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Unit 3 assignment 1, 2, 3
Presentation study book Essential Cell Biology of Bruce Alberts, Dennis Bray - ISBN: 9780393680393 (pass guaranteed)
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Investigate the use of chemical reagents to identify starch, reducing sugars,proteins.
Introduction:
In this practical will be provided with the concentration of starch. Then using
the technique of serial dilution to make a number of other lower
concentration. Then add a constant amount of Iodine indicator to each, using a
colorimeter find how much light its absorbed through each sample. Using this
data, a graph will be used which will show the relationship between the
concentration of the starch and the light level. Finally the samples of food
concentration will be found. Use the colorimeter and calibration curve to find
the concentration of the food samples.
Background Theory:
Starch is an example of a natural polymer. A polymer is a long and repeating
chain of the same molecule stuck together. Starch is a long-chain polymer of
glucose molecules joined together. Starch, a white, granular, organic chemical
that is produced by all green plants. Starch is a soft, white, tasteless powder
that is insoluble in cold water, alcohol, or other solvents. The basic chemical
formula of the starch molecule is (C6H10O5)n. Starch is a polysaccharide
comprising glucose monomers joined in α 1,4 linkages. The simplest form of
starch is the linear polymer amylose; amylopectin is the branched form.
Iodine is a chemical element. The body needs iodine but cannot make it. The
iodine needed by the body must come from the diet. As a rule, there is very
little iodine in food, unless it has been added during processing. Processed
food typically contains more iodine due to the addition of iodised salt. Most of
the world's iodine is found in the ocean, where it is concentrated by sea life,
especially seaweed.
Iodine is the test for starch, when starch is present the yellow/orange iodine
solution becomes a blue/black colour. The iodine will be present in the centre
of the starch helix, forming a starch iodine complex. Starch is a carbohydrate
found in plants. It consists of two different types of polysaccharides that are
made up of glucose units which are connected in two different ways. One is
the linear amylose and the other is the branched amylopectin.
, Amylose is the compound that is responsible for the blue colour. Its chain
forms a helix shape, and iodine can be bound inside this helix.
Amylose forms a colloidal dispersion in hot water whereas amylopectin is
completely insoluble. The structure of amylose consists of long polymer chains
of glucose units connected by an alpha acetal linkage. Starch - Amylose shows
a very small portion of an amylose chain.
Boiling a starch solution causes temporary unwinding of the helix and the
subsequent release of the iodine molecule, the blue/black colour will
disappear, and when the helix cools down the blue/black colour will re-form.
This effect is measured by the quantity of concentration rather than its quality.
Colorimeter This instrument calculates the density of the coloured solution, for
example, when iodine is applied to starch, it can be used to determine the
concentration of a compound solution.
Reference:
https://en.m.wikipedia.org/wiki/Iodine_test
www.chemieunterricht.de 2006.
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