Pasteur's swan-neck flask experiments demonstrated what about microbial growth?

• A. Dust is irrelevant
• B. Air carries microbes and dust trapped in bend prevents contamination
• C. Heat alone prevents growth
• D. Life emerges spontaneously

Answer: (D)

Pasteur’s experiment shows that microbes come from the surrounding environment, not spontaneously arise in the broth. The curved neck of the flask lets air in but traps dust and the microbes it carries in the bend, so the sterile broth remains uncontaminated as long as the neck is intact. When the neck is broken or removed, microbes from the air can reach the broth and grow, proving that contamination comes from outside the broth rather than from nothing within it. This demonstrates biogenesis: life comes from pre-existing life in the environment, not from spontaneous generation.