory, as laxative, stiffening, stabilising and emulsifying agent. It is obtained from cell wall of some red alae like Gracilaria, Gelidium and Gelidiella.
Pectin (Pectic Compounds):
It is an acidic polysaccharide that occurs in the matrix of cell wall and middle lamella (as calcium pectate). Pectin is soluble in water and can undergo ↔ sol gel interchange. Pectin is formed of galacturonic acid, galactose, methylated galacturonic acid and arabinose. It is used in making Jelly and Jams.
It is a mixture of polysaccharides of xylans, mannans, galactans, arabino-galactans and glucomannans. Hemicellulose occurs in the cell wall where it forms a link between pectic compounds and cellulose micro fibrils.
Peptidoglycan (Murein, Mucopeptide):
It is formed of heteropolysaccharide chains cross-linked by short peptides (generally tetra peptides). Heteropolysaccharide chains are formed of two alternate amino-sugar molecules, N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM).
It is a complex formed of lipid and polysaccharide which forms the outer membrane of Gram -ve bacteria. There is a glycolipid responsible for endotoxic activity, a core polysaccharide and an antigen specific variable chain. Lipopolysaccharide induces fever, shock and other toxic effects.
The protein with conjugated monosaccharide’s form mucus. These are found in stomach, nasal secretion, intestine, vagina and are antibacterial and protective in function.
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.
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
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.
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.
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
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.
Carbohydrates are widely distributed molecules in plant and animal tissues. In plants and arthropods, carbohydrates from the skeletal structures, they also serve as food reserves in plants and animals. They are important energy source required for various metabolic activities, the energy is derived by oxidation.
Some of their major functions include
- Living organisms use carbohydrates as accessible energy to fuel cellular reactions. They are the most abundant dietary source of energy (4kcal/gram) for all living beings.
- Carbohydrates along with being the chief energy source, in many animals, are instant sources of energy. Glucose is broken down by glycolysis/ Kreb’s cycle to yield ATP.
- Serve as energy stores, fuels, and metabolic intermediates. It is stored as glycogen in animals and starch in plants.
- Stored carbohydrates act as an energy source instead of proteins.
- They form structural and protective components, like in the cell wall of plants and microorganisms. Structural elements in the cell walls of bacteria (peptidoglycan or murein), plants (cellulose) and animals (chitin).
- Carbohydrates are intermediates in the biosynthesis of fats and proteins.
- Carbohydrates aid in the regulation of nerve tissue and is the energy source for the brain.
- Carbohydrates get associated with lipids and proteins to form surface antigens, receptor molecules, vitamins, and antibiotics.
- Formation of the structural framework of RNA and DNA (ribonucleic acid and deoxyribonucleic acid).
- They are linked to many proteins and lipids. Such linked carbohydrates are important in cell-cell communication and in interactions between cells and other elements in the cellular environment.
- In animals, they are an important constituent of connective tissues.
- Carbohydrates that are rich in fiber content help to prevent constipation.
- Also, they help in the modulation of the immune system.
Cell membranes are selective barriers that separate individual cells and cellular compartments.Membranes are assemblies of carbohydrates, proteins, and lipids held together by binding forces.Carbohydrates are covalently linked to proteins (glycoproteins) or lipids (glycolipids) and also an important part of cell membranes, and function as adhesion and address loci for cells.The Fluid Mosaic Model describes membranes as a fluid lipid bilayer with floating proteins and carbohydrates.Membrane carbohydrates are chemically bound to glycolipids and glycoproteins.However, some membrane carbohydrates are part of proteoglycans that insert their amino acid chain among the lipid fatty acids.Although some carbohydrates can be found associated with intracellular membranes, most of them are located in the outer monolayer of the plasma membrane, facing the extracellular space.
Carbohydrates present in the plasma membrane as short sometimes branched chains of sugars attached either to exterior peripheral proteins (forming glycoproteins) or to the polar ends of phospholipid molecules in the outer lipid layer (forming glycolipids).Carbohydrate chains may consist of 2-60 monosaccharide units and can be either straight or branched.The oligosaccharide chains of membrane glycoproteins and glycolipids are formed by various combinations of six principal sugars D-galactose, D-mannose, L-fucose, N-acetylneuraminic acid (also called sialic acid), N-acetyl-D-glucosamine, and N-acetyl-D- galactosamine. All of these may be derived from glucose.The oligosaccharide side chains of glycoproteins and glycolipids are enormously diverse in their arrangement of sugars.Although they usually contain fewer than 15 sugars, they are often branched, and the sugars can be bonded together by a variety of covalent linkages—unlike the amino acids in a polypeptide chain, which are all linked by identical peptide bonds.Even three sugars can be put together to form hundreds of different trisaccharides.In principle, both the diversity and the exposed position of the oligosaccharides on the cell surface make them especially well-suited to a function in specific cell-recognition processes.
- Membrane carbohydrates perform two main functions: participate in cell recognition and adhesion, either cell-cell signaling or cell-pathogen interactions, and they have a structural role as a physical barrier.
- Blood groups are determined by cell surface carbohydrates of erythrocytes, and they also have the ability to trigger immunological responses.
- After an infection, endothelial cells near the injured tissue expose a type of proteins, known as selectins, in their plasma membranes. They recognize and bind carbohydrates of the plasma membrane of lymphocytes that go through the bloodstream. In this way, lymphocytes get attached to the blood vessel walls, can cross the endothelium and move to the infection focus.
- Carbohydrates as recognition molecules are also important during embryonic development.
- Carbohydrates of the plasma membrane are major recognition and attaching sites for pathogens during infection.
- The glycocalyx also has important functions in humans. It allows cells on the inside of blood vessels to withstand the strong flow of liquid across their surfaces.
- It protects microvilli in the gut, which absorb nutrients, and the glycocalyx even aids in the breakdown of food for this absorption by holding digestive enzymes in its coat.
- Certain plasma transport proteins, hormones, and enzymes are glycoproteins, and in these molecules, carbohydrate is important to physiological activity.
Test for Carbohydrate
|a) To 2 mL of the sample solution, 1 mL of Fehling’s solution is added and warmed.||A brick red ppt is obtained.|
|b) To 2 mL of the sample solution, 2-3 drops of α-naphthol solution is added. After that, conc. sulphuric acid is slowly added along the side of the test tube.||A violet ring is formed at the junction of the two liquids.|
|c) To 1 mL of the sample solution, 1mL of lead acetate solution is added followed by ammonium hydroxide solution. The content is boiled.||A white ppt is formed which turns to salmon pink on boiling with ammonium hydroxide.|