The group of compounds known as carbohydrates received their general name because of early observations that they often have the formula Cx(H2O)y - that is, they appear to be hydrates of carbon.
Limitations of the old definition: The above definition could not survive long due to the following reasons:
(i) A number of compounds such as rhamnose, (C6H12O5) and 2-deoxyribose (C5H10O4) are known which are carbohydrates by their chemical behaviour but cannot be represented as hydrates of carbon.
(ii) There are other substances like formaldehyde (HCHO, CH2O) and acetic acid [CH3COOH, C2 (H2O)2] which do not behave like carbohydrates but can be represented by the general formula, Cx(H2O)y.
Carbohydrates are defined as polyhydroxy aldehydes or polyhydroxy ketones or substances which give these on hydrolysis and contain at least one chiral carbon atom. It may be noted here that aldehydic and ketonic groups in carbohydrates are not present as such but usually exist in combination with one of the hydroxyl group of the molecule in the form of hemiacetals and hemiketals respectively.
Classification Of Carbohydrates
The carbohydrates are divided into three major classes depending upon whether or not they undergo hydrolysis, and if they do, on the number of products formed.
(i) Monosaccharides: The monosaccharides are polyhydroxy aldehydes or polyhydroxy ketones which cannot be decomposed by hydrolysis to give simpler carbohydrates. Examples are glucose and fructose, both of which have molecular formula, C6H12O6.
(ii) Oligosaccharides: The oligosaccharides (Greek, oligo, few) are carbohydrates which yield a definite number (2-9) of monosaccharide molecules on hydrolysis. They include,
(a) Disaccharides, which yield two monosaccharide molecules on hydrolysis.
Examples are sucrose and maltose, both of which have molecular formula, C12H22O11.
(b) Trisaccharides, which yield three monosaccharide molecules on hydrolysis.
Example is raffinose, which has molecular formula, C18H32O16.
(c) Tetrasaccharides, etc.
(iii) Polysaccharides: The polysaccahrides are carbohydrates of high molecular weight which yield many monosaccharide molecules on hydrolysis.
Examples are starch and cellulose, both of which have molecular formula, (C6H10O5)n.
* In general, the monosaccharides and oligosaccharides are crystalline solids, soluble in water and sweet to taste. They are collectively known as sugars. The polysaccharides, on the other hand, are amorphous, insoluble in water and tasteless. They are called non-sugars.
* The carbohydrates may also be classified as either reducing or non-reducing sugars. All those carbohydrates which have the ability to reduce Fehling’s solution and Tollen’s reagent are referred to as reducing sugars, while others are non-reducing sugars. All monosaccharides and the disaccharides other than sucrose are reducing sugars.
* The reaction of glucose with acetic anhydride & tollen’s reagent suggest that it is
(A) a penta hydroxyl aldehyde
(B) hydrate of carbon
(C) a polyhydroxy ketone
(D) an alcohols
The monosaccharides are the basis of carbohydrate chemistry since all carbohydrates are either monosaccharides or are converted into monosaccharides on hydrolysis. The monosaccharides are polyhydroxy aldehydes or polyhydroxy ketones. There are, therefore, two main classes of monosaccharides.
1. The Aldoses, which contain an aldehyde group
2. The Ketoses, which contain a ketone group
The aldoses and ketoses are further divided into sub-groups on the basis of the number of carbon atoms in their molecules, as trioses, tetroses, pentoses, hexoses, etc.
To classify a monosaccharide completely, it is necessary to specify both, the type of the carbonyl group and the number of carbon atoms present in the molecule. Thus monosaccharides are generally referred to as aldotrioses, aldotetroses, aldopentoses, aldohexoses, ketohexoses, etc.
The aldoses and ketoses may be represented by the following general formulas.
Glucose and fructose are specific examples of an aldose and a ketose.
How Kiliani (1886) shows the chemical structure of the monosaccharoses?
He prepared the cyanhydrins of glucose and fructose, hydrolysed them to the corresponding oxy-acids, from which the hydroxy groups were split out by reduction; it was found that glucose yielded normal heptylic acid and fructose methylbutylacetic acid; hence glucose is an aldehyde alcohol, CH2OH(CHOH)4CHO, whilst fructose is a ketone alcohol CH2OHCO(CHOH)3CH2OH. Kiliani also showed that arabinose, CH2OH(CHOH)3CHO a sugar found in cherry gum, was an aldopentose, and thus indicated an extension of the idea of a " sugar." Before proceeding to the actual synthesis of the sugars, it is advisable to discuss their decompositions and transformations.
*Cyanhydrins - The cyanhydrins on hydrolysis give monocarboxylic acids, which yield lactones; these compounds when reduced by sodium amalgam in sulphuric acid solution yield a sugar containing one more carbon atom. This permits the formation of a higher from a lower sugar (E. Fischer) CH20HCH20H / CHCHOH(CHOH)2 -> (CHOH)2CH-OHCHOHCOCHO
-> Lactone -> Hexose.
The configuration of the monosaccharides
We'll take a more detailed look at the cyclic and non-cyclic structures of sugars shortly.
Now let's see what aldotetrose means. Taking the name apart from right to left, the ending "ose" means a sugar, which may be a monosaccharide, a disaccharide or an oligosaccharide (a "short" polysaccharide). The middle part "tetr" means that our sugar has four carbons linked in a straight unbranched chain. Terms like "pent" for five carbons and "hex" for six carbons are also in common use. The beginning "aldo" means that there is an aldehyde in the compound. The alternative would be a ketone group, for which we would use the prefix "keto."
Glucose, the most common monosaccharide, is an aldohexose. We understand that to mean that it is a sugar having six carbons in a straight unbranched chain which ends in an aldehyde group. We can represent that structure in this fashion:
This structure includes four stereogenic carbon atoms (marked with an asterisk *). There are a total of sixteen stereoisomers possible. Eight of these are enantiomers of the other eight. The rest of the relationships are diastereoisomeric. Since the groups at the top and the bottom of the chain are not the same, there are no meso isomers. Eight of the isomers are shown here. The other eight are mirror images of these and may be readily drawn.
The question is "which of the sixteen stereochemical representations (Fischer projections, remember that each stereoisomer shown also has an enantiomer which is not shown) describes the absolute configuration of glucose? When Emil Fischer took up this problem about 100 years ago, he realized that there was no way to determine if glucose was one of the eight structures above or one of the unshown enantiomers. He made the assumption that it was one of the ones above so that he could work on the diastereoisomeric part of the problem, hoping that later work would resolve the question of which enantiomer best represented glucose.
Fischer also developed the D/L system for specifying the structures of sugars. If the OH group on the stereogenic carbon farthest from the aldehyde group is to the right in the Fischer projection, then the compound is a D-sugar. All of the sugars in the figure above are D-sugars. If the OH group on the stereogenic carbon farthest from the aldehyde group is to the left in the Fischer projection, then the compound is an L sugar. The enantiomers of all the sugars in the figure above are L sugars. Fischer's assumption amounts to saying that glucose is a D-sugar. Later work resolved this issue, and Fischer was right.
How did Fischer determine which of the eight structures above was glucose? He had available samples of glucose and mannose, both aldohexoses, and arabinose, an aldopentose. He also learned how to reduce the aldehyde functional group to a primary alcohol. (We'll illustrate this with NaBH4 to avoid learning a new reaction, but he used another reagent.) He developed a method for extending the carbon chain of an aldose (called the Kiliani-Fischer chain extension). He also had a polarimeter so he could determine whether a sample was optically active or not. Perhaps most importantly, he had a group of talented and dedicated students.
Now, some data.
Experimental result: When the aldehyde group of arabinose was reduced to a primary alcohol group, the product was optically active.
Conclusion: Arabinose has either structure 2 or 4 in the scheme below. This is because if arabinose were either 1 or 3, the product would have a plane of symmetry (mirror plane) and would be optically inactive.
Experimental result: When the aldehyde group of glucose was reduced to a primary alcohol group, the product was optically active. The same result was obtained for mannose.
Conclusion: The structures "X'd" out below do not represent either glucose or mannose since the products from these structures would be meso compounds.
Experimental result: Kiliani-Fischer chain extension applied to arabinose produces glucose and mannose.
Conclusion: The bottom three stereogenic carbon atoms of glucose and mannose are have identical configurations to the three stereogenic carbon atoms of arabinose. This means that glucose and mannose differ only in the configuration of the stereogenic carbon atom nearest the aldehyde functional group. We can further conclude that if one member of a pair of aldohexoses (paired because their bottom three stereogenic carbons are identical) is ruled out, so is the other.
Now let's see what we have left. There are four structures remaining as candidates. They are on the right below. If we go back to the possibilities for arabinose, we find that the two on the top come from structure 2 for arabinose, which was a possibility, while the two on the bottom come from structure 3, which was ruled out earlier. The conclusion is that arabinose is represented by structure 2, and glucose and mannose are the two structures to its right.
But which is glucose and which is mannose? Fischer noticed that if reactions could be developed which changed the aldehyde group into a primary alcohol and the primary alcohol into an aldehyde (switch ends) one of these structures would give itself, and the other would give back a new L sugar. The reactions are complex and we will not look at them, but when the chemistry was applied to the sample called mannose, the product was mannose. When the chemistry was applied to the sample called glucose, a new sugar was formed.
There was a great deal more to be done to confirm this conclusion and to synthesize the other six aldohexoses, but Fischer's exercise in logic and dedicated experimentation led to the conclusion that the eight D-aldohexoses are:
Notice that the new sugar which was produced from glucose by the "exchange ends" experiment is L-gulose. The names of the hexoses tell us which diastereoisomer we have; the D or L designation gives us which enantiomer we have.
The corresponding names for the aldopentoses are:
To finish today, we'll see what happens when a hemiacetal is formed between the aldehyde carbon and one of the OH groups on the chain. We'll look at two examples, ribose, which is a key component of RNA, and glucose because of its abundance. (You may wish to review the mechanism for hemiacetal formation.)
Since there are four OH groups in ribose, we could anticipate four different ring sizes. In three atom rings and four atom rings the bond angles are far from 109.5 degree, so these rings are strained, have higher energies and are hard to form. Remember that there is an equilibrium between a hemiacetal and the aldehyde/alcohol it comes from, and that high energy materials don't persist at equilibrium. We are left with rings which have either five or six atoms in them. In the case of ribose the important ring (found in RNA) is the five membered ring.
Notice that the carbon in the newly formed hemiacetal group is stereogenic. This means that there are two possible diastereoisomers for the cyclic structure. Usually both are formed, and they have a special name -- they are anomers of each other. The carbon between the two oxygens in the hemiacetal group is called the anomeric carbon. If the OH group is down (in a drawing with the ring oxygen to the rear center or right), the designation for that anomer is alpha. If the OH group is up, the designation is beta. Since the alpha and beta anomers are diastereoisomers, they have different properties; in particular, different optical activities. The term for a five atom sugar ring is "furanose."
Glucose usually makes hemiacetal cyclic structures with six atom rings, although five membered rings can also be formed when the six membered rings are precluded. Such six membered rings are named by the term "pyranose." The ring forms look like this, keeping in mind that alpha and beta anomers are also involved here:
Again, we have an equilibrium between the open chain form and the two diastereoisomeric anomers.
There is one further point to be made about these glucopyranoses. The structure we have drawn for the ring is flat. The bond angles would be 120 degree, quite far from the normal tetrahedral value of 109.5 degree. The atoms in the ring can have bond angles of about 109.5 degree if the ring puckers as shown here:
Of course, molecules adopt these puckered shapes (called chair conformations from their resemblance to a rather wide lounge chair) automatically. (You can review the material on cyclohexane for a more detailed analysis of this material.) It has been established that in these chair conformations, the molecules have a lower energy if the larger substituents on the carbons are roughly in the plane of the ring itself. These positions are called "equatorial" to distinguish them from the other positions (roughly perpendicular to the ring, called "axial"). A compound which can have all of its larger substituents (everything is larger than hydrogen) in an equatorial position is more stable than one which cannot. beta-D-glucose has all of its substituents in equatorial positions, and is thus the most stable hexopyranose. It is also the most abundant.
Important reactions of monosaccharides
1. Reaction with HCN
Glucose adds a molecule of hydrogen cyamide to give a cyanohydrin.
This is a Kiliani-Fischer synthesis, the Kiliani-Fischer synthesis applied to arabinose gives a mixture of glucose and mannose (see the reaction below):
2. Reaction with hydroxyl amine
Glucose reacts with hydroxylamine to give monoxime.
3. Reaction with phenylhydrazine (Osazone)
D-glucose reacts with phenyl hydrazine to give glucose phenyl hydrazine which is soluble. If excess od phenyl hydrazine is used, a dihydrazone, known as osazone is obtained. The osazone reaction was developed and used by Emil Fischer to identify aldose sugars differing in configuration only at the alpha-carbon. The upper equation shows the general form of the osazone reaction, which effects an alpha-carbon oxidation with formation of a bis-phenylhydrazone, known as an osazone. Application of the osazone reaction to D-glucose and D-mannose demonstrates that these compounds differ in configuration only at C-2.
(a) Oxidation with Cu (II) ion (Fehling, Benedict)
(b) Reaction with Tollen’s reagent
Glucose reduces ammoniacal silver nitrate solution (Tollens reagent) to metallic silver and also Fehlings solution to reddish brown cuprous oxide and itself gets oxidized to gluconic acid. This confirms the presence of an aldehydic group in Glucose.
(c) Reaction with Bromine water (ketoses do not react with bromine water)
(d) Reaction with Nitric acid
On oxidation with nitric acid, glucose as well as gluconic acid both yield a dicarboxylic acid saccharic acid. This indicates that presence of a primary alcoholic group in glucose.
(e) Reaction with body enzyme
D-glucose reacts with body enzyme to give glucuronic acid
*Petose -> Petitol
*Hexose -> Hexitol
(a) With HCl
When treated with concentrated sulphuric acid glucose undergoes dehydration and results in the formation of hydroxy methyl furfural.
* Pentose -> furfural
* Hexose -> mixture of hydroxy methyl furfural and levulinic acid
(b) With strong base
On heating with conc. solution of NaOH, glucose first turns yellow, then brown and finally resinifies (Caramelizing). However, with dilute NaOH, glucose undergoes a reversible isomerisation and is converted into a mixture of D-glucose, D-maltose and D-fructose. This reaction is known as Lobry de Bruyn-van Ekenstein rearrangement. Same results are obtained if maltose. or fructose are treated with alkali. It is probably on account of this isomerisation that fructose reduces Fehling's and Tollen's reagent in alkaline medium although it does not contain a -CHO group.
7. Degradation in monosaccharides
Ruff degradation of the pentose arabinose gives the tetrose erythrose.
If a monosaccharide has a carbonyl function on one of the inner atoms of the carbon chain it is classified as a ketose. Dihydroxyacetone may not be a sugar, but it is included as the ketose analog of glyceraldehyde. The carbonyl group is commonly found at C-2, as illustrated by the following examples (chiral centers are colored red). As expected, the carbonyl function of a ketose may be reduced by sodium borohydride, usually to a mixture of epimeric products. D-Fructose, the sweetest of the common natural sugars, is for example reduced to a mixture of D-glucitol (sorbitol) and D-mannitol, named after the aldohexoses from which they may also be obtained by analogous reduction. Mannitol is itself a common natural carbohydrate.
Although the ketoses are distinct isomers of the aldose monosaccharides, the chemistry of both classes is linked due to their facile interconversion in the presence of acid or base catalysts. This interconversion, and the corresponding epimerization at sites alpha to the carbonyl functions, occurs by way of an enediol tautomeric intermediate.
Because of base-catalyzed isomerizations of this kind, the Tollens' reagent is not useful for distinguishing aldoses from ketoses or for specific oxidation of aldoses to the corresponding aldonic acids. Oxidation by HOBr is preferred for the latter conversion.
Anomeric Forms of Glucose
Fischer's brilliant elucidation of the configuration of glucose did not remove all uncertainty concerning its structure. Two different crystalline forms of glucose were reported in 1895. Each of these gave all the characteristic reactions of glucose, and when dissolved in water equilibrated to the same mixture. This equilibration takes place over a period of many minutes, and the change in optical activity that occurs is called mutarotation. These facts are summarized in the diagram below.
When glucose was converted to its pentamethyl ether (reaction with excess CH3I & AgOH), two different isomers were isolated, and neither exhibited the expected aldehyde reactions. Acid-catalyzed hydrolysis of the pentamethyl ether derivatives, however, gave a tetramethyl derivative that was oxidized by Tollen's reagent and reduced by sodium borohydride, as expected for an aldehyde.
The search for scientific truth often proceeds in stages, and the structural elucidation of glucose serves as a good example. It should be clear from the new evidence presented above, that the open chain pentahydroxyhexanal structure drawn above must be modified. Somehow a new stereogenic center must be created, and the aldehyde must be deactivated in the pentamethyl derivative. A simple solution to this dilemma is achieved by converting the open aldehyde structure for glucose into a cyclic hemiacetal, called a glucopyranose, as shown in the following diagram. The linear aldehyde is tipped on its side, and rotation about the C4-C5 bond brings the C5-hydroxyl function close to the aldehyde carbon. For ease of viewing, the six-membered hemiacetal structure is drawn as a flat hexagon, but it actually assumes a chair conformation. The hemiacetal carbon atom (C-1) becomes a new stereogenic center, commonly referred to as the anomeric carbon, and the α and β-isomers are called anomers.
We can now consider how this modification of the glucose structure accounts for the puzzling facts noted above. First, we know that hemiacetals are in equilibrium with their carbonyl and alcohol components when in solution. Consequently, fresh solutions of either alpha or beta-glucose crystals in water should establish an equilibrium mixture of both anomers, plus the open chain chain form. Note that despite the very low concentration of the open chain aldehyde in this mixture, typical chemical reactions of aldehydes take place rapidly.
Second, a pentamethyl ether derivative of the pyranose structure converts the hemiacetal function to an acetal. Acetals are stable to base, so this product should not react with Tollen's reagent or be reduced by sodium borohydride. Acid hydrolysis of acetals regenerates the carbonyl and alcohol components, and in the case of the glucose derivative this will be a tetramethyl ether of the pyranose hemiacetal. This compound will, of course, undergo typical aldehyde reactions.
Cyclic Forms of Monosaccharides
The open chain structure of monosaccharides (eg. glucose) proposed by Baeyer explained most of its properties. However, it could not explain the following:
1. Despite having an aldehydic group, glucose does not gives Schiff's test and it does not react with sodium bi-sulphite and ammonia.
2. The penta acetate of glucose does not react with hydroxylamine indicating absence of -CHO group.
When glucose was crystallized from a concentrated solution at 30oC it gave a form of glucose (Melting point 146oC) whose optical rotation is 111o. The b form (Melting point 150o) obtained on crystallization of glucose from a hot saturated aqueous solution at a temperature above 98oC has an optical rotation of 19.2o. These two forms of glucose are called anomers.
As noted above, the preferred structural form of many monosaccharides may be that of a cyclic hemiacetal. Five and six-membered rings are favored over other ring sizes because of their low angle and eclipsing strain. Cyclic structures of this kind are termed furanose (five-membered) or pyranose (six-membered), reflecting the ring size relationship to the common heterocyclic compounds furan and pyran. Ribose, an important aldopentose, commonly adopts a furanose structure, as shown in the following illustration. By convention for the D-family, the five-membered furanose ring is drawn in an edgewise projection with the ring oxygen positioned away from the viewer. The anomeric carbon atom (colored red here) is placed on the right. The upper bond to this carbon is defined as beta, the lower bond then is alpha.
The cyclic pyranose forms of various monosaccharides are often drawn in a flat projection known as a Haworth formula, after the British chemist, Norman Haworth. As with the furanose ring, the anomeric carbon is placed on the right with the ring oxygen to the back of the edgewise view. In the D-family, the alpha and beta bonds have the same orientation defined for the furanose ring (beta is up & alpha is down). These Haworth formulas are convenient for displaying stereochemical relationships, but do not represent the true shape of the molecules. We know that these molecules are actually puckered in a fashion we call a chair conformation. Examples of four typical pyranose structures are shown below, both as Haworth projections and as the more representative chair conformers. The anomeric carbons are colored red.
The size of the cyclic hemiacetal ring adopted by a given sugar is not constant, but may vary with substituents and other structural features. Aldolhexoses usually form pyranose rings and their pentose homologs tend to prefer the furanose form, but there are many counter examples. The formation of acetal derivatives illustrates how subtle changes may alter this selectivity. By clicking on the above diagram. the display will change to illustrate this. A pyranose structure for D-glucose is drawn in the rose-shaded box on the left. Acetal derivatives have been prepared by acid-catalyzed reactions with benzaldehyde and acetone. As a rule, benzaldehyde forms six-membered cyclic acetals, whereas acetone prefers to form five-membered acetals. The top equation shows the formation and some reactions of the 4,6-O-benzylidene acetal, a commonly employed protective group. A methyl glycoside derivative of this compound (see below) leaves the C-2 and C-3 hydroxyl groups exposed to reactions such as the periodic acid cleavage, shown as the last step. The formation of an isopropylidene acetal at C-1 and C-2, center structure, leaves the C-3 hydroxyl as the only unprotected function. Selective oxidation to a ketone is then possible. Finally, direct di-O-isopropylidene derivatization of glucose by reaction with excess acetone results in a change to a furanose structure in which the C-3 hydroxyl is again unprotected. However, the same reaction with D-galactose, shown in the blue-shaded box, produces a pyranose product in which the C-6 hydroxyl is unprotected. Both derivatives do not react with Tollens' reagent. This difference in behavior is attributed to the cis-orientation of the C-3 and C-4 hydroxyl groups in galactose, which permits formation of a less strained five-membered cyclic acetal, compared with the trans-C-3 and C-4 hydroxyl groups in glucose. Derivatizations of this kind permit selective reactions to be conducted at different locations in these highly functionalized molecules.
Acetal derivatives formed when a monosaccharide reacts with an alcohol in the presence of an acid catalyst are called glycosides. This reaction is illustrated for glucose and methanol in the diagram below. In naming of glycosides, the "ose" suffix of the sugar name is replaced by "oside", and the alcohol group name is placed first. As is generally true for most aldols, glycoside formation involves the loss of an equivalent of water. The diether product is stable to base and alkaline oxidants such as Tollen's reagent. Since acid-catalyzed aldolization is reversible, glycosides may be hydrolyzed back to their alcohol and sugar components by aqueous acid.
The anomeric methyl glucosides are formed in an equilibrium ratio of 66% alpha to 34% beta. From the structures in the previous diagram, we see that pyranose rings prefer chair conformations in which the largest number of substituents are equatorial. In the case of glucose, the substituents on the beta-anomer are all equatorial, whereas the C-1 substituent in the alpha-anomer changes to axial. Since substituents on cyclohexane rings prefer an equatorial location over axial (methoxycyclohexane is 75% equatorial), the preference for alpha-glycopyranoside formation is unexpected, and is referred to as the anomeric effect.
Glycosides abound in biological systems. By attaching a sugar moiety to a lipid or benzenoid structure, the solubility and other properties of the compound may be changed substantially. Because of the important modifying influence of such derivatization, numerous enzyme systems, known as glycosidases, have evolved for the attachment and removal of sugars from alcohols, phenols and amines. Chemists refer to the sugar component of natural glycosides as the glycon and the alcohol component as the aglycon. Two examples of naturally occurring glycosides and one example of an amino derivative will be displayed above by clicking on the diagram. Salicin, one of the oldest herbal remedies known, was the model for the synthetic analgesic aspirin. A large class of hydroxylated, aromatic oxonium cations called anthocyanins provide the red, purple and blue colors of many flowers, fruits and some vegetables. Peonin is one example of this class of natural pigments, which exhibit a pronounced pH color dependence. The oxonium moiety is only stable in acidic environments, and the color changes or disappears when base is added. The complex changes that occur when wine is fermented and stored are in part associated with glycosides of anthocyanins. Finally, amino derivatives of ribose, such as cytidine play important roles in biological phosphorylating agents, coenzymes and information transport and storage materials.
The Most Important Monosaccharide On Earth
Pentoses and hexoses are biologically most significant and abundant. They exist in open chain as well as ring form.
(1) Glucose (Dextrose, Grape sugar)
(a) So widely used in organisms
- Glucose has a lower tendency, relative to other hexose sugars, to react non-specifically with the amino groups of proteins.
- This reaction (glycation) reduces or destroys the function of many enzymes. The low rate of glycation is due to glucose's preference for the less reactive cyclic isomer.
- Nevertheless, many of the long-term complications of diabetes (e.g., blindness, renal failure, and peripheral neuropathy) are probably due to the glycation of proteins or lipids.
- In contrast, enzyme-regulated addition of glucose to proteins by glycosylation is often essential to their function.
(b) As an energy source; Glucose is a ubiquitous fuel in biology.
- It is used as an energy source in most organisms, from bacteria to humans.
- Use of glucose may be by either aerobic respiration, anaerobic respiration, or fermentation.
- Carbohydrates are the human body's key source of energy, through aerobic respiration, providing approximately 3.75 kilocalories (16 kilojoules) of food energy per gram. Breakdown of carbohydrates (e.g. starch) yields mono- and disaccharides, most of which is glucose.
- Through glycolysis and later in the reactions of the citric acid cycle (TCAC), glucose is oxidized to eventually form CO2 and water, yielding energy sources, mostly in the form of ATP.
- The insulin reaction, and other mechanisms, regulate the concentration of glucose in the blood. A high fasting blood sugar level is an indication of prediabetic and diabetic conditions.
- Glucose is a primary source of energy for the brain, and hence its availability influences psychological processes. When glucose is low, psychological processes requiring mental effort (e.g., self-control, effortful decision-making) are impaired.
- Use of glucose as an energy source in cells is via aerobic or anaerobic respiration. Both of these start with the early steps of the glycolysis metabolic pathway. The first step of this is the phosphorylation of glucose by hexokinase to prepare it for later breakdown to provide energy.
- The major reason for the immediate phosphorylation of glucose by a hexokinase is to prevent diffusion out of the cell. The phosphorylation adds a charged phosphate group so the glucose 6-phosphate cannot easily cross the cell membrane. Irreversible first steps of a metabolic pathway are common for regulatory purposes.
(c) As a precursor
- Glucose is critical in the production of proteins and in lipid metabolism. In plants and most animals, it is also a precursor for vitamin C (ascorbic acid) production.
- It is modified for use in these processes by the glycolysis pathway.
- Glucose is used as a precursor for the synthesis of several important substances.
- Starch, cellulose, and glycogen ("animal starch") are common glucose polymers (polysaccharides).
- Lactose, the predominant sugar in milk, is a glucose-galactose disaccharide. In sucrose, another important disaccharide, glucose is joined to fructose. These synthesis processes also rely on the phosphorylation of glucose through the first step of glycolysis.
(d) Glucose for use in the laboratory; Industrial use
- In industry, glucose is used as a precursor to make vitamin C in the Reichstein process, to make citric acid, gluconic acid, bio-ethanol, polylactic acid, sorbitol.
(e) Sources and absorption
- Most dietary carbohydrates contain glucose, either as their only building block, as in starch and glycogen, or together with another monosaccharide, as in sucrose and lactose.
- In the lumen of the duodenum and small intestine, the glucose oligo- and polysaccharides are broken down to monosaccharides by the pancreatic and intestinal glycosidases.
- Other polysaccharides cannot be processed by the human intestine and require assistance by intestinal flora if they are to be broken down; the most notable exceptions are sucrose (fructose-glucose) and lactose (galactose-glucose). Glucose is then transported across the apical membrane of the enterocytes by SLC5A1, and later across their basal membrane by SLC2A2.
- Some of the glucose is directly utilized as an energy source by brain cells, intestinal cells and red blood cells, while the rest reaches the liver, adipose tissue and muscle cells, where it is absorbed and stored as glycogen (under the influence of insulin).
- Liver cell glycogen can be converted to glucose and returned to the blood when insulin is low or absent; muscle cell glycogen is not returned to the blood because of a lack of enzymes.
- In fat cells, glucose is used to power reactions that synthesize some fat types and have other purposes.
- Glycogen is the body's 'glucose energy storage' mechanism because it is much more 'space efficient' and less reactive than glucose itself.
- It is found in dairy products, sugar beets, and other gums and mucilages.
- It is also synthesized by the body, where it forms part of glycolipids and glycoproteins in several tissues.
(b) Clinical significance
- Chronic systemic exposure of mice, rats, and Drosophila to D-galactose causes the acceleration of senescence and has been used as an aging model.
- Two studies have suggested a possible link between galactose in milk and ovarian cancer.
- Other studies show no correlation, even in the presence of defective galactose metabolism.
- More recently, pooled analysis done by the Harvard School of Public Health showed no specific correlation between lactose containing foods and ovarian cancer, and showed statistically insignificant increases in risk for consumption of lactose at ≥30 g/d.
- More research is necessary to ascertain possible risks.
- There are some ongoing studies that suggest that galactose may have a role in treatment of focal segmental glomerulosclerosis (a kidney disease resulting in kidney failure and proteinuria).
- This effect is likely to be a result of binding of galactose to FSGS factor.
- Galactose is a component of the antigens present on blood cells that determine blood type within the ABO blood group system.
- Natural sources of fructose include fruits, vegetables (including sugar cane), and honey. Fructose is often further concentrated from these sources.
- The highest dietary sources of fructose, besides pure crystalline fructose, are foods containing table sugar (sucrose), high-fructose corn syrup, agave nectar, honey, molasses, maple syrup, and fruit juices, as these have the highest percentages of fructose (including fructose in sucrose) per serving compared to other common foods and ingredients.
- Fructose exists in foods either as a free monosaccharide, or bound to glucose as sucrose, a disaccharide.
- Fructose, glucose, and sucrose may all be present in a food; however, different foods will have varying levels of each of these three sugars.
(b) Commercial sweeteners (carbohydrate content)
- Fructose is also found in the synthetically manufactured sweetener, high-fructose corn syrup (HFCS).
- Hydrolyzed corn starch is used as the raw material for production of HFCS. Through the enzymatic treatment, glucose molecules are converted into fructose.
- There are three types of HFCS, each with a different proportion of fructose: HFCS-42, HFCS-55, and HFCS-90. The number for each HFCS corresponds to the percentage of synthesized fructose present in the syrup. HFCS-90 has the highest concentration of fructose, and is typically used to manufacture HFCS-55; HFCS 55 is used as sweetener in soft drinks, while HFCS-42 is used in many processed foods and baked goods.
- Mannose can be formed by the oxidation of mannitol.
- It can also be formed from glucose in the Lobry-de Bruyn-van Ekenstein transformation
- D-Mannose is a simple sugar that occurs naturally in some plants, including cranberries.
- Although small amounts of D-Mannose are metabolized by the human body, much of it is rapidly excreted in the urine.
- D-mannose is sold as a naturopathic remedy for urinary tract infections, and it is claimed to work through the disruption of adherence of bacteria in the urinary tract.
- Glyceraldehyde is the most common form of triose found in the living cells.
- Erythrose is a tetrose involved in synthesis of lignin and anthocyanin.
- Riboses and ribuloses are most common forms of pentoses. Ribose sugars occur in nucleic acids.
- Sedoheptulose is a common example of heptose found in living cells.
- A sugar containing fewer oxygen atoms than carbon atoms, resulting in one or more carbons in the molecule lacking an attached hydroxyl group.
(i) Nucleic acid found in DNA is 2-deoxy-alpha-D-ribufuranose
(ii) Found in polisachharide of mukoprotein plasm (L-Fucose(6-deoxy-alpha-L-galactopiranose))
- Hexosamines are amino sugars created by adding an amine group to a hexose.
- Examples include:
(1) Fructosamine (based upon fructose)
(2) Galactosamine (based upon galactose)
(3) Glucosamine (based upon glucose)
(4) Mannosamine (based upon mannose): Found in mukoprotein
Vitamin C (L-Ascorbic Acid)
- Derive from hexose
- Water soluble
- Optically active + 49 degree
- Vitamin C or L-ascorbic acid or L-ascorbate is an essential nutrient for humans and certain other animal species, in which it functions as a vitamin.
- In living organisms, ascorbate is an anti-oxidant, since it protects the body against oxidative stress.
- It is also a cofactor in at least eight enzymatic reactions, including several collagen synthesis reactions that cause the most severe symptoms of scurvy when they are dysfunctional.
- In animals, these reactions are especially important in wound-healing and in preventing bleeding from capillaries.
- Ascorbate (an ion of ascorbic acid) is required for a range of essential metabolic reactions in all animals and plants.
- It is made internally by almost all organisms; notable mammalian group exceptions are most or all of the order chiroptera (bats), and one of the two major primate suborders, the Anthropoidea (Haplorrhini) (tarsiers, monkeys and apes, including human beings).
- Ascorbic acid is also not synthesized by guinea pigs, capybaras, and some species of birds and fish. All species that do not synthesize ascorbate require it in the diet. Deficiency in this vitamin causes the disease scurvy in humans.
- It is also widely used as a food additive.
- The disaccharides yield on hydrolysis two monosaccharides. Those disaccharides which yield two hexoses on hydrolysis have a general formula C12H22O11. The hexoses obtained on hydrolysis may be same or different.
- The hydrolysis is done by dilute acids or enzymes. The enzymes which bring hydrolysis of sucrose, lactose and maltose are invertase, lactase and maltase, respectively. Out of the three disaccharides, sucrose (cane-sugar) is the most important as it is an essential constituent of our diet.
- In disaccharides, the two monosaccharides are joined together by glycoside linkage. A glycoside bond is formed when hydroxy group of the hemiacetal carbon of one monosaccharide condenses with a hydroxy group of another monosaccharide giving –O– bond.
- Four examples of disaccharides composed of two glucose units are shown in the following diagram. The individual glucopyranose rings are labeled A and B, and the glycoside bonding is circled in light blue. Notice that the glycoside bond may be alpha, as in maltose and trehalose, or beta as in cellobiose and gentiobiose. Acid-catalyzed hydrolysis of these disaccharides yields glucose as the only product.
- Enzyme-catalyzed hydrolysis is selective for a specific glycoside bond, so an alpha-glycosidase cleaves maltose and trehalose to glucose, but does not cleave cellobiose or gentiobiose. A beta-glycosidase has the opposite activity.
- In order to draw a representative structure for cellobiose, one of the glucopyranose rings must be rotated by 180º, but this feature is often omitted in favor of retaining the usual perspective for the individual rings. The bonding between the glucopyranose rings in cellobiose and maltose is from the anomeric carbon in ring A to the C-4 hydroxyl group on ring B. This leaves the anomeric carbon in ring B free, so cellobiose and maltose both may assume alpha and beta anomers at that site (the beta form is shown in the diagram). Gentiobiose has a beta-glycoside link, originating at C-1 in ring A and terminating at C-6 in ring B. Its alpha-anomer is drawn in the diagram. Because cellobiose, maltose and gentiobiose are hemiacetals they are all reducing sugars (oxidized by Tollen's reagent). Trehalose, a disaccharide found in certain mushrooms, is a bis-acetal, and is therefore a non-reducing sugar. A systematic nomenclature for disaccharides exists, but as the following examples illustrate, these are often lengthy.
Cellobiose : 4-O-β-D-Glucopyranosyl-D-glucose (the beta-anomer is drawn)
Maltose : 4-O-α-D-Glucopyranosyl-D-glucose (the beta-anomer is drawn)
Gentiobiose : 6-O-β-D-Glucopyranosyl-D-glucose (the alpha-anomer is drawn)
Trehalose : α-D-Glucopyranosyl-α-D-glucopyranoside
- Although all the disaccharides shown here are made up of two glucopyranose rings, their properties differ in interesting ways. Maltose, sometimes called malt sugar, comes from the hydrolysis of starch. It is about one third as sweet as cane sugar (sucrose), is easily digested by humans, and is fermented by yeast. Cellobiose is obtained by the hydrolysis of cellulose. It has virtually no taste, is indigestible by humans, and is not fermented by yeast. Some bacteria have beta-glucosidase enzymes that hydrolyze the glycosidic bonds in cellobiose and cellulose. The presence of such bacteria in the digestive tracts of cows and termites permits these animals to use cellulose as a food. Finally, it may be noted that trehalose has a distinctly sweet taste, but gentiobiose is bitter.
- Disaccharides made up of other sugars are known, but glucose is often one of the components. Two important examples of such mixed disaccharides will be displayed above by clicking on the diagram. Lactose, also known as milk sugar, is a galactose-glucose compound joined as a beta-glycoside. It is a reducing sugar because of the hemiacetal function remaining in the glucose moiety. Many adults, particularly those from regions where milk is not a dietary staple, have a metabolic intolerance for lactose. Infants have a digestive enzyme which cleaves the beta-glycoside bond in lactose, but production of this enzyme stops with weaning. Cheese is less subject to the lactose intolerance problem, since most of the lactose is removed with the whey. Sucrose, or cane sugar, is our most commonly used sweetening agent. It is a non-reducing disaccharide composed of glucose and fructose joined at the anomeric carbon of each by glycoside bonds (one alpha and one beta). In the formula shown here the fructose ring has been rotated 180º from its conventional perspective.
- Disaccharides consist of:
(a) Do not have reduction and mutarotation characteristics because do not have free OH lactol. Called as: Glycocyl aldoside, Glycocyl ketoside.
(h) Have reduction and mutarotation characteristics because have free OH lactol. Called as: Glycocyl aldose, Glycocyl ketose.
The Most Important Oligosaccharides On Earth
1. Saccharose (Sucrose, cane sugar):
- The biosynthesis of sucrose proceeds via the precursors glucose 1-phosphate and fructose 6-phosphate.
- Saccharide does not has reduction and mutarotation characteristics because does not has free OH lactol.
- Breaking into monosaccharides by invertase enzyme inside our body, sucrose also called as Inverted sugar and found largely in honey, sugarcane, etc.
- Fermented to form ethanol and CO2 (side product).
- When sucrose heated to 210 degree C, caramel formed.
- Sucrose is formed by plants but not by other organisms. Sucrose is found naturally in many food plants along with the monosaccharide fructose. In many fruits, such as pineapple and apricot, sucrose is the main sugar.
- In others, such as grapes and pears, fructose is the main sugar.
- Sugar forms a major element in confectionery and in desserts. Cooks use it for sweetening, and it can also act as a food preservative when used in sufficient concentrations.
- Sucrose is important to the structure of many foods including biscuits and cookies, cakes and pies, candy, and ice cream and sorbets. It is a common ingredient in many processed and so-called "junk foods."
- In mammals, sucrose is readily digested in the stomach into its component sugars, by acidic hydrolysis. This step is performed by a glycoside hydrolase, which catalyzes the hydrolysis of sucrose to the monosaccharides glucose and fructose.
- Glucose and fructose are rapidly absorbed into the bloodstream in the small intestine. Undigested sucrose passing into the intestine is also broken down by sucrase or isomaltase glycoside hydrolases, which are located in the membrane of the microvilli lining the duodenum. These products are also transferred rapidly into the bloodstream. Sucrose is digested by the enzyme invertase in bacteria and some animals.
- Sucrose is an easily assimilated macronutrient that provides a quick source of energy, provoking a rapid rise in blood glucose upon ingestion. Over consumption of sucrose has been linked with adverse health effects.
- The most common is dental caries or tooth decay, in which oral bacteria convert sugars (including sucrose) from food into acids that attack tooth enamel. Sucrose, as a pure carbohydrate, has an energy content of 3.94 kilocalories per gram (or 17 kilojoules per gram).
- When large amounts of food that contain high percentages of sucrose are consumed, beneficial nutrients can be displaced from the diet, which can contribute to an increased risk for chronic disease.
- It has been suggested that sucrose-containing drinks may be linked to the development of obesity and insulin resistance. Most soft drinks in the USA are now made with high fructose corn syrup, not sucrose. HFCS 55 contains 55% fructose and 45% glucose.
- The rapidity with which sucrose raises blood glucose can cause problems for people suffering from defective glucose metabolism, such as persons with hypoglycemia or diabetes mellitus. Sucrose can contribute to the development of metabolic syndrome.
- In an experiment with rats that were fed a diet one-third of which was sucrose, the sucrose first elevated blood levels of triglycerides, which induced visceral fat and ultimately resulted in insulin resistance.
- Another study found that rats fed sucrose-rich diets developed high triglycerides, hyperglycemia, and insulin resistance.
- Consists of 2 molecules of glucose
- Made from hydrolysis of starch: Starch -> (amilo)dextrin -> maltose
- White in color, solid, soluble in water and alcohol.
- Can be fermented.
- Converted to glucose by maltase enzyme inside our body.
- Forms oxazone: Maltoxazone
- Reduction (+) and mutarotation (+)
- Chemically maltose rotates to the right.
- Does not found naturally in any sources.
- Plain maltose has a sweet taste, about half as sweet as glucose and about one-sixth as sweet as fructose.
- In Southern China, Taiwan, Hong Kong and Macau, maltose is a common ingredient in confectionery. The most common way to consume it is to put a layer of maltose between two pieces of biscuit (usually crackers)
3. Lactose (Milk sugar)
- Lactose is a sugar that is found most notably in milk.
- Lactose makes up around 2~8% of milk (by weight), although the amount varies among species and individuals. It is extracted from sweet or sour whey.
- The name comes from lac, the Latin word for milk, plus the -ose ending used to name sugars. It has a formula of C12H22O11.
- Lactose is a disaccharide that consists of galactose and glucose fragments bonded through a β-1→4 glycosidic linkage.
- Its systematic name is β-D-galactopyranosyl-(1→4)-D-glucose. The glucose fragment can be in either the α-pyranose form or the β-pyranose form, whereas the galactose fragment can only have the β-pyranose form: hence α-lactose and β-lactose refer to anomeric form of the glucopyranose ring alone.
- As it gives free radicals by mechanochemistry, it is possible to use lactose to follow by electron spin resonance(ESR) the energy used during a milling process.
- Lactose is hydrolysed to glucose and galactose, isomerised in alkaline solution to lactulose, and catalyticaly hydrogenated to the corresponding polyhydric alcohol, lactitol.
- Reduction (+) and mutarotation (+)
- Infant mammals nurse on their mothers to drink milk, which is rich in the carbohydrate lactose.
- The intestinal villi secrete an enzyme called lactase (β-D-galactosidase) to digest it. This enzyme cleaves the lactose molecule into its two subunits, the simple sugars glucose and galactose, which can be absorbed.
- Since lactose occurs mostly in milk, in most mammals the production of lactase gradually decreases with maturity due to a lack of constant consumption.
- Many people with ancestry in Europe, West Asia, India, and parts of East Africa maintain lactase production into adulthood. In many of these areas, milk from mammals such as cattle, goats, and sheep is used as a large source of food.
- People who are lactose intolerant may suffer uncomfortable or socially unacceptable symptoms of too much lactose consumption. In these people, lactose is not broken down and provides food for gas-producing gut flora. This can lead to bloating, flatulence, and other gastrointestinal symptoms.
- Lactose can also be bought in pure form, as an assist in high calorie diets
- It has been estimated that the annual worldwide availability of lactose as a by-product of the dairy industry is several million tons.
- Whey contains about 4.8% of lactose, which may be purified by crystallisation.
- Food industry applications, both of pure lactose and lactose-containing dairy by-products, have markedly increased since the 1960s. For example, its bland flavour has lent to its use as a carrier and stabiliser of aromas and pharmaceutical products.
- Lactose is little fermented by baker's yeast and during brewing, which may be used to advantage.
- Lactose is sometimes used in stout beers to sweeten the beer and is non-fermentable in beer.
- A disaccharide with the formula [HOCH2CHO(CHOH)3]2O. The molecule is derived from the condensation of two glucose molecules linked in a β(1→4) bond.
- It can be hydrolyzed by bacteria or cationic ion exchange resins to give glucose. Hydrolyzed by emulsinase enzyme.
- Cellobiose has eight free alcohol (COH) groups and three ether linkages, which give rise to strong -inter- and intra-molecular hydrogen bonds.
- It can be obtained by enzymatic or acidic hydrolysis of cellulose and cellulose rich materials such as cotton, jute, or paper.
- Cellulose is a polymer of glucose units linked by β(1→4) bonds.
- Reduction (+) and mutarotation (+)
- Treatment of cellulose with acetic anhydride and sulfuric acid, gives cellobiose octoacetate, which cannot engage in hydrogen bonding and is soluble in nonpolar organic solvents.
- As the name implies, polysaccharides are large high-molecular weight molecules constructed by joining monosaccharide units together by glycosidic bonds. They are sometimes called glycans.
- Amorphous, tasteless.
- 2 groups of polysaccharide:
(1) As a storage carbohydrate, hydrolyzed easily, alpha bonds. Examples: Starch and glycogen.
(2) As a strengthened compound to strenghten cell wall, not easily hydrolyzed, beta bonds. Example: Cellulose
- The most important compounds in this class, cellulose, starch and glycogen are all polymers of glucose.
- This is easily demonstrated by acid-catalyzed hydrolysis to the monosaccharide.
- Since partial hydrolysis of cellulose gives varying amounts of cellobiose, we conclude the glucose units in this macromolecule are joined by beta-glycoside bonds between C-1 and C-4 sites of adjacent sugars.
- Partial hydrolysis of starch and glycogen produces the disaccharide maltose together with low molecular weight dextrans, polysaccharides in which glucose molecules are joined by alpha-glycoside links between C-1 and C-6, as well as the alpha C-1 to C-4 links found in maltose.
- Polysaccharides built from other monosaccharides (e.g. mannose, galactose, xylose and arabinose) are also known.
- Over half of the total organic carbon in the earth's biosphere is in cellulose.
- Cotton fibers are essentially pure cellulose, and the wood of bushes and trees is about 50% cellulose.
- As a polymer of glucose, cellulose has the formula (C6H10O5)n where n ranges from 500 to 5,000, depending on the source of the polymer.
- The glucose units in cellulose are linked in a linear fashion, as shown in the drawing below.
- The beta-glycoside bonds permit these chains to stretch out, and this conformation is stabilized by intramolecular hydrogen bonds.
- A parallel orientation of adjacent chains is also favored by intermolecular hydrogen bonds.
- Although an individual hydrogen bond is relatively weak, many such bonds acting together can impart great stability to certain conformations of large molecules.
- Most animals cannot digest cellulose as a food, and in the diets of humans this part of our vegetable intake functions as roughage and is eliminated largely unchanged.
- Some animals (the cow and termites, for example) harbor intestinal microorganisms that breakdown cellulose into monosaccharide nutrients by the use of beta-glycosidase enzymes.
- Cellulose is commonly accompanied by a lower molecular weight, branched, amorphous polymer called hemicellulose.
- In contrast to cellulose, hemicellulose is structurally weak and is easily hydrolyzed by dilute acid or base.
- Also, many enzymes catalyze its hydrolysis.
- Hemicelluloses are composed of many D-pentose sugars, with xylose being the major component.
- Mannose and mannuronic acid are often present, as well as galactose and galacturonic acid.
Some important polysaccharide
- A polymer of glucose, found in roots, rhizomes, seeds, stems, tubers and corms of plants, as microscopic granules having characteristic shapes and sizes.
- Most animals, including humans, depend on these plant starches for nourishment. The structure of starch is more complex than that of cellulose.
- The intact granules are insoluble in cold water, but grinding or swelling them in warm water causes them to burst.
- The released starch consists of two fractions. About 20% is a water soluble material called amylose.
- Molecules of amylose are linear chains of several thousand glucose units joined by alpha C-1 to C-4 glycoside bonds.
- Amylose solutions are actually dispersions of hydrated helical micelles.
- The majority of the starch is a much higher molecular weight substance, consisting of nearly a million glucose units, and called amylopectin.
- Molecules of amylopectin are branched networks built from C-1 to C-4 and C-1 to C-6 glycoside links, and are essentially water insoluble.
- The branching in this diagram is exaggerated, since on average, branches only occur every twenty five glucose units.
- Hydrolysis of starch, usually by enzymatic reactions, produces a syrupy liquid consisting largely of glucose. When cornstarch is the feedstock, this product is known as corn syrup.
- It is widely used to soften texture, add volume, prohibit crystallization and enhance the flavor of foods.
- Glycogen is the glucose storage polymer used by animals. It has a structure similar to amylopectin, but is even more highly branched (about every tenth glucose unit).
- The degree of branching in these polysaccharides may be measured by enzymatic or chemical analysis
- Optically active, rotation to the right.
- Consists of 2 components: Amylose and amylopectin
(a) Amylose is a linear polymer of α-D-glucose. Soluble in water. Reacts with iodine will form a blue color substance. Linear chain. Molecular weight: 50 000 - 200 000. It contains about 200 glucose units which are linked to one another through α-linkage involving C1 of one glucose unit with C4 of the other as shown below:
(b) Amylopectin, on the other hand, is a highly branched polymer. Insoluble in water, 80-90% of starch, reacts with iodine will forms a reddish violet, branched chain, molecular weight: 70 000 - 1 000 000. It consists of a large number (several branches) of short chains each containing 20-25 glucose units which are joined together through α-linkages involving C1 of one glucose unit with C4 of the other. The C1 of terminal glucose unit in each chain is further linked to C6 of the other glucose unit in the next chain through C1 – C6 α-linkage. This gives amylopectin a highly branched structure as shown below.
Hydrolysis: Hydrolysis of starch with hot dilute acids or by enzymes gives dextrins of varying complexity, maltose and finally D-glucose. Starch does not reduce Tollen’s reagent and Fehling’s solution.
Uses: It is used as a food. It is encountered daily in the form of potatoes, bread, cakes, rice etc. It is used in coating and sizing paper to improve the writing qualities. Starch is used to treat textile fibres before they are woven into cloth so that they can be woven without breaking. It is used in manufacture of dextrins, glucose and ethyl alcohol. Starch is also used in manufacture of starch nitrate, which is used as an explosive.
- The storage form of glucose in animals and humans which is analogous to the starch in plants. Glycogen is synthesized and stored mainly in the liver and the muscles. Structurally, glycogen is very similar to amylopectin with alpha acetal linkages, however, it has even more branching and more glucose units are present than in amylopectin. Various samples of glycogen have been measured at 1,700-600,000 units of glucose.
- The structure of glycogen consists of long polymer chains of glucose units connected by an alpha acetal linkage. The graphic on the left shows a very small portion of a glycogen chain. All of the monomer units are alpha-D-glucose, and all the alpha acetal links connect C1 of one glucose to C4 of the next glucose.
- The branches are formed by linking C1 to a C6 through an acetal linkages. In glycogen, the branches occur at intervals of 8-10 glucose units, while in amylopectin the branches are separated by 12-20 glucose units.
- Reduction (-)
- Optically active, rotation to the right, consists of alpha-glucose bonds
- Hydrolyzed with diluted acid. Hydrolyzed also with amylase enzyme to form maltose.
- Molecular weight is greater than starch: Around 5 000 000
- Glycogen in muscles: While muscles working, glycogen converted to form pyruvate acid and lactate acid.
- Glycogen in liver: Control blood sugar level (normal: 120 mg%). If blood sugar level decreases glycogen from the liver will be used, if blood sugar level increased glucose will be converted to glycogen and stored inside the liver.
- Inulins are a group of naturally occurring polysaccharides produced by many types of plants.
- They belong to a class of fibers known as fructans.
- Inulin is used by some plants as a means of storing energy and is typically found in roots or rhizomes.
- Most plants that synthesize and store inulin do not store other materials such as starch.
- Inulins are polymers composed mainly of fructose units, and typically have a terminal glucose.
- The fructose units in inulins are joined by a β(2→1) glycosidic bond. In general, plant inulins contain between 20 and several thousand fructose units. Smaller compounds are called fructooligosaccharides, the simplest being 1- kestose, which has 2 fructose units and 1 glucose unit.
- Inulins are named in the following manner, where n is the number of fructose residues and py is the abbreviation for pyranosyl:
(a) Inulins with a terminal glucose are known as alpha-D-glucopyranosyl-[beta-D-fructofuranosyl](n-1)-D-fructofuranosides, abbreviated as GpyFn.
(b) Inulins without glucose are beta-D-fructopyranosyl-[D-fructofuranosyl](n-1)-D-fructofuranosides, abbreviated as FpyFn.
- Hydrolysis of inulins may yield fructooligosaccharides, which are oligomers with a degree of polymerization (DP) of <= 10.
- Hydrolized by acid and inulase enzyme.
- Rotating the polarization field to the left.
- Reacts with iodine to form a yellow substance.
- Inulin is increasingly used in processed foods because it has unusually adaptable characteristics.
- Its flavour ranges from bland to subtly sweet (approx. 10% sweetness of sugar/sucrose).
- It can be used to replace sugar, fat, and flour.
- This is particularly advantageous because inulin contains a quarter to a third of the food energy of sugar or other carbohydrates and a ninth to a sixth of the food energy of fat.
- While inulin is a versatile ingredient, it also has health benefits.
- Inulin increases calcium absorption and possibly magnesium absorption, while promoting the growth of intestinal bacteria.
- In terms of nutrition, it is considered a form of soluble fiber and is sometimes categorized as a prebiotic.
- Due to the body's limited ability to process polysaccharides, inulin has minimal increasing impact on blood sugar, and—unlike fructose—is not insulemic and does not raise triglycerides, making it considered suitable for diabetics and potentially helpful in managing blood sugar-related illnesses.
- The consumption of large quantities (in particular, by sensitive or unaccustomed individuals) can lead to gas and bloating, and products that contain inulin will sometimes include a warning to add it gradually to one's diet.
- Nonhydrolyzed inulin can also be directly converted to ethanol in a simultaneous saccharification and fermentation process, which may have great potential for converting crops high in inulin into ethanol for fuel.
- Inulin is used to help measure kidney function by determining the glomerular filtration rate (GFR). GFR is the volume of fluid filtered from the renal (kidney) glomerular capillaries into the Bowman's capsule per unit time.
- Inulin is of particular use as it is not secreted or reabsorbed in any appreciable amount at the nephron allowing GFR to be calculated, rather than total renal filtration.
- However, due to clinical limtations, inulin is rarely used for this purpose and creatinine values are the standard for determining an approximate GFR.
- Most important in economic and industrial sectors.
- Hydrolyzed by sellulase enzyme.
- Insoluble in water and ordinary solvents Soluble in Scheitzer reactor.
- Molecular weight: 400 000, consists of around 250 monomers (glucose).
- Cellulose is an organic compound with the formula (C6H10O5)n, a polysaccharide consisting of a linear chain of several hundred to over ten thousand β(1→4) linked D-glucose units.
- Cellulose is the structural component of the primary cell wall of green plants, many forms of algae and the oomycetes.
- Some species of bacteria secrete it to form biofilms.
- Cellulose is the most common organic compound on Earth. About 33 percent of all plant matter is cellulose (the cellulose content of cotton is 90 percent and that of wood is 40-50 percent).
- For industrial use, cellulose is mainly obtained from wood pulp and cotton. It is mainly used to produce paperboard and paper; to a smaller extent it is converted into a wide variety of derivative products such as cellophane and rayon.
- Converting cellulose from energy crops into biofuels such as cellulosic ethanol is under investigation as an alternative fuel source.
- Some animals, particularly ruminants and termites, can digest cellulose with the help of symbiotic micro-organisms that live in their guts.
- Humans can digest cellulose to some extent, however it is often referred to as 'dietary fiber' or 'roughage' (e.g. outer shell of Maize) and acts as a hydrophilic bulking agent for feces.
*Synthetic Modification of Cellulose
- Cotton, probably the most useful natural fiber, is nearly pure cellulose. The manufacture of textiles from cotton involves physical manipulation of the raw material by carding, combing and spinning selected fibers. For fabrics the best cotton has long fibers, and short fibers or cotton dust are removed. Crude cellulose is also available from wood pulp by dissolving the lignan matrix surrounding it. These less desirable cellulose sources are widely used for making paper.
- In order to expand the ways in which cellulose can be put to practical use, chemists have devised techniques for preparing solutions of cellulose derivatives that can be spun into fibers, spread into a film or cast in various solid forms. A key factor in these transformations are the three free hydroxyl groups on each glucose unit in the cellulose chain, --[C6H7O(OH)3]n--. Esterification of these functions leads to polymeric products having very different properties compared with cellulose itself.
(a) Cellulose Nitrate, first prepared over 150 years ago by treating cellulose with nitric acid, is the earliest synthetic polymer to see general use. The fully nitrated compound, --[C6H7O(ONO2)3]n--, called guncotton, is explosively flammable and is a component of smokeless powder. Partially nitrated cellulose is called pyroxylin. Pyroxylin is soluble in ether and at one time was used for photographic film and lacquers. The high flammability of pyroxylin caused many tragic cinema fires during its period of use. Furthermore, slow hydrolysis of pyroxylin yields nitric acid, a process that contributes to the deterioration of early motion picture films in storage.
(b) Cellulose Acetate, --[C6H7O(OAc)3]n--, is less flammable than pyroxylin, and has replaced it in most applications. It is prepared by reaction of cellulose with acetic anhydride and an acid catalyst. The properties of the product vary with the degree of acetylation. Some chain shortening occurs unavoidably in the preparations. An acetone solution of cellulose acetate may be forced through a spinneret to generate filaments, called acetate rayon, that can be woven into fabrics.
(c) Viscose Rayon, is prepared by formation of an alkali soluble xanthate derivative that can be spun into a fiber that reforms the cellulose polymer by acid quenching.
- Hemicelluloses are polysaccharides — long chains of sugars — that are part of the plant cell wall. They can be very complex molecules and be comprised of a number of different types of sugars. They differ from the plant cell wall structural compound cellulose, which forms a much longer chain and is comprised entirely of glucose molecules. Hemicellulose is a cross-linking agent and interconnects cellulose microfibrils. It can also link the cellulose to other cell wall components.
- The complexity of hemicellulose means that there are many different types of hemicellulase. Various types of this enzyme have different applications in biotechnology, particularly in the food sciences. The enzyme is often used in combination with other enzymes that degrade plant cell walls or starches.
- A primary use of hemicellulase is in the baking industry. It is used in cake mixes, baked goods, and frozen dough. The enzyme enhances the quality of the dough and helps with storage life.
- Fruit juices and alcoholic beverages can be produced using this type of enzyme. In particular, hemicellulases are used in the production of wines. They help remove unwanted compounds from the skins of the grapes that might contaminate the taste of the wine.
- Although plants make hemicellulases for growth and development, most of the commercial interest is in the enzymes produced by microorganisms. The enzymes are produced industrially from bacteria and fungi. In some cases, these organisms have been genetically engineered to produce optimal amounts of hemicellulase.
- The important role of hemicellulases in many biotechnological applications has led to a great deal of interest in determining their structure and function. The enzymes are often in a modular form; different parts of their structures have varying functions. For this type of enzyme, part is specialized to bind to the hemicellulose, while another part breaks down the bond connecting the sugars by a process known as hydrolysis. This involves adding a molecule of water to the bond, which causes it to break open.
- Microorganisms that produce hemicellulases are also found in the digestive tract of humans and higher animals. Since humans cannot degrade hemicellulose, it is considered fiber. The microorganisms in the human body break down some of the hemicellulose, helping it to be digested. Supplements of hemicellulase are available, but as with any supplement, one should consult with a doctor before consuming them.
- Partially oxidized polysaccharide
- Amorphous, soluble in water: Gel
(a) Gum Arabic
- Also known as gum acacia, chaar gund, char goond or meska, is a natural gum made of hardened sap taken from two species of the acacia tree; Acacia senegal and Acacia seyal.
- The gum is harvested commercially from wild trees throughout the Sahel from Senegal and Sudan to Somalia, although it has been historically cultivated in Arabia and West Asia.
- Gum arabic is a complex mixture of polysaccharides and glycoproteins that is used primarily in the food industry as a stabilizer.
- It is edible and has E number E414. Gum arabic is a key ingredient in traditional lithography and is used in printing, paint production, glue, cosmetics and various industrial applications, including viscosity control in inks and in textile industries, although less expensive materials compete with it for many of these roles.
- While gum arabic is now produced throughout the African Sahel, it is also still harvested and used in the Middle East. For example, Palestinian Arabs use the natural gum to make a chilled, sweetened, and flavored gelato-like dessert.
- Gum arabic's mixture of saccharides and glycoproteins gives it the properties of a glue, and binder which is edible by humans.
- Other substances have replaced it in situations where toxicity is not an issue, as the proportions of the various chemicals in gum arabic vary widely and make it unpredictable.
- Still, it remains an important ingredient in soft drink syrups, "hard" gummy candies such as gumdrops, marshmallows, M&M's chocolate candies and edible glitter, a very popular, modern cake-decorating staple.
- For artists it is the traditional binder used in watercolor paint, in photography for gum printing, and it is used as a binder in pyrotechnic compositions.
- It has been investigated for use in intestinal dialysis.
- Pharmaceuticals and cosmetics also use the gum as a binder, emulsifying agent and a suspending or viscosity increasing agent.
- It is an important ingredient in shoe polish, and can be used in making homemade incense cones.
- It is also used as a lickable adhesive, for example on postage stamps and cigarette papers.
- Printers employ it to stop oxidation of aluminium printing plates in the interval between processing of the plate and its use on a printing press.
- Powdered gum arabic for artists. One part gum arabic is dissolved in four parts distilled water to make a liquid suitable for adding to pigments.
- A selection of Gouaches containing gum arabic.
Gum arabic is used as a binder for watercolor painting because it dissolves easily in water. Pigment of any color is suspended within the gum arabic in varying amounts, resulting in watercolor paint. Water acts as a vehicle or a diluent to thin the watercolor paint and helps to transfer the paint to a surface such as paper. When all moisture evaporates, the gum arabic binds the pigment to the paper surface.
- The historical photography process of gum bichromate photography uses gum arabic mixed with ammonium or potassium dichromate and pigment to create a coloured photographic emulsion that is sensitive to ultraviolet light. In the final print, the gum arabic permanently binds the pigments onto the paper.
- Gum arabic is also used to protect and etch an image in lithographic processes. Ink tends to fill into whitespace on photosensitive aluminum plates if they do not receive a layer of gum. In lithography the gum etch is used to etch the most subtle gray tones. Phosphoric acid is added in varying concentrations to the gum arabic to etch the darker tones up to dark blacks. Multiple layers of gum are used after the etching process to build up a protective barrier that ensures the ink does not fill into the whitespace of the image being printed.
- Gum arabic is also used as a water soluble binder in firework composition.
- Gum arabic reduces the surface tension of liquids, which leads to increased fizzing in carbonated beverages. This can be exploited in what is known as a Diet Coke and Mentos eruption.
- A natural gum obtained from the dried sap of several species of Middle Eastern legumes of the genus Astragalus, including A. adscendens, A. gummifer, and A. tragacanthus.
- Some of these species are known collectively under the common names "goat's thorn" and "locoweed".
- The gum is sometimes called "shiraz gum" , "gum elect" or gum dragon. The name derives from tragos and akantha, which means in Greek "goat" and "thorn", respectively.
- Iran is the biggest producer of the best quality of this gum.
- Gum tragacanth is a viscous, odorless, tasteless, water-soluble mixture of polysaccharides obtained from sap which is drained from the root of the plant and dried.
- The gum seeps from the plant in twisted ribbons or flakes which can be powdered. It absorbs water to become a gel, which can be stirred into a paste.
- The gum is used in veg-tanned leatherworking as an edge slicking and burnishing compound and is occasionally used as a stiffener in textiles.
- It contains an alkaloid that has historically been used as an herbal remedy for such conditions as cough and diarrhea. As a mucilage or paste it has been used as a topical treatment for burns. It is used in pharmaceuticals and foods as an emulsifier, thickener, stabilizer, and texturant additive (code E413).
- Also, it is the traditional binder used in the making of artist's pastels, as it does not adhere to itself the same way other gums (such as gum arabic) do when dry.
- Gum tragacanth is also used to make a paste used in floral sugarcraft to create life-like flowers on wires used as decorations for cakes. It makes a paste which dries brittle in the air and can take colorings. It enables users to get a very fine, delicate finish to their work.
- Gum tragacanth is less common in products than other gums, such as gum arabic or guar gum, largely because most tragacanth is grown in Middle Eastern countries which have shaky trade relations with countries where the gum is to be used. Commercial cultivation of tragacanth plants has generally not proved economically worthwhile in the west, since other gums can be used for similar purposes.
- Gum tragacanth is also used in incense making as a binder to hold all the powdered herbs together. Its water-solubility is ideal for ease of working and an even spread. Only half as much is needed, compared to gum arabic or something similar.
(c) Agar or agar-agar
- A gelatinous substance derived from red algae.
- Historically and in a modern context, it is chiefly used as an ingredient in desserts throughout Asia and also as a solid substrate to contain culture medium for microbiological work.
- The gelling agent is an unbranched polysaccharide obtained from the cell walls of some species of red algae, primarily from the genera Gelidium and Gracilaria, or seaweed (Sphaerococcus euchema).
- Commercially it is derived primarily from Gelidium amansii.
- Agar (agar-agar) can be used as a laxative, a vegetarian gelatin substitute, a thickener for soups, in jellies, ice cream and other desserts, as a clarifying agent in brewing, and for paper sizing fabrics.
- Chemically, agar is a polymer made up of subunits of the sugar galactose. Agar polysaccharides serve as the primary structural support for the algae's cell walls.
- Nutrient agar is used throughout the world to provide a solid surface containing medium for the growth of bacteria and fungi. Agar is typically sold commercially as a powder that can be mixed with water and prepared similarly to gelatin before use as a growth medium. The basic agar formula can be used to grow most of the microbes whose needs are known. More specific nutrient agars are available, because some microbes prefer certain environmental conditions over others.
- As a gel, an agarose medium is porous and therefore can be used to measure microorganism motility and mobility. The gel's porosity is directly related to the concentration of agarose in the medium, so various levels of effective viscosity (from the cell's "point of view") can be selected, depending on the experimental objectives.
- A common identification assay involves culturing a sample of the organism deep within a block of nutrient agar. Cells will attempt to grow within the gel structure. Motile species will be able to migrate, albeit slowly, throughout the gel and infiltration rates can then be visualized; whereas non-motile species will only show growth along the now-empty path introduced by the invasive initial sample deposition.
- Another setup commonly used for measuring chemotaxis and chemokinesis utilizes the under-agarose cell migration assay whereby a layer of agarose gel is placed between a cell population and a chemoattractant. As a concentration gradient develops from the diffusion of the chemoattractant into the gel, various cell populations requiring different stimulation levels to migrate can then be visualized over time using microphotography as they tunnel upward through the gel against gravity along the gradient.
- Agar is a heterogeneous mixture of two classes of polysaccharide: agaropectin and agarose. Although both polysaccharide classes share the same galactose-based backbone, agaropectin is heavily modified with acidic side-groups, such as sulfate and pyruvate.
- The neutral charge and lower degree of chemical complexity of agarose make it less likely to interact with biomolecules and therefore agarose has become the preferred matrix for work with proteins and nucleic acids. Gels made from purified agarose have a relatively large pore size, making them useful for separation of large molecules, such as proteins and protein complexes >200 kilodaltons, as well as DNA fragments >100 basepairs. Agarose has been used widely for immunodiffusion and immunoelectrophoresis as the agarose fibers functions as an anchor for immunocomplexes. Agarose is used generally as the medium for analytical scaleelectrophoretic separation in agarose gel electrophoresis and for column-based preparative scale separation as in gel filtration chromatography and affinity chromatography.
- Research grade agar is used extensively in plant biology as it is supplemented with a nutrient and vitamin mixture that allows for seedling germination in petri dishes under sterile conditions (given that the seeds are sterilized as well). Nutrient and vitamin supplementation for Arabidopsis thaliana is standard across most experimental conditions. Murashige & Skoog (MS) nutrient mix and Gamborg's B5 vitamin mix are generally used. A 1.0% agar/0.44% MS+vitamin dH20 solution is suitable for growth media between normal growth temps.
- The solidification of the agar within any growth media (GM) is pH-dependent, with an optimal range between 5.4-5.7. Usually, the application of KOH is needed to increase the pH to this range. A general guideline is about 600 µl 0.1M KOH per 250 ml GM. This entire mixture can be sterilized using the liquid cycle of an autoclave.
- This medium nicely lends itself to the application of specific concentrations of phytohormones etc. to induce specific growth patterns in that one can easily prepare a solution containing the desired amount of hormone, add it to the known volume of GM, and autoclave to both sterilize and evaporate off any solvent that may have been used to dissolve the often polar hormones. This hormone/GM solution can be spread across the surface of petri dishes sown with germinated and/or etiolated seedlings.
- Experiments with the moss Physcomitrella patens, however, have shown that choice of the gelling agent — agar or Gelrite - does influence phytohormone sensitivity of the plant cell culture.
- Agar-agar is a natural vegetable gelatin counterpart. White and semi-translucent, it is sold in packages as washed and dried strips or in powdered form. It can be used to make jellies, puddings, and custards. For making jelly, it is boiled in water until the solids dissolve. Sweetener, flavouring, colouring, fruit or vegetables are then added and the liquid is poured into molds to be served as desserts and vegetable aspics, or incorporated with other desserts, such as a jelly layer in a cake.
- Agar-agar is approximately 80% fiber, so it can serve as an intestinal regulator. Its bulk quality is behind one of the latest fad diets in Asia, the kanten (the Japanese word for agar-agar) diet. Once ingested, kanten triples in size and absorbs water. This results in the consumer feeling more full. Recently this diet has received some press coverage in the United States as well. The diet has shown promise in obesity studies.
- One use of agar in Japanese cuisine is anmitsu, a dessert made of small cubes of agar jelly and served in a bowl with various fruits or other ingredients. It is also the main ingredient in Mizuyōkan, another popular Japanese food. (See very top image.) In Indian cuisine, agar agar is known as "China grass" and is used for making desserts. In Burmese cuisine, a sweet jelly known as kyauk kyaw is made from agar.
- Agar is also used: As an impression material in dentistry; To make salt bridges for use in electrochemistry; In formicariums as a transparent substitute for sand and a source of nutrition.
- Polymer of galacturonic acid
- Pectin (from Greek πηκτικός - pektikos, "congealed, curdled") is a structural heteropolysaccharide contained in the primary cell walls of terrestrial plants.
- It is produced commercially as a white to light brown powder, mainly extracted from citrus fruits, and is used in food as a gelling agent particularly in jams and jellies.
- It is also used in fillings, sweets, as a stabilizer in fruit juices and milk drinks and as a source of dietary fiber.
- Sources: Apples, guavas, quince, plums, gooseberries, oranges and other citrus fruits, contain large amounts of pectin, while soft fruits like cherries, grapes and strawberries contain small amounts pectin.
- Typical levels of pectin in plants are (fresh weight):
*carrots approx. 1.4%
*citrus peels, 30%
- The main raw-materials for pectin production are dried citrus peel or apple pomace, both by-products of juice production. Pomace from sugar-beet is also used to a small extent.
- From these materials, pectin is extracted by adding hot dilute acid at pH-values from 1.5 – 3.5. During several hours of extraction, the protopectin loses some of its branching and chain-length and goes into solution. After filtering, the extract is concentrated in vacuum and the pectin then precipitated by adding ethanol or isopropanol. An old technique of precipitating pectin with aluminium salts is no longer used (apart from alcohols and polyvalent cations; pectin also precipitates with proteins and detergents).
- Alcohol-precipitated pectin is then separated, washed and dried. Treating the initial pectin with dilute acid leads to low-esterified pectins. When this process includes ammonium hydroxide, amidated pectins are obtained. After drying and milling pectin is usually standardised with sugar and sometimes calcium-salts or organic acids to have optimum performance in a particular application.
- Worldwide, approximately 40,000 metric tons of pectin are produced every year.
- The main use for pectin is as a gelling agent, thickening agent and stabilizer in food.
- The classical application is giving the jelly-like consistency to jams or marmalades, which would otherwise be sweet juices. For household use, pectin is an ingredient in gelling sugar (also known as "Jam Sugar") where it is diluted to the right concentration with sugar and some citric acid to adjust pH. In some countries, pectin is also available as a solution or an extract, or as a blended powder, for home jam making.
- For conventional jams and marmalades that contain above 60% sugar and soluble fruit solids, high-ester pectins are used.
- With low-ester pectins and amidated pectins less sugar is needed, so that diet products can be made.
- Pectin can also be used to stabilize acidic protein drinks, such as drinking yogurt, and as a fat substitute in baked goods. Typical levels of pectin used as a food additive are between 0.5 – 1.0% - this is about the same amount of pectin as in fresh fruit.
- In medicine, pectin increases viscosity and volume of stool so that it is used against constipation and diarrhea. Until 2002, it was one of the main ingredients used in Kaopectate, along with kaolinite. Pectin is also used in throat lozenges as a demulcent.
- In cosmetic products, pectin acts as stabilizer. Pectin is also used in wound healing preparations and specialty medical adhesives, such as colostomy devices.
- Also, it is considered a natural remedy for nausea. Pectin rich foods are proven to help nausea.
- In ruminant nutrition, depending on the extent of lignification of the cell wall, pectin is up to 90% digestible by bacterial enzymes. Ruminant nutritionists recommend that the digestibility and energy concentration in forages can be improved by increasing pectin concentration in the forage.
- In the cigar industry, pectin is considered an excellent substitute for vegetable glue and many cigar smokers and collectors will use pectin for repairing damaged tobacco wrapper leaves on their cigars.
- Pectin is also used in jellybeans.
Carbohydrate qualitative tests
- To detect a substance is a carbohydrate or not, we use Molisch test. When the substance tested positive with molisch test (substance turns to reddish violet), the substance is a carbohydrate, while non-carbohidrate will shows a negative result.
- To determine type of a carbohydrate, we use Iodine test. A starch turns to blue, glycogen erythrodextrin turns to red, while mono or disaccharide turns to colourless.
- To classify mono or disaccharide type, we use barfoed test. A sucrose does not produces sediment, disaccharide produces reddish orange sediment in 7-12 minutes, while monosaccharide produces reddish orange sediment in 5-7 minutes.
- If we have a monosaccharide substance and we want to know if it is a pentose or a hexose, we will use orcinol test (HCl-Bial). Pentose forms a mixture of green solution and sediment, while hexose is not.
- To classify a hexose substance, we use seliwanoff test. Fructose turns to red-reddish orange, while glucose and galactose turn to colorless.
- To differentiate glucose and galactose substance, we need fermentation test. Galactose produces CO2, while Glucose is not.
- Back to disaccharide, to differentiate maltose and lactose, we need a fermentation test. Maltose produces CO2, while Lactose is not.