Pre AP Biology -CELLS 2.1 Cell Structure and Function- FRQ Exam Style Questions -New Syllabus

(a) Design of the Experiment (Based on Pasteur’s Swan-neck Flask)

To differentiate between the effect of gases and airborne microorganisms, the student needs a method that allows gas exchange but prevents particle entry. The following method ensures validity and safety:

• Step 1 (Setup): Place equal volumes of beef stock into two separate flasks.
• Step 2 (Manipulation): Bend the neck of one flask into an “S” shape (swan-neck). Leave the second flask with a straight, open neck. This acts as the control.
• Step 3 (Sterilization): Boil the beef stock in both flasks vigorously for several minutes. This ensures that any existing microorganisms in the stock are killed (sterilization).
• Step 4 (Variable Control): The swan-neck design allows air (gases like \(O_2\)) to pass freely into the flask but traps heavier airborne dust and microorganisms in the lower bend of the neck. The straight neck allows both air and microorganisms to enter.
• Step 5 (Incubation): Leave both flasks to sit at room temperature (e.g., \(25^\circ\text{C}\)) or in an incubator for several days/weeks.
• Step 6 (Safety): Wear safety goggles to protect eyes from boiling liquid. Use heat-resistant gloves when handling hot glassware. Ensure the Bunsen burner is turned off when not in use.

(b) Prediction and Justification

• Prediction: The beef stock in the straight-neck flask will become cloudy and develop a foul odor, indicating microbial growth. The beef stock in the swan-neck flask will remain clear and sterile indefinitely, despite being exposed to air.
• Conclusion: Since both flasks had access to air gases, but only the straight-neck flask showed growth, it can be concluded that airborne microorganisms (carried on dust), not the gases themselves, are responsible for contamination.
• Contribution to Understanding: This experiment disproves the theory of spontaneous generation.
• It demonstrates that microbial life comes from pre-existing life (biogenesis).
• This knowledge is fundamental to preventing contamination: it highlights that preventing the physical entry of microbes (using seals, filters, or bent tubes) preserves food and ensures sterility.
• This leads to modern aseptic techniques in medicine and food preservation methods like pasteurization and canning.