Carbohydrates – Structure, Classification & Tests

A comprehensive set of vocabulary flashcards covering definitions, classifications, structural concepts, reactions, diagnostic tests and representative examples of carbohydrates taught in the lecture.

Carbohydrate

Organic compound defined as a polyhydroxy aldehyde or ketone, or a substance that yields such units on hydrolysis.

Monosaccharide

Simplest carbohydrate (simple sugar) that cannot be hydrolysed further; general formula (CH₂O)n.

Oligosaccharide

Carbohydrate that yields 2–10 monosaccharide units on hydrolysis; usually tasteless and colloidal.

Polysaccharide

High-molecular-weight polymer of many monosaccharide units linked by glycosidic bonds.

Aldose

Monosaccharide whose functional group is an aldehyde (-CHO); e.g., glucose, glyceraldehyde.

Ketose

Monosaccharide whose functional group is a keto (carbonyl) group (-C=O-); e.g., fructose, dihydroxyacetone.

Triose

Monosaccharide containing three carbon atoms (C₃H₆O₃).

Tetrose

Monosaccharide containing four carbon atoms (C₄H₈O₄).

Pentose

Monosaccharide containing five carbon atoms (C₅H₁₀O₅).

Hexose

Monosaccharide containing six carbon atoms (C₆H₁₂O₆).

Heptose

Monosaccharide containing seven carbon atoms (C₇H₁₄O₇).

Stereoisomerism (in sugars)

Phenomenon where compounds share the same structural formula but differ in spatial arrangement around asymmetric carbons.

Asymmetric Carbon

Carbon atom attached to four different atoms or groups; creates chirality in sugars.

D-Isomer

Sugar whose OH on the penultimate (reference) carbon is on the right in Fischer projection.

L-Isomer

Sugar whose OH on the penultimate carbon is on the left in Fischer projection.

Optical Activity

Ability of chiral compounds to rotate plane-polarized light; measured as specific rotation.

Dextrorotatory (+)

Compound that rotates plane-polarized light to the right (clockwise).

Levorotatory (–)

Compound that rotates plane-polarized light to the left (counter-clockwise).

Racemic Mixture

Equimolar mixture of D- and L-isomers; overall optical rotation is zero.

Epimer

Two sugars differing in configuration around only one specific (non-anomeric) carbon atom.

Anomer

α or β cyclic form of a sugar that differs in configuration at the anomeric (carbonyl-derived) carbon.

Mutarotation

Change in specific optical rotation as α- and β-anomers interconvert to an equilibrium mixture (e.g., glucose from +112.2° to +52.7°).

Reducing Sugar

Sugar with a free aldehyde or keto group capable of reducing Cu²⁺ or Ag⁺ ions (e.g., glucose, lactose).

Non-Reducing Sugar

Sugar in which the carbonyl groups are tied up in glycosidic bonds and cannot act as reducing agents (e.g., sucrose).

Benedict’s Test

Alkaline Cu²⁺ test producing brick-red Cu₂O precipitate; detects reducing sugars.

Fehling’s Test

Similar to Benedict’s but uses Fehling A & B solutions; brick-red precipitate indicates reducing sugar.

Barfoed’s Test

Cu²⁺ test in acidic medium; positive (red precipitate) within 2 min indicates monosaccharide.

Seliwanoff’s Test

Resorcinol–HCl reaction; cherry-red colour indicates ketose (e.g., fructose).

Molisch’s Test

α-Naphthol + H₂SO₄ gives violet ring with furfural derivatives; general test for all carbohydrates.

Osazone

Yellow needle-shaped crystals formed when phenylhydrazine reacts with the first two carbons of a reducing sugar.

Disaccharide

Carbohydrate composed of two monosaccharides joined by a glycosidic bond.

Maltose

Reducing disaccharide of two α-D-glucose units linked α(1→4).

Sucrose

Non-reducing disaccharide of α-D-glucose and β-D-fructose linked α(1→2)β; cane sugar.

Lactose

Reducing disaccharide of β-D-galactose and β-D-glucose linked β(1→4); milk sugar.

Invert Sugar

Equimolar mixture of glucose and fructose produced by hydrolysis of sucrose; overall levorotatory.

Homopolysaccharide

Polysaccharide that yields only one type of monosaccharide on hydrolysis (e.g., starch, glycogen).

Heteropolysaccharide

Polysaccharide yielding two or more different monosaccharides or derivatives on hydrolysis (e.g., heparin).

Starch

Plant storage homopolysaccharide of α-D-glucose; composed of amylose (linear) and amylopectin (branched).

Amylose

Linear component of starch with α(1→4) linkages; gives blue colour with iodine.

Amylopectin

Branched starch component with α(1→4) chains and α(1→6) branch points; reddish-violet with iodine.

Inulin

Fructose homopolymer (fructosan) found in onion, garlic; used to measure glomerular filtration rate.

Glycogen

Animal storage polysaccharide of glucose with α(1→4) chains and α(1→6) branches; ‘animal starch’.

Cellulose

Structural plant polysaccharide of β-D-glucose linked β(1→4); major dietary fibre component.

Dietary Fibre

Indigestible plant carbohydrates (e.g., cellulose) that reduce intestinal absorption of glucose and cholesterol.

Hyaluronic Acid

Mucopolysaccharide in synovial fluid and vitreous humour; acts as lubricant and shock absorber.

Heparin

Highly sulfated heteropolysaccharide that functions as a natural anticoagulant in tissues.

Dermatan Sulfate

Heteropolysaccharide rich in skin; composed of L-iduronic or D-glucuronic acid and N-acetyl-D-galactosamine-4-sulfate.