Eduqas Biology for A Level Book 1: 2nd Edition

Study point Amylose molecules have α -1,4- glycosidic bonds and amylopectin molecules have both α -1,4- and α -1,6-glycosidic bonds. Knowledge check Match the tests 1–4 with colours A–D, the final colours of the test solutions. 1. Glucose + Benedict’s test 2. Sucrose + Benedict’s test 3. Glucose + iodine / potassium iodide 4. Starch + iodine / potassium iodide A. Orange–brown B. Brick–red C. Blue-black D. Blue 1.2 Stretch & challenge As temperature increases, the colour intensity decreases so above about 35ºC, this test is unreliable. It is also unreliable at very low pH, as the starch is hydrolysed. Exam tip At very low iodine or starch concentration, the colour produced is violet rather than blue-black. ▪ Amylopectin also has chains of glucose monomers joined with α -1,4-glycosidic bonds. They are cross-linked with α -1,6-glycosidic bonds and fit inside the amylose. When a glycosidic bond forms between the C1 atom on one glucose molecule and the C6 atom on another, a side branch is seen. These occur every 24–30 glucose molecules. α -1,4- glycosidic bonds continue on from the start of the branch. Structure of a molecule of amylose α -1,4 and α -1,6 bonds in amylopectin and glycogen Comparing amylose and amylopectin Testing for the presence of starch: iodine–potassium iodide test Iodine solution (iodine dissolved in aqueous potassium iodide) interacts with starch. There is a colour change from the orange–brown of the iodine solution to blue–black. The depth of blue–black colour gives an indication of relative concentration of starch. An accurate concentration cannot be determined so, like the Benedict's test, this test is qualitative. Starch CH OH CH OH CH OH CH OH CH OH CH OH O O CH OH O O CH OH O O CH OH O O Cellulose O 4 1 4 1 4 1 4 1 4 1 O O O O O O O O CH OH CH OH O CH OH O O α-glucose molecules arranged in a helix glycosidic bond ⎧ ⎨ ⎩ { ⎧ ⎨ ⎩ X O O 1 4 1 4 1 4 1 O O O O O CH 2 6 Amylose Amylopectin Amylose 19  1.1 Chemical elements and biological compounds