Write on Pasteur’s Swan-neck flask experiment to disprove Spontaneous Generation. [5 Marks]
Introduction: Louis Pasteur (1859–1861) designed an elegant and decisive experiment using swan-neck (S-shaped) flasks to permanently disprove the theory of spontaneous generation.
Method: He poured nutrient broth into the flasks and heated the glass necks, drawing them out into long, curved “S” shapes. The broth was then thoroughly boiled to sterilize it and kill any pre-existing microorganisms.
The Mechanism: The open, curved neck was the brilliant part of the design: it allowed fresh air to enter freely, which satisfied critics who believed air contained a “vital force” necessary for life. However, airborne dust particles and microorganisms became trapped in the curve of the neck and could not reach the sterile broth.
Observation: As long as the flasks remained intact and upright, the broth stayed clear and completely sterile indefinitely. But, if the flask was tilted so the broth touched the trapped dust, or if the neck was broken off, microbial growth appeared rapidly and the broth turned cloudy.
Conclusion: This experiment conclusively proved that microorganisms do not generate spontaneously from non-living broth. Instead, life only arises from pre-existing microbes present in the environment, establishing the principle of biogenesis.
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