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Monosaccharides – Definition, Structure, Types, Examples

Last Updated : 23 Mar, 2022
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The branch of chemistry that deals with the molecules involved in living things is called biochemistry. Carbohydrates, proteins, vitamins, and nucleic acids are some of the major components of our body. These are collectively called biomolecules.

A biomolecule is sometimes associated as a biological molecule, a term that refers to molecules found in living objects that are important for one or additional biological processes, analogous to cell division, morphogenesis, or development. Large macromolecules (or polyanions) similar to proteins, carbohydrates, lipids, and nucleic acids, as well as primary metabolites, secondary metabolites, and smaller moieties, similar to natural products, are all examples of biomolecules. Natural material is the more broad term for this type of material. Biomolecules are essential factors of living organisms. While endogenous biomolecules are made within the organism, organisms usually require external biomolecules, such as specific nutrients, to be present.

Carbohydrates are optically active polyhydroxy aldehydes or polyhydroxy ketones or substances that upon hydrolyzed would produce these types of compounds.

Monosaccharides

Monosaccharides are polyhydric aldehydes and ketones that cannot be hydrolyzed into simple carbohydrates. Monosaccharides are classified as:

  • Aldoses-Monosaccharides containing an aldehyde (-CHO) group are called aldoses. This group (-CHO) is always present at one end of the CH₁₂0 carbon chain, that is, at C₁.
  • Ketoses- Monosaccharides containing the keto (C = 0) group are called ketoses. In all naturally occurring ketoses, the keto is on carbon next to the terminal carbon, that is, at C₂. group exists.

predicated on the composition of carbon atoms, they’re else broke down as triose, tetraose, pentose, hexose, heptose, etc. Thus, when nominating these monosaccharides, the prefix indicates the composition of carbon atoms similar as tetra- (4), Penta- (5), Hexa- (6), hepta- (7), etc. is comprehended in the expression aldose or is done. ketos. For illustration, an aldopentose means that it’s an aldehyde carbohydrate- bearing five carbon atoms. Also, ketohexose means a ketone holding six carbon atoms.  

Most monosaccharides are found in nature. They are colorless, crystalline solids, soluble in water, and have a sweet taste. These are quite stable and are not hydrolyzed. They sizzle when heated and give off a distinctive odor. Optionally active.

All monosaccharides and disaccharides are sweet, the so-called sugars. All monosaccharides and disaccharides (except sucrose) are reduced by Fehling’s solution or Tollen’s reagent, so they are called reducing sugars. These sugars have free aldehyde and ketonic groups. If the reducing group i.e. aldehyde or ketonic group is bonded then they are called reducing sugars. These sugars, such as sucrose, do not reduce tollen or Fehling’s solution.

Structures of Monosaccharides

The simplest monosaccharides are trioses such as glyceraldehyde and dihydroxyacetone, both of which have the molecular formula C3H6O3, glyceraldehyde is aldose while dihydroxyacetone is ketose as shown below:

The majority of famed monosaccharides are ribose, C5H10O5, glucose C6H12O6, and fructose C6H12O6. Ribose is aldopentose, glucose is aldohexose while fructose is ketohexose.

D- and L- Designation

Sugars are divided into two families: the D-family and the L-family which have definite configurations. These configurations are indicated with respect to glyceraldehyde as the standard. Glyceraldehyde can be presented in two forms:

In the D-configuration -OH is attached to the carbon adjacent to -CH2OH while in the L-configuration – OH is attached to the carbon adjacent to the -OH on the left. The sugar is referred to as D- or L- depending on whether the configuration of the molecule is related to D-glyceraldehyde or L-glyceraldehyde.

It has been found that all naturally occurring sugars are related to the D-chain D-glucose, D-ribose, and D-fructose.

However, it may be noted that D- and L- do not represent dextrorotatory levorotatory. The optical activity of the molecule is represented by (+) and (−) which represents the direction of rotation of the plane polarized light ether dextrorotatory or levorotatory.

Presence of Asymmetric Carbon Atoms

On careful examination of monosaccharide molecules, we see that they contain one or more chiral carbon atoms. For example, glucose has four chiral carbon atoms (carbons 2, 3, 4, and 5). We know that if the molecule has n chiral carbon atoms, it will have 2n optical isomers. Therefore, glucose has 24 or sixteen optical isomers. Three of these are sixteen aldohexoses which are D-glucose, D-galactose, and D-galactose, D-mannose.

 

It may be noted that in all three of these molecules, the configuration of C-5 is the same (-OH on the right) and hence, they belong to the D-family.

Examples of Monosaccharide

Glucose

Glucose occurs in nature in an autonomous as well as a related fashion. It is present in sweet fruits and honey. Ripe grapes contain about 20% glucose and that is why it is also known as grape sugar. Glucose in related form is substantial in polysaccharides such as cane sugar and starch and cellulose.

Preparation of Glucose

  • From Sucrose (Cane sugar): When sucrose is boiled in an alcoholic solution with dilute HCl or H2SO4 equal amounts of glucose and fructose are obtained.

C6H22O11(sucrose) + H2O → C6H12O6(glucose) + C6H12O6(fructose)

  • From Starch: Glucose is produced commercially by the hydrolysis of starch by boiling it with dilute H₂SO4 at 393 K under the pressure of 2–3 atm.

(C6H10O5)(starch) + nH2O → C6H12O6(glucose)

In this process, an aqueous solution of starch obtained from corn is acidified with dilute H2SO4 then it is heated in an autoclave under 2-3 atm pressure steam. When hydrolysis is complete, the liquid is neutralized with sodium carbonate to a pH of 4–5. The resulting solution glucose Xe is concentrated under reduced pressure to obtain crystals of glucose.

Fructose

Fructose is found in fruits and is called fruit sugar. It is also present in honey and sweet fruits along with glucose. In the combined state, it is also present in disaccharides (sucrose) and polysaccharides (insulin).

It is obtained by hydrolysis of cane sugar with dilute H2SO4 with glucose.

C12H22O11 + H2O → C6H12O6(D- glucose) + C6H12O6(D- fructose)

This solution containing equal molecules of D-glucose and D-fructose is called invert sugar.

Sample Questions

Question 1: Why are carbohydrates usually optically active?

Answer:

Carbohydrates are usually optically active because they contain one or more chiral carbon atoms.

Due to the presence of one or more chiral or asymmetric carbon atoms and the absence of a plane of symmetry, carbohydrates are usually optically active.

Question 2: What are monosaccharides?

Answer:

Monosaccharides are polyhydric aldehydes and ketones that cannot be hydrolyzed into simple carbohydrates. Monosaccharides with aldehyde group (-CHO) are called aldoses while monosaccharides with ketonic group (C=0) are called ketoses. For example, glucose, fructose, ribose, etc.

Question 3: Explain what is meant by the pyranose structure of glucose?

Answer:

The six-membered cyclic structure of glucose is called the pyranose structure (α or β) in analogy to pyran. Pyran is a cyclic compound consisting of one oxygen atom and five carbon atoms in the ring.

Question 4: What are the structural features of reducing sugars?

Answer:

Reducing sugars contain free aldehyde or ketonic groups.

Question 5: What are the two functions of carbohydrates in plants?

Answer:

The two functions of carbohydrates in plants are:

  • Carbohydrates are used as storage molecules in plants in the form of starch.
  • The cell wall of bacteria and plants is made up of cellulose.


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