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Biology for Advanced Level Secondary Schools


                                                      Acetamide
                                              CH                                    CH
                 Glucosamine ring                2                                     2
                                              C   O                                  C  O
                     CH OH              H     N   H        CH OH               H     N  H
                        2                                     2
                 H                                H    H                                H
                     H                  OH    H             H                 OH    H
                     OH    H            H                   OH   H             H
          FOR ONLINE READING ONLY
                               H    H                                H     H
                     H     N  H         CH OH β-1,4-glycosidic  H  N  H        CH OH
                                           2
                                                                                 2
                                                    bond
                        O  C                                  O  C
                           CH                      Chitin        CH
                              2                                    2
                                        Figure 1.27: Structure of chitin

           Chitin has also been proven to be useful  the presence of starch, reducing sugars and
           for several medical and industrial purposes.  non-reducing sugars (Table 1.3).
           Chitin  is  used  as  a  flocculating  agent
           for waste water, wound-healing agent,  (a) Biochemical test for starch
           a thickener and stabiliser of foods and    The presence of starch in biological
           pharmaceuticals. Also, it used as a binder   materials can be tested by using Iodine
           for dyes, fabrics, and adhesives, and a    solution. The use of iodine to test for
           sizing and strengthening agent for paper.   the presence of starch is one of common
           In butterfly wing scales, chitin is often   experiments. A solution of iodine (I ) and
                                                                                        2
           organised into stacks of nano-layers or    potassium iodide (KI) in water has a light
           nano-sticks made up of chitin nanocrystals   orange-brown colour. If is added to a sample
           that produce various iridescent colours by   that contains starch, the color changes to a
           thin-film interference.                    dark blue (or blue black). In the absence

                                                      of starch, the orange-brown colour of the
           Biochemical test for carbohydrates         aqueous solution remains unchanged.
           Carbohydrates are carbon compound
           that  have  polyhydroxy  aldehydes  and    The basis for starch test
           polyhydroxy ketones, with reducing         The Iodine bound inside the helical structure
           properties. Carbohydrates may be present   of the amylose forms a dark blue colour.
           as isolated molecules or they may be       The reaction is due to the formation of
           physically associated or chemically bound   polyiodide chains (complex) from the
           to other molecules. The specific test for a   reaction of starch and iodine. The amylose
           particular type of carbohydrate is based   in the starch forms helices where iodine
           on the presence of specific component of   molecules are assembled, forming a dark
           carbohydrate or the reducing properties    blue colour (Figure 1. 28).
           resulting from aldehyde or ketone groups.
           Different methods can be used to test for






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