Quantitative Reasoning – an example

I asked my intro biology students, working in groups to analyze parts of the Wolfe-Simon et al. 2011 arsenic life paper, to calculate how much bacterial growth the background level of phosphate would support:

The basal culture medium contained 3 x 10-6 (3 micromolar) phosphate; this level of phosphate was present in the no-phosphate medium used to grow GFAJ-1 cells. Marine bacteria grown under P-limited conditions contain 2 femtograms (2 x 10-15 g) phosphate per cell. How much cell growth will 3 micromolar phosphate support, in cells/mL, assuming that GFAJ-1 has the same phosphate requirements as the marine bacteria? Express your answer to 2 significant digits, in the form “3.7 x 105 cells/ml”

I saw that many were struggling with this question, so I gave the students additional information, that the molar mass of phosphate is 95, that they could round up to 100.

In the end, 51% arrived at a correct solution. Is this good, bad, satisfactory, unsatisfactory?

Only about 20% of the students are Biology majors; the rest are largely engineering majors, some biochemistry, and assorted other majors.


About jchoigt

I'm an Associate Professor in the School of Biology at Georgia Tech, and Faculty Coordinator of the Professional MS Bioinformatics degree program.
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