Experiment for Starch
Experiment 1: Preparation of biological material for microscopic examination Introduction: Microscopes are instruments used for observing and/or imaging cells. A simple light microscope [used in bright field microscopy] consists of a light source, microscope stage, coarse adjuster, fine adjuster, objective lens, and last but not least eyepiece lenses. A magnified image of a cell which is examined by a microscope is observed directly by looking into the eyepiece lens. Coarse and fine adjusters permit up-and-down movement of the microscope stage. This facilitates precise focus.
Objective lenses produce an image from the light rays that are transmitted from the light source and through the microscope stage which holds the sample under examination. The visibility of objects in a sample can be enhanced by staining. In this experiment three different samples of biological material are observed and examined by a microscope. Their images are recorded before and after staining, after observation under 10? and 40? power. Experiment Objection: I. Observation of starch grains under microscope (before and after staining) II. Observation of onion cells under microscope (before and after staining) III.
Observation of cheek cells under microscope (before and after staining) Materials: * iodine, * methylene blue * a small piece of potato, * an onion * water Apparatus: * A light microscope * 3 slides * 3 cover slips * filter paper * forceps * several toothpicks Methods: I. Observation of starch grains: 1. A small piece of potato was cut and placed on a slide. Enough pressure was applied to the potato to force its juice out. The potato was discarded afterwards. 2. A drop of water was added to the slide and then it was covered by a cover slip. 3.
The slide was put on the microscope stage and the sample was examined under low power (10? ) and high power (40? ). Magnified images of the sample were drawn. 4. Afterwards the sample was stained by iodine using irrigation technique. (A drop of iodine was placed on one edge of the slide and a filter paper was placed on the opposite edge. The water on the edge and under the cover slip was absorbed by the filter paper, drawing the iodine on the opposite edge under the cover slip. ) 5. The third step was repeated. II. Observation of onion cells: 1. An onion was cut in half.
Forceps were used to tear a piece of thin lining from inside the onion . The lining was placed on a slide. 2. One drop of water was added to the slide. Afterwards, it was covered by a cover slip. Then, the slide was examined under the microscope under high and low power. Magnified images of the sample were drawn and labeled. 3. Later on, the lining was stained by iodine using irrigation technique [which was mentioned earlier]. 4. The slide was put on the microscope stage again, and examined under high and low power. Magnified images of the sample were drawn and labeled. III.
Observation of cheek cells: 1. Inside of a cheek was scraped by a toothpick. And the scrapings were mounted in a drop of water on a microscope slide. 2. The slide was covered with a cover slip. 3. The slide was put on the microscope stage and the sample was observed under high and low power focus. Magnified images of the cells were sketched and labeled. 4. Afterwards, the sample was stained by methylene blue by irrigation technique. 5. The third step was repeated. Results: I. Observation of starch grains: 1) Before staining: a) Under 10? power b) Under 40? ower 2) After Staining with iodine: a) Under 10? power b) Under 40? power II. Observation of onion cells: 1) Before staining: a) Under 10? power b) Under 40? power 2) After staining with iodine: a) Under 10? power b) Under 40? power III. Observation of cheek cells: 1) Before staining: a) Under 10? power b) Under 40? power 3) After staining with methylene 4) After staining with methylene blue a) Under 10? ower b) Under 40? power Discussion: Starch is a polysaccharide which is found in most parts of the plant in the form of small granules. It is a reserve food formed from any excess glucose produced during photosynthesis. In this experiment, potato cells themselves were not examined under the microscope but their juice, which contained a lot of starch grains, was. Starch grains themselves have a yellow color under microscopic examination. Their color is light and distinguishing them requires precision. After staining a sample of starch grains, they become uch more visible under microscopic examination; for they take on a purplish dark blue color. When examining an onion lining under a microscope, the first thing that attracts one’s attention is the organized and ordered arrangement of the cells. Like most plant cells, onion cells show a high degree of order and organization in their structure, and most of them have fixed shapes. After being stained by iodine onion cells gain a tint of yellow color under microscopic examination, which makes them a lot more visible than before being stained. Cheek cells, unlike onion cells, do not have fixed shapes.
Their big nuclei can be easily distinguished under microscope examination. Staining by methylene blue, turns the color of cheek cells from light pink, to dark blue, resulting in clearer images of the cells under microscope. Conclusion: Staining samples by specific substances, enhances their visibility under microscopic examination. References: Beck S, Liem K, and Simpson A, (1991), Life: An Introduction To Biology, third edition, Harper Collins Publishers. Glenn and Susan Toole, (1999), New Understanding Biology For Advanced Level, fourth edition, Stanley Thornes Publishers.
