2. CARBOHYDRATES

CARBOHYDRATES: A TEACHER’S COMPREHENSIVE GUIDE

Welcome, future pharmacists and healthcare professionals!

Carbohydrates are organic compounds with the general formula C_n(H₂O)_n. They are composed of carbon, hydrogen, and oxygen having the ratio of hydrogen and oxygen atoms as 2:1. These are regarded as hydrates of carbon, but structurally they show resemblance with polyhydroxy aldehydes and ketones. They are the primary source of energy as cells utilise carbohydrate directly for cellular respiration in the presence of oxygen.

Dpharmguru’s exam insights:

In my years of teaching biochemistry, I have observed that students often get confused between the different types of carbohydrates. Remember: “Monosaccharides are the simplest sugars, disaccharides have two units, and polysaccharides have many units.” This is a fundamental concept that appears in almost every biochemistry exam!

CLASSIFICATION OF CARBOHYDRATES

Carbohydrates can be divided in two ways: on the basis of hydrolysis and on the basis of reactivity.

1. On the Basis of Hydrolysis

• Monosaccharides: Single unit carbohydrates (polyhydroxy aldehydes or ketones) that cannot be hydrolysed further. Examples: glucose and fructose.
• Oligosaccharides: Made up of 2-10 units of monosaccharide or simple sugars.
  • Disaccharides: Two monosaccharide units. Example: sucrose.
  • Trisaccharides: Three monosaccharide units. Example: raffinose.
• Polysaccharides: Formed by polymerisation of more than ten monosaccharide units. Examples: starch and cellulose.

2. On the Basis of Reactivity

• Reducing Sugars: Contain an aldehyde (-CHO) or a ketone (C=O) functional group. They act as reducing agents. All monosaccharides and disaccharides (except sucrose) are reducing sugars. Examples: glucose, fructose, lactose.
• Non-reducing Sugars: Do not have free aldehyde or ketone functional group. They do not reduce Fehling’s and Tollen’s reagents. Example: sucrose.

Chemical Properties of Carbohydrates

• They undergo dehydration and charring with a characteristic burning sugar smell (caramelisation) when heated or dry with strong sulphuric acid.
• They yield characteristic reactions of hydroxyl functional group, like esterification, etherification, oxidation, dehydration, substitution with halogens, etc.
• They reduce Fehling’s or Benedict’s reagent, precipitating reddish-brown cuprous oxide.
• Reducing sugars react with Tollen’s reagent, forming a silver mirror on the glass wall of the tube.
• Bromine water is decolourised by aldoses but not ketoses.
• Concentrated nitric acid oxidises both the aldehyde and primary alcohol groups to dicarboxylic acids.
• All reducing sugars react with phenylhydrazine forming osazones.
• On heating with strong acid, aldohexoses and aldopentoses are dehydrated to yield hydroxymethyl furfural and furfural respectively.

QUALITATIVE TESTS FOR CARBOHYDRATES

• Fehling’s Solution Test: A yellowish-red colour appears confirming the presence of a reducing sugar.
• Molisch’s Test: All carbohydrates give a positive result. A colour develops at the junction of the two liquids.
• Barfoed’s Test: Used for detecting monosaccharides. A red precipitate indicates the presence of a monosaccharide.
• Osazone Formation: Sugar on heating with phenylhydrazine hydrochloride forms yellow crystals of osazone.
• Resorcinol Test for Ketoses (Selivanoff’s Test): Pink colour is produced in case of ketoses.
• Test for Pentoses: Red colour is formed in case of pentoses.
• Furfural Test: A pink or red stain appears on the reagent paper.
• Benedict’s Test: A yellow or brownish-red precipitate confirms the presence of a reducing sugar.
• Tollen’s Test: A shining silver mirror indicates the presence of reducing carbohydrates.
• Iodine Test: Only starch gives this test. A deep blue colour appears due to the presence of starch.

Dpharmguru’s exam insights:

Remember the key tests: Molisch’s test is positive for all carbohydrates, Benedict’s and Fehling’s tests are positive for reducing sugars, and the Iodine test is specific for starch. These are frequently tested in practical exams!

BIOLOGICAL ROLE OF CARBOHYDRATES

• Source of Energy: They are an immediate source of energy for the living cell.
• Protection: The bacterial cell wall is made up of polysaccharides, providing protection to bacterial cells.
• Storage Form: They are stored as glycogen in the cells of liver and muscles of animals.
• Absorption: Proteoglycans are important for absorption by fibroblasts.
• Disease Treatment: Carbohydrates are involved in the breakdown of fatty acids and prevent ketosis. Inulin is used for clinical purposes in clearance tests.
• Biological Recognition: The glycocalyx is involved in biological recognition processes.

MONOSACCHARIDES

Monosaccharides are polyhydroxy aldehydes or ketones that cannot be hydrolysed further to produce simple sugars. They have a sweet flavour and are water soluble. Their general formula is C_nH_2nO_n.

1. Structure of Glucose

Glucose is an aldohexose with molecular formula C₆H₁₂O₆. It exists in two forms: open-chain (acyclic) and ring (cyclic) form. In an aqueous solution, both forms are present in equilibrium.

Open Chain Structure

• Glucose has a 6-carbon unbranched chain.
• It has five -OH groups (penta-acetyl derivative).
• It has a carbonyl group (forms oxime).
• It has a terminal -CHO functional group.

Cyclic Structure

• Glucose fails to restore the pink colour of Schiff’s reagent.
• It fails to form additional products with sodium bisulphite.
• Two stereoisomeric forms (α- and β-forms) exist.
• Mutarotation is exhibited by both forms.

Mutarotation

Mutarotation is defined as the change in specific rotation of an optically active compound that accompanies the interconversion of α and β-anomers in an aqueous solution.

• α-D-glucose: Melting point 146°C, specific rotation +112°.
• β-D-glucose: Melting point 148-150°C, specific rotation +18.5°.
• Equilibrium mixture: Specific rotation +52.6°.

Dpharmguru’s exam insights:

A common exam question is: “What is mutarotation?” Remember: It is the change in specific rotation when α and β-anomers interconvert in solution. Also, glucose has a 6-membered pyranose ring—this is frequently tested!

2. Structure of Fructose

Fructose is a ketohexose with molecular formula C₆H₁₂O₆. It is the sweetest of all sugars and is also known as laevulose because it is optically active laevorotatory (-) sugar.

• Open Chain Structure: Contains 5 -OH groups, a carbonyl group, and a ketonic group at C-2 position.
• Cyclic Structure: Fructose exhibits mutarotation and forms 5-membered furanose ring.

3. Structure of Galactose

Galactose is a monosaccharide and glucose epimer with molecular formula C₆H₁₂O₆. It is found in lactose (milk sugar). The chair form of galactose follows a similar pattern to glucose.

DISACCHARIDES

1. Maltose

Maltose is a disaccharide with chemical formula C₁₂H₂₂O₁₁. It is formed from two glucose units linked by α(1→4) bonds. It is also known as malt sugar.

• It reduces Tollen’s reagent and Fehling solution.
• On hydrolysis, it produces only D-(+)-glucose.
• It exhibits mutarotation.

2. Lactose

Lactose is the sugar present in milk. Its molecular formula is C₁₂H₂₂O₁₁.

• On hydrolysis, it produces D-glucose and D-galactose.
• It reduces Tollen’s reagent and Fehling solution.
• It exhibits mutarotation.

3. Sucrose

Sucrose is common table sugar present in sugarcane, sugar beet, etc. Its molecular formula is C₁₂H₂₂O₁₁.

• On hydrolysis, it produces D(+) glucose and D(-) fructose.
• It is a non-reducing sugar.
• It is named as 1-α-D-glucopyranoside-2-β-D-fructofuranoside.

Dpharmguru’s exam insights:

Remember: Sucrose is the only non-reducing disaccharide. Maltose is made of two glucose units, lactose is made of glucose + galactose, and sucrose is made of glucose + fructose. This is a common exam question!

POLYSACCHARIDES

1. Starch

Starch is a significant storage form of polysaccharides present in plants. It comes in two forms:

• Amylose: Made up of 250-300 glucose residues joined by α-1,4 glycosidic linkages. Unbranched and coiled.
• Amylopectin: Glucose polymer with α-1,4 glycosidic linkages that is extensively branched. Branches linked by α-1,6 glycosidic connections.

2. Glycogen

Glycogen is a storage polysaccharide found in animal tissues, including the liver and muscle. It is a highly branched polymer with around 8 to 10 glucose units per branch.

• Linear chains joined by α-1,4 glycosidic links.
• Branches joined by α-1,6 glycosidic bonds.
• More highly branched than amylopectin.

Dpharmguru’s exam insights:

Carbohydrates are essential biomolecules that serve as the primary energy source for the body. In exams, pay special attention to: (1) The classification of carbohydrates, (2) The differences between reducing and non-reducing sugars, (3) The structures of glucose, fructose, and sucrose, and (4) The qualitative tests for carbohydrates. Remember: “Carbohydrates are the fuel of life!”

REFERENCES AND FURTHER READING

• Lehninger, A. L., Nelson, D. L., & Cox, M. M. (2017). Lehninger Principles of Biochemistry (7th ed.). W. H. Freeman.
• Berg, J. M., Tymoczko, J. L., & Stryer, L. (2019). Biochemistry (9th ed.). W. H. Freeman.
• Satyanarayana, U., & Chakrapani, U. (2021). Biochemistry (5th ed.). Elsevier.
• Vasudevan, D. M., Sreekumari, S., & Vaidyanathan, K. (2022). Textbook of Biochemistry for Medical Students (9th ed.). Jaypee Brothers.
• National Institutes of Health (NIH). (2022). Carbohydrate Metabolism Resources. Retrieved from https://www.nih.gov.

Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns.

written by:
Dr. N. Sujith Kumar

Dr. N. Sujith Kumar is a pharmacy educator, Pharm.D graduate, and academic content creator from Jawaharlal Nehru Technological University Kakinada (JNTUK), where he completed his Doctor of Pharmacy (Pharm.D) degree between 2015 and 2021. He has more than 7 years of experience creating pharmacy educational content, writing study materials, and reviewing academic articles for pharmacy students. He has also contributed guest articles to pharmacy education platforms, including PharmD Guru. At D.PharmGuru, his work focuses on simplifying complex Diploma in Pharmacy (D.Pharmacy) subjects into easy-to-understand notes, practical explanations, and exam-oriented educational resources for students across India. His areas of focus include Human Anatomy and Physiology, Pharmaceutics, Pharmacology, Pharmaceutical Chemistry, Hospital and Clinical Pharmacy, and other core D.Pharmacy subjects.