A technician sterilises two capped tubes of nutrient broth, X and Y. Using a separate sterile dropping pipette for each tube, the technician adds a few drops of sterile water from the same bottle to each tube.
The tip of the pipette used for tube Y touches the laboratory bench just before it is used. The tip of the pipette used for tube X touches nothing except the water.
Both tubes are recapped and incubated under the same conditions. After incubation, microorganisms are found in tube Y but not in tube X.
Which conclusion is best supported?
The pipette tip carried microorganisms from the bench into tube Y.
The sterile water added to tube Y contained microorganisms.
Airborne microorganisms entered tube Y while its cap was removed.
Microorganisms in the incubator entered tube Y through its replaced cap.
87 exam-style questions on Edexcel GCSE Biology 5.3 Immune system and medicines, covering 5.3.1 The specific immune system, 5.3.2 Response to immunisation, 5.3.3 Advantages and disadvantages of immunisation, 5.3.4 Antibiotics and bacterial infections, 5.3.5 Aseptic techniques in culturing, 5.3.6 Cross-sectional areas of cultures, 5.3.7 Stages of developing new medicines, and 5.3.8 Production of monoclonal antibodies. Each one has a worked solution and a mark scheme showing where the marks go.