AQA A Level Biology

Revision Notes

7.4.7 Growth Rate of Microorganisms

Apparatus & Techniques: Investigating Growth Rate Using Turbitity Measurements

  • The population growth rate of microorganisms, such as bacteria or yeast, can be investigated by growing the microorganisms in a broth culture
  • The turbidity of the suspension can then be used as a way of estimating the number of cells (i.e. the population size) of the microorganisms in the broth culture
    • Turbidity is simply a measure of the cloudiness of a suspension (i.e. how much light can pass through it)
  • As the microorganisms in the broth culture reproduce and their population grows, the suspension becomes progressively more turbid (cloudy)
  • This changing turbidity can be monitored by measuring how much light can pass through the suspension at fixed time intervals after the initial inoculation of the nutrient broth with the microorganisms
    • A turbidity meter, a light sensor or a colorimeter (connected to a datalogger) can be used to take these measurements
  • The results can then be used to plot a population growth curve to show how the population of microorganism grew over time

Maths Skill: Using Logarithms When Investigating Bacteria

  • Bacterial colonies can grow at rapid rates when in culture with very large numbers of bacteria produced within hours
  • Dealing with the experimental data relating to large numbers of bacteria can be difficult when using traditional linear scales
    • There is a wide range of very small and very large numbers
    • This makes it hard to work out a suitable scale for the axes of graphs
  • Logarithmic scales can be very useful when investigating bacteria

Using logarithms to deal with orders of magnitude

  • Logarithmic scales allow for a wide range of values to be displayed on a single graph
  • For example, yeast cells were grown in culture over several hours. The number of cells increased very rapidly from the original number of cells present
  • The results from the experiment are shown in the graph below, using a log scale
    • The number of yeast cells present at each time interval was converted to a logarithm before being plotted on the graph
    • The log scale is easily identifiable as there are not equal intervals between the numbers on the y-axis
    • The wide range of cell numbers fit easily onto the same scale

Yeast log scale graph, downloadable AS & A Level Biology revision notes

Image showing the number of yeast cells grown in culture over 10 hours, using a logarithmic scale

  • The pH scale is logarithmic
    • The concentration of hydrogen ions varies massively between each pH level

Logarithmic-Ph-Scale, downloadable AS & A Level Biology revision notes

Image showing the range of hydrogen ion concentrations within the pH scale.

Exam Tip

You won’t be expected to convert values into logarithms or create a log scale graph in the exam. Instead you might be asked to interpret results that use logarithmic scales or explain the benefit of using one! Remember that graphs with a logarithmic scale have uneven intervals between values on one or more axes.


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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