Carbohydrate: Structure, Functions and Types

Carbohydrate: Structure, Functions and Types

  • Carbohydrates are most abundant biomolecules in the nature.
  • They can hydrolyze Polyhydroxy aldehyde, ketones, or compounds.
  • Carbon, hydrogen, and oxygen are the primary component of carbohydrates.
  • They are also known as hydrates of carbon because in most of the carbohydrates H and O are present in the same ratio as in water.


Carbohydrates are involved in various functions:

  • They also act as precursor for many organic compounds.
  • It also involved in structural components of many organisms.
  • It also involves in immediate source of energy demands of the body by serving as the storage form of energy (glycogen).
  • They also involved in the structural and cellular functions of cell such as- fertilization, cell growth and adhesion.


Classification of Carbohydrates

  • Carbohydrates are commonly referred to as a saccharides (greek: sakcharon means sugar).
  • Carbohydrates are classified into three major groups depending upon whether these undergo hydrolysis and if so on then the number of products formed by them.
    1. Monosaccharides
    2. Oligosaccharides
    3. Polysaccharides


  • Greek: mono means one.
  • It consists of single polyhydroxy aldehyde or ketone unit.
  • They are the simplest sugar and generally, referred as simple sugars.
  • General formula is cnh2no
  • They are colorless, crystalline solids that are soluble in water but insoluble in nonpolar solvents.
  • They cannot further hydrolyzed to form simpler molecules.


  • It is divided into two categories on the basis of functional groups:
    1. Aldoses: monosaccharides with aldehyde groups e.g. glyceraldehyde, glucose.
    2. Ketoses: monosaccharides with ketone groups e.g. dihydroxyacetone, fructose.
  • D-glucose is the most abundant monosaccharides on the earth.

Structural aspects of carbohydrates

  • One of the most important character of monosaccharides are stereoisomerism.
  • Stereoisomerism may be defined as compounds which have same structural formula but differ in their spatial configuration.
  • All the monosaccharides except dhap (Dihydroxyacetone phosphate) contains one or more asymmetric carbon atoms.


  • Chiral molecules can exist in two configurations that are not super imposable mirror images to each other.
  • The two members are designated as d- and l- sugars.

Methamphetamine EnantiomersMethamphetamine Enantiomers


  • Most of the sugars are closely related, but differs only by the stereo-chemistry at a single carbon atom.
  • Example: D-glucose and D-mannose they are differ at only carbon number 2.



  • Cyclic structure exists in two different configurational forms. If the OH- groups in the anomeric carbon is below the plane of the ring, then it is termed as alpha position; if the OH- groups is above the plane of ring then they are termed as beta position.
  • These two diastereomers are termed as anomers.

Derivatives of monosaccharides

There are various monosaccharides, which are physiologically important:

  1. Sugar acids.
  2. Sugar alcohols
  3. Alditols
  4. Amino sugars.
  5. Deoxysugars
  6. L-ascorbic acid


  • Greek: oligo means few.
  • It contains 2-10 monosaccharides molecules which are liberated on hydrolysis.
  • It can be further divided into several groups depending upon the number of monosaccharides units present in them.



  • Disaccharides are the most common, among oligosaccharides.
  • It consists of two monosaccharides units which is held together by glycosidic bond.
  • It is crystalline, soluble in water and sweet in taste.

Maltose a disaccharides

  • It is of two types:
    1. Reducing; free aldehyde or keto group
    2. Non reducing: no free aldehyde or keto group.

Occurrence and biochemical roles of some important disaccharides:

Disaccharides Structure Roles
                  Sucrose Glucose (α1→2β) fructose A product of photosynthesis.
                  Lactose Galactose β(1→4) glucose A major animal energy source.
                  Trehalose Glucose α (1→1)α glucose A major circulatory sugar in insects, used for energy.
                  Maltose Glucose α(1→4) glucose The dimer derived from the starch and glycogen.
                  Cellobiose Glucose β(1→4) glucose The dimer of cellulose polymer.
                  Gentiobiose Glucose β(1-6) glucose Constituent of plant glycosides and some polysaccharides.


  • Greek: poly means many.
  • They are generally polymers of monosaccharide units with high molecular weight.
  • They are tasteless and most often form colloids with water.
  • It is linear as well as branched polymer.
  • It is of two types:
    1. Homopolysaccharides: it yields only a single type of monosaccharide on hydrolysis.
    2. Heteropolysaccharides: it yields a mixture of a few monosaccharides or their derivatives on hydrolysis.


  • Starch, glycogen, cellulose and chitin are homopolysaccharides.
  • Glycosaminoglycan, peptidoglycan are heteropolysaccharides.

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Carbohydrate: Structure, Functions and Types

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