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12th Biology 2016 Set1 Delhi Board Paper Solution

Question 13

Describe the experiment that helped Louis Pasteur to dismiss the theory of spontaneous generation of life.

Answer

Leuis Pasteur suggested that life is propagated only through pre-existing life forms. The swan-necked flask experiment helped him prove his point on the continuity of life. His experiment is explained step by step as follows:

Experiment was carried out by two swan-necked flasks containing nutrient broth. Broth in the both flask was boiled to kill any existing microbes and was made sterile. After sterilising, the swan neck of one of the glass was broken and the flask was left undisturbed in the open air for some time. After some time the dust particles along with bacteria in the air entered the flask with broken neck and got accumulated at the neck of the unbroken flask. The broth in the broken flask became cloudy,while the broth in the unbroken flask remain clear. Cloudiness of the broth indicated the presence of microbial life in the broken flask. Appearance of life even after sterilisation, in the broken flask concluded that life in the flask arose only from pre-exising life.Thus, it discarded the theory of spontaneous generation.