Interpreting Graphs 9

Some biological quantities cover a very wide range of values, for example the number of bacteria in a culture, which can increase from 10 to 10 000 000 cells. These are often plotted on a logarithmic scale.On a log scale each major division represents a tenfold increase: 1, 10, 100, 1000 and so on. The divisions are equally spaced even though the values are not.The values may be written as powers of ten, for example 10³ = 1000, or as log₁₀ values, for example log₁₀ 1000 = 3.When bacteria divide by binary fission at a constant rate, their number doubles in each generation. This exponential growth gives a curve on a normal scale but a straight line on a log scale.When reading a log scale, remember that the small divisions between two major lines are not equal steps. Read major divisions carefully and use standard form in your answer.Example: if a population rises from 10² to 10⁵ cells, it has increased 10³ times, or 1000-fold.