Monosaccharides, Disaccharides, and Polysaccharides

  • Carbohydrates are molecules that contain carbon, hydrogen, and oxygen.
  • There are twice as many hydrogen atoms as carbon or oxygen atoms.
  • The general formula for a carbohydrate can be written as Cx(H2O)y.
  • They act as the source of energy (e.g. glucose), as a store of energy (e.g. starch and glycogen) and as structural units (e.g. cellulose in plants and chitins in insects).

  • Most carbohydrates are polymers.
  • Polymers are large, complex molecules composed of long chains of monomers.
  • Monomers are small, basic molecular units.
  • Carbohydrates can be divided into three groups: monosaccharides, disaccharides, and polysaccharides.

Monosaccharides – Structure, Properties, and Examples

  • Monosaccharides are simple sugars in which there are one oxygen atom and two hydrogen atoms for each carbon atom present in the molecule.
  • They have general formula as (CH2O)n.
  • Monosaccharides are reducing sugars.
  • The test for reducing sugar is called Benedict’s test.
  • They are sugars, which taste sweet, are soluble in water and are insoluble in non-polar solvents.
  • They exist in straight chains or in the ring or cyclic forms.
  • They are classified according to the number of carbon atoms in each molecule as trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), heptoses (7) and so on.
  • The names of all sugars end with -ose.
  • Examples: Glyceraldehyde (triose), Erythrose (tetrose), Ribose (pentose), Glucose (hexose), Fructose (hexose), Galactose (hexose), Sedoheptulose (heptose), etc.
  • They are used as a source of energy in respiration.
  • They are important building blocks for large molecules.

Disaccharides – Structure, Properties, and Examples

  • Disaccharides are made up of two monosaccharides joined together by a condensation reaction.

  • The condensation reaction is the joining of two molecules with the formation of a new chemical bond and a water molecule is released when the bond is formed.
  • A glycosidic bond is formed between two monosaccharides. If carbon 1 on one monosaccharide joins to carbon 4 on another monosaccharide, it is called a 1,4-glycosidic bond.
Disaccharides examples

  • Examples: Maltose is formed from two α-glucose molecules joined together by a glycosidic bond. Sucrose is formed from a condensation reaction between a glucose molecule and a fructose molecule. Lactose is formed from glucose and a galactose molecule.
  • Sucrose is a non-reducing sugar.
  • Disaccharides can be split apart into two monosaccharides by breaking the glycosidic bond by adding water molecules, which is known as hydrolysis reaction. The water provides a hydroxyl group (-OH) and hydrogen (-H), which helps the glycosidic bond to break.
  • Sucrose is the transport sugar and Lactose is the sugar found in milk which an important constituent of the diet of young mammals.

Polysaccharides – Structure, Properties, and Examples

  • Polysaccharides are polymers formed by combining many monosaccharide molecules (more than two) by condensation reactions.
  • Molecules with 3-10 sugar units are known as oligosaccharides while molecules containing 11 or more monosaccharides are true polysaccharides.
  • Polysaccharides do not taste sweet.
  • Because their molecules are so enormous, the majority of polysaccharides do not dissolve in water.
  • Polysaccharides made solely from one kind of monosaccharides are called homopolysaccharides (Starch) while those made of more than one monomer are called heteropolysaccharides (Hyaluronic acid).
Polysaccharides Examples

  • Starch is made up of long chains of α-glucose (Amylose and Amylopectin). Glycogen is made of α-glucose linked together by glycosidic bonds. Cellulose is also made of many β-glucose molecules linked by glycosidic bonds between carbon 1 and carbon 4.
  • Starch is the main energy storage materials in plants. Glycogen is the main energy storage materials in animals. Cellulose is the major component of cell walls in plants.
  • The test for starch is called an Iodine test.
starch glycogen and cellulose


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About Author

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Sagar Aryal

Sagar Aryal is a microbiologist and a scientific blogger. He is currently doing his Ph.D. from the Central Department of Microbiology, Tribhuvan University in collaboration with Helmholtz-Institute for Pharmaceutical Research Saarland (HIPS), Saarbrucken, Germany. He did his M.Sc. in Microbiology and B.Sc. in Microbiology from St. Xavier’s College, Kathmandu, Nepal. He worked as a Lecturer at St. Xavier’s College, Maitighar, Kathmandu, Nepal, from March 2017 to June 2019. He is interested in research on actinobacteria, myxobacteria, and natural products. He has published more than 15 research articles and book chapters in international journals and well-renowned publishers.

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