Starch, cellulose, glycogen and food tests
Starch and glycogen store glucose in plants and animals; cellulose builds plant cell walls. They differ in α or β links and in branching.
Comparing the three polysaccharides
| Feature | Starch | Cellulose | Glycogen |
|---|---|---|---|
| Monomer | α-D-glucose | β-D-glucose | α-D-glucose |
| Linkage | C1 to C4 (amylose); C1 to C4 with C1 to C6 branches (amylopectin) | C1 to C4 only, unbranched | C1 to C4 with many C1 to C6 branches |
| Where found | Cereals, roots, tubers, some vegetables | Plant cell walls, cotton, wood | Liver, muscles and brain; also yeast and fungi |
| Role | Food store of plants | Structure and support | Food store of animals |
Amylose vs amylopectin
Importance of carbohydrates
Carbohydrates are essential for life in both plants and animals. They form a major part of our food: rice, wheat, potatoes and sugar. Honey has long been used as a quick source of energy in Ayurvedic medicine. Carbohydrates are stored as starch in plants and as glycogen in animals. Cellulose forms plant cell walls, and we use it as wood for furniture and cotton for clothes. Carbohydrates are raw materials for the textile, paper, lacquer and brewing industries. Two aldopentoses, D-ribose and 2-deoxy-D-ribose, are parts of the nucleic acids.
Activity: tests for carbohydrates, fats and proteins in pure samples and foodstuffs
Pure samples: glucose, starch, groundnut oil and egg albumin. Foods: grape juice, boiled potato, ghee, milk and moong dal. Make water extracts of the solids, and record each result in a table: test, sample, observation, inference.
- 1Molisch's test (any carbohydrate): 2 mL of solution + 2 to 3 drops of alcoholic α-naphthol; pour concentrated H₂SO₄ slowly down the side of the tube.A violet ring forms where the two layers meet.
- 2Fehling's test (reducing sugars): equal volumes of Fehling's A and B + the solution; heat in a water bath.Orange-red precipitate of Cu₂O with glucose, grape juice and honey. Sucrose and starch give no such precipitate.
- 3Iodine test (starch): add a few drops of iodine solution.Blue colour with starch and potato. It fades on heating and returns on cooling.
- 4Tests for fats and oils: (a) rub a little on filter paper; (b) shake with water, and separately with chloroform; (c) heat with solid KHSO₄.(a) A translucent greasy spot that does not dry off. (b) Insoluble in water, soluble in chloroform. (c) A pungent smell of acrolein.
- 5Biuret test (proteins): solution + 2 mL of sodium hydroxide solution + 2 to 3 drops of 1% CuSO₄ solution.Violet colour with egg albumin, milk and dal extract.
- 6Xanthoproteic test (proteins): heat with a few drops of concentrated HNO₃, cool, then add sodium hydroxide solution.Yellow colour, turning orange with the alkali.
- 7Ninhydrin test (proteins and amino acids): boil with a few drops of ninhydrin solution.Blue-violet colour.
- 8★Write an inference for each food: which of carbohydrate, reducing sugar, starch, fat and protein it contains.
★ key step
We cannot digest the cellulose in vegetables and whole grains, because our bodies cannot break its β links. Cattle can, with the help of microbes in their stomachs. For us, this undigested fibre keeps digestion healthy.
2 marks: “Differentiate between amylose and amylopectin.” Amylose: water-soluble, 15 to 20% of starch, unbranched chain of α-D-glucose joined C1 to C4. Amylopectin: insoluble, 80 to 85%, branched, with C1 to C4 links and C1 to C6 links at branches.
2 marks: “Starch and cellulose are both polymers of glucose. How do their structures differ?” Starch is made of α-D-glucose, with C1 to C4 links and C1 to C6 branches (amylopectin). Cellulose is made of β-D-glucose joined C1 to C4 in straight, unbranched chains.
2 marks: “Name the storage polysaccharide of animals. Where is it found, and how is its structure related to starch?” Glycogen; in the liver, muscles and brain. It is like amylopectin but more highly branched.
α stores, β builds. Amylose is straight and soluble, amylopectin is branched; glycogen is amylopectin with extra branches.