Carbohydrates: classification and reactions
Carbohydrates are grouped by how many sugar units they have: mono, di, oligo and polysaccharides. Free reducing sugars give tests like Benedict's and osazone.
Classification by number of sugar units
| Group | Sugar units | Examples | Indian food sources |
|---|---|---|---|
| Monosaccharides | 1 (cannot be split further) | Glucose, fructose, galactose, ribose | Honey, grapes, mango, ripe banana (glucose, fructose) |
| Disaccharides | 2 | Sucrose (glucose + fructose), lactose (glucose + galactose), maltose (glucose + glucose) | Sugar, jaggery (sucrose); milk (lactose); malted ragi (maltose) |
| Oligosaccharides: trisaccharides | 3 | Raffinose (galactose + glucose + fructose) | Rajma, soybean, Bengal gram |
| Oligosaccharides: four units and above | 4–10 | Stachyose (4), verbascose (5) | Soybean and other pulses |
| Polysaccharides | More than 10 | Starch, glycogen, cellulose (one kind of unit: homopolysaccharides); heparin, hyaluronic acid (mixed units: heteropolysaccharides) | Rice, wheat, ragi, potato (starch); vegetables, whole grains (cellulose) |
Monosaccharides by carbon number
| Carbons | Aldoses (aldehyde group, –CHO) | Ketoses (keto group, C=O) |
|---|---|---|
| Trioses (3C) | Glyceraldehyde | Dihydroxyacetone |
| Tetroses (4C) | Erythrose | Erythrulose |
| Pentoses (5C) | Ribose, xylose | Ribulose, xylulose |
| Hexoses (6C) | Glucose, galactose, mannose | Fructose |
Reducing vs non-reducing sugars
Benedict's test is a simple screening test for sugar in urine. Glucose in urine (glycosuria) suggests diabetes.
General reactions
| Reaction | What happens | Example or use |
|---|---|---|
| Reducing property | Free –CHO or C=O group reduces Cu2+ (blue) to Cu2O (brick-red) on heating | Benedict's and Fehling's tests for urine sugar |
| Osazone formation | Reducing sugar boiled with excess phenylhydrazine forms yellow crystals | Glucose, fructose, mannose: needle-shaped; lactose: powder-puff; maltose: sunflower-shaped. Sucrose forms none. |
| Fermentation | Yeast enzymes (zymase) turn glucose into ethanol and CO2 | Bread dough rising; toddy |
| Oxidation | Mild (bromine water): glucose → gluconic acid. Strong (nitric acid): glucose → saccharic acid. In the body: glucose → glucuronic acid | Glucose oxidase strips for blood sugar; glucuronic acid helps remove bilirubin and drugs |
| Reduction | C=O group → –OH, giving sugar alcohols: glucose → sorbitol, galactose → galactitol (dulcitol), mannose → mannitol | Sorbitol as a sweetener; sorbitol build-up in the eye lens adds to diabetic cataract |
| Esterification | –OH groups join with acids, especially phosphoric acid, to form sugar phosphates | Glucose-6-phosphate, the first step of glycolysis |
Starch from rice, wheat and millets is the main energy source in the Indian diet (about 4 kcal/g). Cellulose cannot be digested by humans but gives dietary fibre. Raffinose and stachyose in pulses reach the colon undigested and cause flatulence (gas); soaking, sprouting and cooking reduce them. Low lactase causes lactose intolerance; curd is usually better tolerated than milk.
2 marks: “What are reducing sugars? Give two examples.” Sugars with a free aldehyde or keto group that reduce Benedict's reagent, e.g. glucose and lactose.
5 marks: “Classify carbohydrates with suitable examples.” Draw a chart of mono, di, oligo and polysaccharides with units, two examples each and a food source.
10 marks: “Classify carbohydrates and explain the general reactions of monosaccharides.” Classification chart with examples, then reducing property, osazone, fermentation, oxidation, reduction and esterification, one example each.
Every common disaccharide contains glucose: Sucrose adds fructose, Lactose adds galactose, Maltose adds another glucose.