Carbohydrates: Definition, Structure, Types, Examples, Functions

What are Carbohydrates?

The category of carbonyl compounds known as carbohydrates includes aldehydes and ketones that have multiple hydroxyl groups. It could also include their derivatives, which, when hydrolyzed, produce chemicals that are identical. In addition to being the most prevalent organic compounds in nature, they go by the name “saccharides” as well. “Sugars” are any carbohydrates that may dissolve in water and have a sweet taste.

Structure of Carbohydrates

  • Carbon, hydrogen, and oxygen make up carbohydrates.
  • The typical empirical configuration of carbohydrates is (CH2O)n.
  • They are hydroxyl-rich organic compounds having aldehyde or ketone structures that extend from the carbon chain.
  • All carbohydrates are composed of monosaccharides, which are simple sugars.
  • A polyhydroxy aldehyde (aldose) or a polyhydroxy ketone may be a monosaccharide (ketose).

The three structural expressions of carbohydrates are as follows:

  • An open chain arrangement
  • Hemi-acetal composition.
  • Haworth structure

Open chain structure.- Long, straight chains of carbohydrates are those having an open chain structure.

Hemi-acetal structure –The OH group of the fifth carbon and the first carbon of glucose unite to form a ring.

Haworth structure -.A pyranose ring structure is a distinctive feature of the Haworth structure.

Properties of Carbohydrates

Physical Properties of Carbohydrates

Stereoisomerism – Compounds that exhibit stereoisomerism have identical structural formulae but different spatial configurations. Glucose contains two isomers with regard to the penultimate carbon atom, for instance. D-glucose and L-glucose are their names.

Optical Activity – It is the rotation of plane-polarized light that produces (+) glucose and (-) glucose.

Diastereo isomers – changes in the configuration of glucose’s C2, C3, or C4 atoms. Example: Mannose, galactose.

Annomerism – It is the spatial arrangement of the first carbon atom in aldoses and the second carbon atom in ketoses.

Chemical Properties of Carbohydrates

Osazone formation: When too much phenylhydrazine is present in the reaction between sugars and phenylhydrazine, osazone is the result. Glucosazone

Benedict’s test: Reducing sugars are converted into powerful lower species termed enediols when burned in the vicinity of an alkali. The mixture becomes orange-red or brick red when Benedict’s reagent and reducing sugar are cooked together.

Oxidation: Monosaccharides are reducing sugars if their carbonyl groups are reduced to become carboxylic acids as a result of oxidation. Consequently, D-glucose is oxidised to D-gluconic acid in Benedict’s test; consequently, glucose is regarded as a low sugar.

Reduction to alcohols: Sodium borohydride, NaBH4, or catalytic hydrogenation (H2, Ni, EtOH/H2O) may reduce the C=O groups in open-chain forms of carbohydrates to alcohols. The products are referred to as “alditols.”

Properties of Monosaccharides

  • Most monosaccharides have a sweet flavour (the sweetest sugar is fructose, which is 73% sweeter than sucrose).
  • They exist in solid form at a normal temperature.
  • These compounds are particularly water-soluble: Monosaccharides are far more water-soluble than the bulk of molecules with equivalent MW, despite having high molecular weights due to the presence of many OH groups.
  • 1 gram of glucose may be dissolved in 1 millilitre of water to create a syrup.

Simple carbohydrates consist of monosaccharides, oligosaccharides, and polysaccharides.


  • The simplest category of carbohydrates is frequently referred to as simple sugars, since they cannot undergo further hydrolysis.
  • Solids that are crystalline and polar solvents won’t dissolve them, but water will.
  • These compounds include free ketone or aldehyde groups.
  • Cn (H2O) or CnH2nOn is the general formula.
  • They are divided into categories according to the number of carbon atoms and the presence of active groups.
  • Based on if they include an aldehyde or ketone group, the monosaccharides containing 3, 4, 5, 6, 7… carbons are referred to as trioses, tetroses, pentoses, hexoses, heptoses, etc.
  • Examples: Glucose, Fructose, Erythrulose, Ribulose.


  • Hydrolysis of oligosaccharides generates two to ten molecules of the same or distinct monosaccharides.
  • The glycosidic bond joins the monosaccharide units together.
  • It is further classified as a disaccharide, trisaccharide, tetrasaccharide, etc. depending on the quantity of monosaccharide molecules.
  • On hydrolysis, oligosaccharides produce two molecules of monosaccharides, which are known as disaccharides. The terms trisaccharides and tetrasaccharides, respectively, are used to describe compounds that provide three or four monosaccharides.
  • Cn (H2O) n-1 is the general formula for disaccharides, whereas Cn (H2O) n-2 is the formula for trisaccharides, and so on.
  • Examples: disaccharides include sucrose, lactose, maltose, etc.
  • Trisaccharides are Raffinose, Rabinose.


  • They go by the name “glycans” as well.
  • Greater than ten monosaccharide units make up polysaccharides, which may even include hundreds more sugar units.
  • Approximately 10 monosaccharide molecules are produced during hydrolysis.
  • Polysaccharides are distinguished from one another by the characteristics of their repeating monosaccharide units, the distance of their chains, the types of bonds joining units, and the degree of splitting.
  • They are primarily focused on two fundamental goals: structural integrity and energy storage.
  • They are further divided into groups according to the kinds of molecules produced during hydrolysis.
  • They may be homopolysaccharides or heteropolysaccharides, including monosaccharides of various kinds.
  • Homopolysaccharides include starch, glycogen, cellulose, and pectin.
  • Chondroitin and Hyaluronic acid are heteropolysaccharides.

Functions of Carbohydrates

Carbohydrate molecules are widespread in plant and animal tissues. Animals and plants both store food in the form of carbohydrates derived from skeletal components of plants and arthropods. They are essential energy sources for a variety of metabolic processes, and their energy is produced via oxidation.

Among their primary roles are

  • In living things, carbohydrates provide easily accessible energy to drive cellular functions. For all living creatures, they provide the greatest amount of energy (4 kcal/gram).
  • Carbohydrates are not only the main energy source for many animals, but they also provide energy right away. Through the glycolysis/cycle, Kreb’s glucose is metabolised to create ATP.
  • Function as a source of energy, fuel, and metabolic intermediaries. It is stored in plants as starch and in mammals as glycogen.
  • Carbohydrates are used as an energy source instead of proteins.
  • They serve as structural and protective elements, much like in plant and microbial cell walls. Components of bacterial (peptidoglycan or murein), plant (cellulose), and mammalian (cellulose) cell walls (chitin).
  • Carbohydrates function as intermediaries in the production of lipids and proteins.
  • Carbohydrates help in the control of nerve tissue and serve as the brain’s energy supply.
  • Carbohydrates combine with lipids and proteins to generate antigens on the surface of cells, receptor molecules, vitamins, and antibiotics.
  • Formation of the RNA and DNA structural frameworks (ribonucleic acid and deoxyribonucleic acid).
  • They are associated with several proteins and lipids. These connected carbohydrates are essential for cell-cell communication and cell interactions with other cellular environmental components.
  • They are an essential component of connective tissues in mammals.
  • Fiber-rich carbohydrates aid in the prevention of constipation.
  • In addition, they help modulate the immune system.


  • Lehninger, A. L., Nelson, D. L., & Cox, M. M. (2000). Lehninger principles of biochemistry. New York: Worth Publishers.
  • Madigan, M. T., Martinko, J. M., Bender, K. S., Buckley, D. H., & Stahl, D. A. (2015). Brock biology of microorganisms (Fourteenth edition.). Boston: Pearson.
  • Rodwell, V. W., Botham, K. M., Kennelly, P. J., Weil, P. A., & Bender, D. A. (2015). Harper’s illustrated biochemistry (30th ed.). New York, N.Y.: McGraw-Hill Education LLC.
  • https://biology.tutorvista.com/biomolecules/carbohydrates.html
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