Scientific investigation
Bacteria that glow in a crowd
- Unit 4
- 4 points
- About 10 minutes
You can use a calculator on this question, just like on exam day.
A short question about a described lab investigation. You describe a concept behind it, identify part of the method such as a control or a variable to hold constant, state a null hypothesis or predict a result, and justify a prediction. On the exam: Question 3 of 6 in Section II; about 10 minutes suggested.
The question and its sources
A species of marine bacterium glows (gives off light) only when its cells are crowded together at high density. Each cell releases small amounts of a signaling molecule into the surrounding water. Researchers investigated whether this molecule controls when the bacteria glow.
Investigation
The researchers grew one culture of the bacteria to a high density, at which the cells glowed, and another culture to a low density, at which the cells did not glow. They filtered each culture to remove all of the cells, keeping the liquid medium.
They then divided a new low-density culture, which was not glowing, into three groups of five flasks. To each flask they added an equal volume of one of three liquids: fresh medium that had never held bacteria, filtered medium from the low-density culture, or filtered medium from the high-density culture. After 1 hour, they measured the light given off by each flask.
Source: Hypothetical study
Suggested time: 10 minutes
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Part (a)
1 pointDescribe how bacteria use quorum sensing to respond to the density of their population.
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Part (b)
1 pointIdentify the treatment that serves as the negative control in this investigation.
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Part (c)
1 pointPredict the amount of light given off by the flasks that received filtered medium from the high-density culture compared with the flasks that received fresh medium.
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Part (d)
1 pointJustify your prediction in part (c).
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