There is another reason to use semi-log paper: it reminds you that the
correct way of determining whether an organism is growing exponentially is
by plotting Log (cell no.) vs. time. All too often students fall into the
logical trap of saying that the A value is already a log function (due to
the Beer-Lambert law) and therefore they can use an arithmetic plot. They
forget that they are not really measuring A, but using it as a surrogate
measure of light scattering. They also forget that if your eye cannot see
turbidity,neither can the spectrophotometer. I have seen many a growth curve
plottted with A values of 0.001, 0.002, etc. I don't know what is measured,
pobably scratches in the glass. The rule I learned was that A values are
linearly proportional to cell mass(or volume) between A=0.1 and A =0.8.
Thus the log of a value in this range is equivilant to (Log cell #l) as far
as plotting is concerned.
David Kafkewitz, Department of Biological Sciences,
Rutgers University, Newark N.J. 07102, U.S.A.
201 648 5865; fax: 201 648 1007
kafkwtz at andromeda.rutgers.edu