AQA · GCSE · Biology

    Course version: 8461 · Version from Sept 2018

    Students should be able to calculate the number of bacteria in a…

    Full specification wording

    Culturing microorganisms (biology only): Students should be able to calculate the number of bacteria in a population after a certain time if given the mean division time.

    Bacteria reproduce by binary fission, so the population can double each mean division time.

    Checked against the AQA 8461 specification (opens in a new tab) on .

    • Point 7 of 11
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Bacteria reproduce by binary fission, so the population can double each mean division time. To find the number after a given time, divide the total time by the mean division time to find the number of divisions, n. The population is then starting number × 2ⁿ. For example, if a bacterium divides every 20 minutes and 2 hours pass, n = 120 ÷ 20 = 6 divisions, so one starting cell becomes 1 × 2⁶ = 64 bacteria. If the starting population is larger, multiply that starting number by 2ⁿ. This assumes ideal conditions with unlimited nutrients and no deaths, so it gives a theoretical maximum.

    What earns the marks

    • Identify the mean division time and the total time over which growth occurs.
    • Convert times to the same unit before dividing, for example both in minutes.
    • Calculate the number of divisions, n, by dividing total time by mean division time.
    • Use the relationship final population = starting number × 2ⁿ.
    • Carry out the power of 2 correctly and multiply by the starting population.
    • Recognise that the calculation assumes ideal conditions and no cell death.

    Common mistakes and how to put them right

    • Multiplying the starting number by 2n instead of 2ⁿ.

      Put it right: Use 2 raised to the power n.

    • Forgetting to convert units so that total time and division time match.

      Put it right: Convert both to the same unit before dividing.

    • Using the total time as the number of divisions.

      Put it right: Divide total time by the mean division time first.

    Exam tips

    • Write the formula final population = starting number × 2ⁿ before substituting values.
    • Show the division step that gives n, then show the power calculation.
    • Check that the answer is realistic and state that it is a theoretical maximum under ideal conditions.

    Try the lesson

    The start of the Culturing microorganisms (biology only): Students should be able to calculate the number of bacteria in a population after a certain time if given the mean division time. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A bacterium has a mean division time of 20 minutes. Starting from one bacterium, how many bacteria are present after 60 minutes?

    Listen

    Bacteria reproduce by binary fission, so the population can double each mean division time. To find the number after a given time, divide the total time by the mean division time to find the number of divisions, n. The population is then starting number × 2ⁿ. For example, if a bacterium divides every 20 minutes and 2 hours pass, n = 120 ÷ 20 = 6 divisions, so one starting cell becomes 1 × 2⁶ = 64 bacteria. If the starting population is larger, multiply that starting number by 2ⁿ. This assumes ideal conditions with unlimited nutrients and no deaths, so it gives a theoretical maximum. A strong answer earns marks for these points. Identify the mean division time and the total time over which growth occurs. Convert times to the same unit before dividing, for example both in minutes. Calculate the number of divisions, n, by dividing total time by mean division time. Use the relationship final population = starting number × 2ⁿ. A common slip: Multiplying the starting number by 2n instead of 2ⁿ. Instead, use 2 raised to the power n. Exam tip: Write the formula final population = starting number × 2ⁿ before substituting values.

    See it

    A top-to-bottom flowchart with six boxes joined by arrows. The first box says identify mean division time and total time. An arrow leads to a box saying convert times to the same unit. An arrow leads to a box saying calculate number of divisions n = total time ÷ mean division time. An arrow leads to a box saying use final population = starting number × 2ⁿ. An arrow leads to a box saying carry out the power of 2 and multiply by starting number. A final arrow leads to a box saying recognise this is a theoretical maximum under ideal conditions with no cell death.
    The sequence of steps for calculating bacterial population size from the mean division time, ending with the reminder that the result is a theoretical maximum.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Calculate number of divisions n = total time ÷ mean division time
    2. 2. Convert times to the same unit
    3. 3. Identify mean division time and total time
    4. 4. Recognise this is a theoretical maximum under ideal conditions with no cell death
    5. 5. Carry out the power of 2 and multiply by starting number
    6. 6. Use final population = starting number × 2ⁿ

    Key words

    Say what each word means before you turn the card.

    Binary fission

    Try answering aloud or in your head before turning the card.

    Mean division time

    Try answering aloud or in your head before turning the card.

    Ideal growth conditions

    Try answering aloud or in your head before turning the card.

    See every definition
    Binary fission
    The process by which bacteria reproduce, so that the population can double each mean division time.
    Mean division time
    The time taken for a bacterium to divide, used to find the number of divisions by dividing the total time by it.
    Ideal growth conditions
    Conditions with unlimited nutrients and no deaths, which the calculation assumes and which give a theoretical maximum.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-to-bottom flowchart with six boxes joined by arrows. The first box says identify mean division time and total time. An arrow leads to a box saying convert times to the same unit. An arrow leads to a box saying calculate number of divisions n = total time ÷ mean division time. An arrow leads to a box saying use final population = starting number × 2ⁿ. An arrow leads to a box saying carry out the power of 2 and multiply by starting number. A final arrow leads to a box saying recognise this is a theoretical maximum under ideal conditions with no cell death.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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