A student prepares two dishes using the same sterile growth medium. No bacteria are deliberately added. One dish is kept covered. The other is left open for ten minutes before being covered. Only the second dish develops colonies.
Which conclusion is best supported?
Microorganisms were produced by the medium after the lid was removed.
Microorganisms entered the exposed dish after the medium had been sterilised.
Microorganisms survived sterilisation equally in both dishes containing the growth medium.
Microorganisms require uncovered dishes throughout incubation to form any visible colonies.
45 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.