Monosaccharides are the easiest of every sugars, save at the very least three carbon atoms, and also are the building blocks of every carbohydrates.

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From: reference Module in Food Science, 2016

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James N. BeMiller, in Carbohydrate Chemistry because that Food researchers (Third Edition), 2019

Structures and nomenclature


Carbohydrate (a term derived from the German kohlenhydrate and also the comparable French hydrate de carbone) expresses the truth that most simple carbohydrates have the general elemental ingredient Cx(H2O)y (that is, they space molecules the contain carbon atom plus hydrogen and oxygen atom in the same proportion as they occur in water). Your composition is related to the truth that they are produced by photosynthesis indigenous carbon dioxide and water as shown by the adhering to unbalanced equation:


However, the great majority of natural carbohydrate compounds uncovered in living organisms carry out not have actually the an easy empirical formula Cx(H2O)y. Rather many naturally emerging carbohydrates room oligomers (oligosaccharides ) or polymers (polysaccharides ) make by involvement sugars with the basic empirical formula or sugars through modified structures pertained to the simple empirical formula. When lower–molecular load carbohydrates because that food use room often derived by depolymerization of herbal polymers, this book starts with a presentation the the basic sugars and builds from over there to larger and more complicated structures.


A characteristic of carbohydrates (which are additionally called saccharides) is that they save on computer chiral carbon atoms. A chiral carbon atom is a carbon atom that have the right to exist in two different spatial kinds (configurations). Chiral carbon atoms can be recognized easily because they room carbon atom that have actually each of their four tetrahedral bonds linked to different atoms or groups of atoms. The two different arrangements the the 4 groups in an are (configurations) space what are called nonsuperimposable mirror images (Fig. 1.1). In various other words, one is the reflection of the various other that one would check out in a mirror, with everything on the best in one construction on the left in the other and vice versa (Box 1.2).



Figure 1.1. Chiral carbon atom. (A), (B), (D), and also (E) represent various atoms or sensible groups2 attached to carbon atom (C). Wedges suggest chemical binding projecting exterior from the airplane of the page; dashes suggest chemical bonds projecting right into or listed below the plane of the page.


Box 1.2

Mirror Images

Nonsuperimposable winter images deserve to be depicted by pressing the palms that your two hands with each other in front of friend (fingers up). One is the enjoy of the other that friend would view in a mirror with every little thing that is on the appropriate in one on the left in the other and vice versa, that is, her thumbs and each that your four fingers oppose each other. Now if you orient your hands in the very same direction, for example, through the palms dealing with away from you, girlfriend will watch that the thumbs are on the opposite sides and that your two hands space different, the is, they are chiral and nonsuperimposable winter images. You would certainly soon discover this the end if you tried to placed a glove for the left hand on the right hand, for example.


Monosaccharides space carbohydrate molecules that cannot be damaged down by hydrolysis2 into much easier (smaller) carbohydrate molecules. Hence, monosaccharides space at times described as “simple sugars” or just :sugars,\" i beg your pardon infers the they space the simplest (smallest) that the carbohydrates. (The ax saccharide is obtained from saccharose, which is an old term for cane sugar. Now, it refers to any type of carbohydrate, particularly a monosaccharide; but as subsequent chapters top top oligo- and polysaccharides indicate, it have the right to be used to any size carbohydrate. Mono is obtained from the Greek word for one. In chemistry, it often means containing just one, so the term monosaccharide way one saccharide or one sugar, indicating that it is a molecule created of just one street unit and not of 2 or an ext sugar units joined together). Monosaccharides are the monomeric systems of oligosaccharides (Chapter 3) and also polysaccharides (Chapter 4), both of i m sorry contain much more than one saccharide (sugar) unit and also can be hydrolyzed to relax their ingredient monosaccharides. The common monosaccharides offered as structure blocks for oligo- and polysaccharides found in foods contain a team termed together the saccharose group.



Discussion of particular carbohydrate structures starts with d-glucose, the many common, most widely distributed, and most abundant carbohydrate (if every its combined forms space considered). D-Glucose (known commercially as dextrose) is a monosaccharide. D-Glucose is both a polyalcohol (polyhydroxy compound) and also an aldehyde. It and also all sugars containing an aldehydic team are classified as aldoses (Table 1.1). The prefix ald- indicates that they space aldehydes; the suffix -ose commonly (but not always as friend will discover out) signifies a nonpolymeric carbohydrate (that is, a monosaccharide). D-Glucose contains six carbon atoms, making it a hexose (Table 1.1); more specifically, that is one aldohexose. As soon as the structure of d-glucose is composed in a upright straight-chain fashion (termed one acyclic or open-chain structure) through the aldehydic group (position 1 ) at the top and also the carbon atom v the major hydroxyl team attached come it at the bottom (at place 6 ), it can be viewed that all secondary hydroxyl teams are ~ above carbon atom C2, C3, C4, and also C5. To make the determination/assignment the which side of the carbon chain the hydroxyl groups are on, a convention because that orientation of the carbon chain is used. In this convention, the carbon chain is oriented so that each vertical (carbon-to-carbon) shortcut projects into the airplane of the page and each horizontal shortcut projects exterior from the aircraft of the page as in Fig. 1.1 (although in equipment there is rotation around the upright bonds that permits a hydroxyl group to it is in in any position through respect to the one above it, so the the molecules have the right to actually i think a huge number of various conformations ). Each of the 4 carbon atom that have actually a secondary hydroxyl group attached come it (C2, C3, C4, C5) room chiral carbon atoms due to the fact that each has actually four different substituents attached come it. Every chiral center has a mirror image (Fig. 1.1), and mirror image chiral carbon atoms space not superimposable on every other, simply as a person's 2 hands room mirror images and also are no superimposable.


Number of carbon atomsKind the carbonyl groupAldehydeKetone
3triosetriulose
4tetrosetetrulose
5pentosepentulose
6hexosehexulose
7heptoseheptulose
8octoseoctulose
9nonosenonulose


Because each chiral carbon atom has actually a winter image, there are 2n (where n is the variety of chiral carbon atoms) feasible arrangements of these atoms. Thus, in a six-carbon aldose (of i beg your pardon d-glucose is one), in which over there are 4 chiral carbon atoms, there are 24 or 16 different arrangements of the carbon atoms containing four second hydroxyl teams (theoretically enabling formation of 16 different six-carbon sugars with an aldehydic group). Eight of these sugars belong to what is known as the D series; eight are their winter images and belong to the l series. (All 16 of this aldohexoses have the empirical formula C6H12O6.) every sugars that have the hydroxyl group on the highest-numbered chiral carbon atom (C5 in the case of d-glucose) positioned on the right-hand next (using this projection and also convention because that the chain conformation) room termed D sugars, and all through the highest-numbered chiral atom having its hydroxyl team on the left space classified as L sugars. (Note that l-glucose is the finish molecular mirror photo of d-glucose and also not the substance v the opposite construction of just C5. If just the construction of C5 is changed, the product is l-idose .) The framework of d-glucose is shown in the open-chain (acyclic) type (called the Fischer projection) v the carbon atoms numbered in the traditional manner. Together is customary, the horizontal currently indicating the covalent chemical bonds come the hydrogen atoms and hydroxyl teams are skipping in the structure on the right. Because the lowermost carbon atom (C6 in the situation of d-glucose) is no chiral, the loved one positions of the atoms and groups attached come it require not be designated, and it is composed as –CH2OH. The great majority that carbohydrates discovered in foods items are composed mostly of aldohexoses. Shown listed below is the aldopentose arabinose in both the D and also L forms, both the which take place in nature. Glucose is found only in the form of d-glucose.



An organic chemist say the d-glucose and all various other carbohydrate molecule as highly functionalized because there is a practical group3 on each carbon atom. The finish functionalization that the backbone carbon atom structure with hydroxyl groups, a carbonyl oxygen atom (in the instance of the straightforward sugars), and also a high percent of chiral carbon atoms are differentiating features that carbohydrates.

d-Glucose, together its O6 phosphate ester4 (d-glucose 6-phosphate), is the an initial sugar that photosynthesis. D-Glucose 6-phosphate (Chapter 2) is present in only tiny amounts due to the fact that it is rapidly converted right into sucrose (Chapter 3) for deliver to assorted parts that the plant wherein it is provided for synthetic of various other substances. Due to the fact that d-glucose is the monomeric building unit the cellulose (Chapter 8), it have the right to be thought about to it is in the most abundantly easily accessible organic compound on earth (if its combined forms are considered). D-Glucose 6-phosphate is likewise used together an energy resource in the plant's metabolism.


There space two aldoses containing three carbon atoms: d-glyceraldehyde and l-glyceraldehyde (d- and l-glycerose according to formal carbohydrate nomenclature, although this names are rarely used). Each possesses only one chiral carbon atom. Aldoses with four carbon atoms (the tetroses) have two chiral carbon atoms. Aldoses with 5 carbon atoms (the pentoses) have three chiral carbon atoms and also comprise the second most common group of aldoses. Expanding the collection above six carbon atoms provides heptoses, octoses, and nonoses (seven, eight, and nine carbon atoms, respectively). Nine carbon atom is the dimension limit that naturally developing sugars. Only pentoses and also hexoses are uncovered in the common carbohydrates that food products. Advancement of the eight d-hexoses native d-glyceraldehyde is shown below using the Rosanoff shorthand forecast (Fig. 1.2). In the Rosanoff projection, a circle represents the aldehydic group; horizontal lines indicate the place of every hydroxyl group on that chiral carbon atom, and also the bottom the the vertical lines shows the terminal, nonchiral hydroxymethyl (-CH2OH) team (a major hydroxyl group). Sugars who names room in italics in Fig. 1.2 are commonly found in plants, virtually exclusively in combined forms. Thus, castle are existing in our diet in combined forms. Only a little amount the d-glucose in the free monosaccharide form (section ~ above Glycosides in this chapter) is existing in natural foods items (except because that honey), and it is normally the only complimentary aldose present.



d-Glyceraldehyde occurs normally as the O3 phosphate ester. Most other herbal sugars, consisting of the common d-glucose, have actually the very same configuration (D) of their highest-numbered chiral carbon atom (C5 in the case of a hexose). L-Sugars room much less numerous and also abundant in nature 보다 the D forms. The principal l-sugar uncovered in foods is l-arabinose, which wake up in a combined form in some polysaccharides.

To make a molecular version of a sugar, two basic rules must be followed. The an initial is to take into consideration only one carbon atom at a time in copying a projected framework such together a Fischer estimate or a Rosanoff structure. The second is to keep in mind the all horizontal bonds in the projected structure are envisioned as protruding towards you from the carbon atom, whereas upright bonds are envisioned together protruding far from you.


So far aldoses (in i beg your pardon the carbonyl unit the the saccharose team is the of an aldehyde) have been discussed. In another form of monosaccharide, the carbonyl unit in the saccharose group is a ketone. This sugars are well-known as ketoses, v the prefix ket- identify them as having a ketone group. D-Fructose is the prime example of this street group. D-Fructose is one of the 2 monosaccharide devices of the disaccharide sucrose (Chapter 3) (the other being d-glucose) and also makes increase ca. 55% that a usual commercial high-fructose syrup (Chapter 7) used in soft drinks. About 40% that the carbohydrate of honey are d-fructose.


d-Fructose is the principal commercial ketose and the only among importance in foods. (In the past, d-fructose was called both levulose and also fruit sugar, yet these designations space rarely provided today.) d-Fructose has only three chiral carbon atoms (C3, C4, and C5). Thus, there are just 23 or 8 ketohexoses. The miscellaneous ketotetroses, -pentoses, and also -hexoses are pertained to nonchiral dihydroxyacetone. The suffix designating a ketose in methodical carbohydrate nomenclature is –ulose (Table 1.1). In methodical nomenclature, d-fructose is d-arabino-hexulose since its 3 chiral carbon atoms have the very same configuration as those in d-arabinose. The Rosanoff forecast of a ketopentose (pentulose) with the D-threo construction (that is the configuration of the 2 chiral carbon atom in d-threose) is provided in Fig. 1.3 as an additional demonstration the the nomenclature principle.

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Figure 1.3. Rosanoff projection of a ketopentose (D-threo-pentulose, “d-xylulose”) reflecting the construction of the two chiral carbon atoms.