Monosaccharides, Chemistry tutorial


The term monosaccharide is derived from mono; stand for 'one', and saccharide, stands for 'sugar'. Monosaccharides are the carbohydrates made up from a single sugar molecule. Such simple sugars impart a sweet taste to foods. Monosaccharides don't require to be broken down throughout digestion, as they are already small adequate to be absorbed. Such sugars are absorbed in the small intestine, where they pass via the mucosal lining and to the bloodstream. The level of sweetness and capability to influence blood sugar levels will fluctuate based on the kind of monosaccharide.

Monosaccharides all encompass the similar basic structure, represented by the chemical formula (CH2O)n, in which 'n' signifies for the number of carbon atoms. Monosaccharide (that is, simple sugar) - a carbohydrate which can't be split to smaller units via the action of dilute acids. Monosaccharides are categorized according to the number of carbon atoms they have: Trioses encompass three carbon atoms; Tetroses, four; pentoses, five; hexoses, six; and so on. Each of these is further categorized into aldoses and ketoses, based on whether the molecule includes an aldehyde group (-CHO) or a ketone group (-CO-). For illustration glucose, containing six carbon atoms and an aldehyde group, is an aldohexose while fructose is a ketohexose. Such aldehyde and ketone groups confer reducing properties on the monosaccharides: they can be oxidized to result sugar acids. They as well react with phosphoric acid to form phosphate esters (example in ATP), which are significant in the cell metabolism. Monosaccharides can exist as either straight-chain or ring-shaped molecules. They as well show optical activity, giving mount to both dextrorotatory and laevorotatory forms.

Function of Monosaccharide in Biology:

Biological forms of Monosaccharides:

Monosaccharides are characterized via the number of carbon atoms their molecules have. Monosaccharides having the formula C6H12O6 comprise glucose, galactose and fructose that are sugars utilized by organisms for energy. Xylose is a five-carbon monosaccharide, known as pentose, found in the plant cells; this monosaccharide joins with xylan to prepare woody materials, like those that make up trees. Likewise, arabinose is found in the coniferous trees. Ribose and deoxyribose are the components of ribonucleic and deoxyribonucleic acids, generally termed as RNA and DNA.

Fuel for Metabolism:

One major function of a monosaccharide is its use for energy in a living organism. Glucose is a generally known carbohydrate that is metabolized within cells to create fuel. In the presence of oxygen, glucose breaks down to carbon-dioxide and water, and energy is discharged as a byproduct. Glucose is the product of photosynthesis, and plants get energy from glucose via respiration. Humans obtain glucose from food, and the body converts this monosaccharide to energy.

Building Blocks:

Monosaccharides are as well the base for more complex carbohydrates, or they serve up as components to amino acids. The ribose and deoxyribose monosaccharides are very important elements of RNA and DNA that are the building blocks of life. As monosaccharides can't be broken down to smaller sugars, disaccharides and polysaccharides are broken down to monosaccharides in procedures such as digestion. For illustration, the disaccharide lactose is degraded to monosaccharides that can be absorbed to the human body.

Derivatives of Monosaccharides:

The derivatives of monosaccharides are found in the nature. One illustration of a monosaccharide derivative is vitamin C, as well termed as ascorbic acid that is derived from glucose. Sugar replaces, like sorbitol and mannitol, are employed as sweeteners, and they form naturally in plants and berries from monosaccharides such as glucose and mannose. Amino sugars, like glucosamine, a derivative of glucose, generate cartilage, connective tissue and chitin, a component of the insect's exoskeleton.

Classification of monosaccharides:

Monosaccharides are the commonest form of carbohydrates and might be sub classified as aldoses or ketoses. The sugar is an aldose if it consists of an aldehyde functional group. A ketose shows that the sugar consists of a ketone functional group. Monosaccharides might be further categorized based on the number of carbon atoms in the backbone, that can be designated by the prefixes tri-(3), tetr-(4), pent-(5), hex-(6), hept-(7), and so on in the name of sugar.

Monosaccharides are often symbolized by a Fischer Projection, a shorthand notation specifically helpful for illustrating stereochemistry in straight chained organic compounds. The L and D confirmations stand for the absolute configuration of the asymmetric carbon farthest away from the ketone or aldehyde group on the monosaccharide. On Fischer projection, if the farthest hydroxyl (-OH) group is on the right, then it is categorized as D sugar, if the hydroxyl group is on the left, then it is L sugar.

Types of Monosaccharide Sugars:

1) Galactose:

The carbon and oxygen atoms in sugar galactose prepare a hexagon - a six-sided ring. Galactose hardly ever occurs by itself in foods; however it occurs in the peas. Whenever galactose combines to glucose, it makes the milk sugar lactose. An unusual genetic condition, galactosemia, arises in 1 out of every 60,000 births. In this situation, the baby is not able to digest galactose and it builds up in the body, causing a series of health problems. Avoidance of dairy products or infant formula with lactose prevents such health problems.

2) Glucose:

Glucose, as well termed as dextrose, is the most general carbohydrate in nature. Glucose is the fundamental unit of more complex sugars, or polysaccharides, and is gently sweet sugar. In nature, glucose hardly ever exists as a single molecule. However, each and every disaccharide consists of glucose as one of its two sugar molecules. Glucose is as well part of starch and fiber molecules. The Glucose supplies energy to your cells and is the favored fuel for your brain. Your liver stores and discharges glucose whenever required to regulate blood sugar levels. Glucose is the sugar tested for by diabetics whenever they measure blood sugar levels. Glucose consists of six carbon atoms, and consists of the chemical formula C6H12O6.

3) Fructose:

Similar to galactose and glucose, fructose as well consists of six carbon atoms. Dissimilar to galactose and glucose, the carbon and oxygen atoms of fructose make a pentagon - a five-sided structure. The other names for fructose comprise levulose and fruit sugar. This monosaccharide is the sweetest tasting sugar. Both vegetables and fruits encompass fructose. Honey consists of both glucose and fructose, though the sweet taste of honey comes mostly from the fructose. Corn syrup that comprises of 50 % fructose, sweetens foods such as desserts, soft drinks and jams.

4) Pentoses:

The sugar molecules which have five carbon atoms are known as pentoses. Pentoses occur in food in extremely small amounts; however they are an important part of the genetic material of each and every cell. The monosaccharide ribose is a part of RNA, or ribonucleic acid. Deoxyribose, the other five-carbon monosaccharide, is a part of DNA, or deoxyribonucleic acid.

5) Sugar Alcohols:

Whenever a hydroxyl group, or -OH, substitutes a hydrogen atom in the sugar molecule, it becomes a sugar alcohol or polyol. Our body doesn't fully absorb sugar alcohols, and as such, they don't produce much of a rise in the blood sugar levels. Such monosaccharides as well encompass fewer calories per gram than other sugars. The sugar alcohols comprise erythritol, lactitol, isomalt, mannitol, sorbitol and xylitol. Xylitol consists of a sweetness level equivalent to table sugar. Sugar alcohols do take place in extremely small quantities in several fruits. Though, these monosaccharides are prepared in large quantities synthetically. Sugar alcohols are helpful as low-calorie sweeteners in foods like chewing gum, hard candies, sugar-free candies, chocolate and tooth-paste.

Properties of Monosaccharides:

Monosaccharides encompass lots of physical and chemical properties; some of them are as follows:

1) Asymmetric carbon atom:

It is that carbon that is joined to four different groups or atoms.

1071_Asymetric carbon atom.jpg

Fig: Asymmetric carbon atom

Each and every monosaccharide includes one or more asymmetric carbon atom apart from the ketotriose dihydroxyacetone. Any compound containing one or more asymmetric carbon atom exhibits optical activity and stereoisomerism.

2) Stereoisomerism:

The compounds which encompass the similar structural formula however differ in spatial configuration are termed as stereoisomers and the phenomenon is known as stereoisomerism. The number of possible isomers of a compound based on the number of asymmetric carbon atoms (n) and is equivalent to 2n. Glucose includes four asymmetric carbon atoms so it consists of 24 =16 isomers. The more significant kinds of isomers are:

a) Aldose ketose isomers:

In this kind of isomerism the two isomers have similar structural formula however differ in the carbonyl carbon; one includes aldehyde group and the other includes ketone group example: glucose and fructose each of them consists of the similar structural formula C6(H2O)6 however glucose consists of aldehyde group and fructose consists ketone group.

1198_Aldose ketose isomers.jpg

Fig: Aldose ketose isomers

b) D and L isomers:

The monosaccharide can be found either in D form or L form; in D form the hydroxyl group at the penultimate carbon (that is, the carbon prior to the last carbon) is on the right side whereas in L form the hydroxyl group at penultimate carbon is on the left side example: D-glyceraldehyde and L-glyceraldehyde, as well 1D-glucose and L-glucose. D-sugars are the richest forms in humans.

c) Epimers:

Epimers vary in configuration around one carbon atom other than the carbonyl carbon and the penultimate carbon. Glucose and galactose encompass the similar structural formula C6(H2O)6 however differ only in the position of the hydroxyl group at C4; glucose consists of hydroxyl group at C4 on the right side whereas galactose consists of hydroxyl group at C4 on the left side. Therefore glucose and galactose are C4 epimers. As well, glucose and mannose encompass the similar structural formula C6(H2O)6 however vary merely in the position of the hydroxyl group at C2; glucose consists of hydroxyl group at C2 on the right side whereas mannose consists of hydroxyl group at C2 on the left side. Therefore glucose and mannose are C2 epimers.

d) α and β isomers:

These are the monosaccharides which vary in configuration only around the carbonyl carbon in cyclic structure. A monosaccharides in cyclic structure can exist either in a or p configuration. In a form the hydroxyl group linked to the carbonyl carbon is on the right side. In 13 forms, the hydroxyl group linked to the carbonyl carbon is on the left side. In monosaccharide solutions, the 13 forms are in equilibrium and can be readily convened to one other.

3) Optical activity:

It is the capability of the compound to rotate plane polarized light to the right or to the left Plane polarized light is the light which its waves pass in the similar plane via the action of special prisms. Whenever the compound rotates plane polarized light to the right, it is dextrorotatory (d or +). When the compound rotates plane polarized light to the left, it is levorotatory (I or -). The angle of rotation is assessed by polarimeter and is influenced by numerous factors like the source of light, nature and concentration of substance and temperature.

Each and every optically active substance consists of a specific angle of rotation. For illustration, the specific rotation for glucose is + 52.5o and for fructose is -91o

4) Reducing properties:

Each and every monosaccharide has reducing property because of the presence of free aldehyde or ketone group. They decrease Fehling solution providing red precipitate of cuprous oxide.

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