Biomolecules

Syllabus Unit: Organic Chemistry

Why it matters

Chapter overview

Biomolecules bridges organic chemistry and biology, covering the structure and classification of the molecules that build and run living systems: carbohydrates (energy storage and structure), proteins and amino acids (structure and function through four levels of folding), nucleic acids (the hereditary code), and vitamins/hormones (regulation and micronutrients). JEE tests this unit through structural classification and a handful of signature reactions/tests rather than deep biochemistry.

Chemistry Syllabus Structure Organic Chemistry ~34% weightage

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MonosaccharidesSingle unit: glucose, fructose

Disaccharides2 units: sucrose, maltose, lactose

Oligosaccharides3-10 units

PolysaccharidesMany units: starch, cellulose, glycogen

1Carbohydrates & Their Classification

Carbohydrates are polyhydroxy aldehydes or ketones (or compounds that hydrolyse to them), classified by hydrolysability into monosaccharides, oligosaccharides and polysaccharides — with glucose's cyclic hemiacetal structure and mutarotation a JEE perennial.

Learning Objectives

Classify carbohydrates as monosaccharides, oligosaccharides and polysaccharides based on hydrolysis behaviour.
Distinguish aldoses from ketoses, classifying glucose (aldohexose) and fructose (ketohexose) accordingly.
Explain the cyclic hemiacetal (pyranose) structure of glucose and define mutarotation.
Compare the structures and biological roles of starch, glycogen and cellulose.

Key Terms

MonosaccharideThe simplest carbohydrate unit that cannot be hydrolysed further, e.g. glucose, fructose, ribose.

HemiacetalThe cyclic structure formed when glucose's C5-OH attacks its own C1 aldehyde intramolecularly, creating a new stereocentre (the anomeric carbon).

MutarotationThe spontaneous interconversion between α- and β-anomers of a cyclic sugar in solution, observed as a change in optical rotation until equilibrium is reached.

PolysaccharideA carbohydrate polymer of many monosaccharide units joined by glycosidic bonds (starch, cellulose, glycogen); generally not sweet and not readily soluble.

Carbohydrates are defined as polyhydroxy aldehydes or ketones, or compounds that yield them on hydrolysis.

Classification by hydrolysis

Monosaccharides — cannot be hydrolysed further (glucose, fructose, ribose).
Oligosaccharides — hydrolyse to give 2-10 monosaccharide units; disaccharides like sucrose, maltose, lactose are the most common.
Polysaccharides — hydrolyse to give many monosaccharide units (starch, cellulose, glycogen); generally not sweet, insoluble or only sparingly soluble.

Glucose is an aldohexose (6-carbon sugar with an aldehyde at C1); fructose is a ketohexose (6-carbon sugar with a ketone at C2).

Cyclic structure and mutarotation

In aqueous solution, glucose's open-chain form is in equilibrium with a cyclic hemiacetal (pyranose, 6-membered ring), formed when the C5-OH attacks the C1 aldehyde intramolecularly. This creates a new stereocentre at C1 (the anomeric carbon), giving two anomers, α-glucose and β-glucose.

Exam TipFreshly prepared solutions of pure α- or β-glucose show a slowly changing optical rotation that settles at an intermediate value as the two anomers interconvert through the open-chain form — this is mutarotation.

Polysaccharides: structure and role

Starch — amylose (unbranched, α-1,4 links) + amylopectin (branched, α-1,4 and α-1,6 links); the plant energy-storage carbohydrate.
Glycogen — highly branched, similar to amylopectin but more so; the animal energy-storage carbohydrate ('animal starch').
Cellulose — unbranched chains of β-1,4 links; structural polysaccharide in plant cell walls, not digestible by humans.