AQA GCSE Biology

Revision Notes

1.1.7 Culturing Microorganisms

Binary Fission

  • Bacteria multiply by a type of simple cell division known as binary fission
  • In the right conditions, a bacterial cell prepares to divide by replicating its genetic material before it increases in size
  • A copy of each piece of circular DNA moves to each end of the cell before the cytoplasm divides, and new cell walls form around each daughter cell

Binary fission in bacteria, IGCSE & GCSE Biology revision notes

Each division of one cell produces two cells, so the number of cells increases by a power of 2 each time binary fission occurs

Growing Bacterial Cultures in the Lab

  • The effect of disinfectants and antibiotics on microorganisms can be investigated using cultures of bacteria grown in the lab
  • In the right conditions, some species of bacteria (such as coli) can multiply as much as once every 20 minutes. This is ideal as large cultures of bacteria for study can be grown in relatively short periods of time
  • To multiply this quickly, bacteria require an adequate supply of nutrients (carbohydrates, proteins, minerals and vitamins) and an appropriate temperature (which varies depending on the species being grown)
    • Warmer temperatures promote faster growth, but in a school lab the maximum allowed temperature for growth is 25°C
    • Above this temperature, more harmful pathogens are likely to grow
  • Bacteria can be grown in a nutrient broth solution or as colonies on an agar gel plate

Uncontaminated Cultures & Aseptic Techniques

  • It is vital that uncontaminated cultures of microorganisms are grown in the lab
  • The presence of competing species can affect the growth of cultures, as well as the validity of any study performed on them
  • Some important aseptic techniques are outlined in the table below:

Uncontaminated culture preparation table

Uncontaminated culture preparation table, downloadable IGCSE & GCSE Biology revision notes

Exam Tip

Only lifting the lid of the petri dish a little is vital to reduce the risk of contamination by other microorganisms. It is not to prevent air from entering, as air is still required by bacteria grown in school labs (more harmful bacteria tend to be anaerobic).

Calculating Inhibition Zone Area

  • The effectiveness of different antibiotics, antiseptics or disinfectants can be determined by calculating the area of an inhibition zone around a disc of the substance being tested
  • To calculate the area of an inhibition zone you should use the equation:

 

Calculating area, downloadable IGCSE & GCSE Biology revision notes

Calculating area

Worked example

Calculating zones of inhibition, downloadable IGCSE & GCSE Biology revision notes

Calculating the area of a clear zone is a far more accurate way of comparing the effect of different substances on bacterial growth than trying to judge by sight

Exam Tip

It is far more accurate to measure the diameter of an inhibition zone than the radius, but remember to half it before using the area equation above.

You will need to take at least two measurements of the diameter of an inhibition zone and find the mean to calculate if the clear zone is not perfectly circular.

Calculating Bacteria in a Population

  • The average amount of time it takes for a bacterial cell in a population to divide is the mean division time
  • The number of times a cell has divided and how many cells it produces can be determined if you know the mean division time and how long division has been occurring

Worked example

Worked Example_ Bacterial Growth, downloadable IGCSE & GCSE Biology revision notes

Exam Tip

Check that both the mean division time and the time the cell has been dividing have the same unit (either hours or minutes).

Higher Tier Only

Calculations in Standard Form

  • If you are calculating the number of bacteria present in a population, you are likely to be handling very large numbers (as bacteria can multiply very quickly in certain conditions)
  • You should be able to express an answer for the number of bacteria in standard form

Worked example

Worked Example: Standard Form, downloadable IGCSE & GCSE Biology revision notes

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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