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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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