Biology Lab

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Aim: to observe and find out how much starch grains are distributed within a transverse section of a banana Apparatus: banana, iodine solution, ruler, white tile, stopwatch, microscope slide, coverslip, filter paper, Light microscope

1) A piece of banana was place on a tile and a section was cut across the piece of banana about 5mm thick. 2) The surface of the banana was covered with iodine solution and left for about one minute. 3) The distribution of starch in the section of the banana was then illustrated in the outline below. 4) With the remaining piece of banana a microscope slide was wiped across its surface. 5) Iodine solution was then added to the tissue smear that was on the microscope slide. 6) The coverslip was then added to microscope slide carefully and filter paper was used to absorb the excess iodine solution from the slide. 7) The banana tissue was then examined under the low power of the microscope. 8) After the banana tissue was examined with the high power of the microscope, a labeled drawing of one cell was made. Discussion:

Starch is a polymer of alpha-glucose. It is formed in plant cells from glucose and is stored in the form of grains. As it has compact and insoluble molecules, it is an ideal form in which to store glucose molecules and these can be reformed by hydrolysis later when needed. Starch is a mixture of two types of chain (amylose and amylopectin) formed from alpha-glucose. Amylose is formed when many alpha-glucose molecules condense together forming 1, 4 glycosdic bonds, so producing long unbranched chains. These chains take up a coiled or helical configuration as each monomer has a bulky side group which has to be accommodated. Amylose forms about 20% of starch. Amylopectin is branched and consist of alpha-glucose condensed together in two ways forming 1, 4 glycosidic bonds and 16 glycosidic bonds. The coiled chains of amylopectin may contain 1500 monomers...
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