OCR · A-Level · Biology

    Course version: H420 · Version from Sept 2025

    (ii) practical investigations into respiration rates in yeast, under…

    Full specification wording

    Respiration: (ii) practical investigations into respiration rates in yeast, under aerobic and anaerobic conditions

    Respiration rate in yeast can be measured by the rate of carbon dioxide production or by the rate of oxygen uptake.

    Checked against the OCR H420 specification (opens in a new tab) on .

    • Point 10 of 13
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Respiration rate in yeast can be measured by the rate of carbon dioxide production or by the rate of oxygen uptake. Under aerobic conditions yeast respires aerobically, taking up oxygen and releasing carbon dioxide; under anaerobic conditions it ferments, releasing carbon dioxide and ethanol without net oxygen uptake. A common method suspends yeast in a glucose solution and collects gas in a syringe or gas jar, or uses a respirometer to measure oxygen uptake; a dye such as hydrogencarbonate indicator or limewater can detect carbon dioxide. Anaerobic conditions are created by sealing the vessel or layering oil over the culture. Rate is calculated by dividing the volume of gas evolved or taken up by the time taken, and controlled variables include temperature, glucose concentration, yeast mass and pH.

    What earns the marks

    • Respiration rate can be measured as the rate of carbon dioxide production or the rate of oxygen uptake over a measured time.
    • A respirometer measures oxygen uptake, whereas gas collection or an indicator measures carbon dioxide release.
    • Anaerobic conditions are produced by sealing the vessel or covering the culture with a layer of oil.
    • Rate is calculated by dividing the gas volume by the time, and controlled variables include temperature, glucose concentration, yeast mass and pH.
    • A water bath keeps temperature constant because respiration is enzyme-controlled, and repeats plus a control improve reliability.

    Common mistakes and how to put them right

    • Measuring only carbon dioxide and assuming it proves aerobic respiration.

      Put it right: Carbon dioxide is released in both aerobic and anaerobic respiration, so oxygen uptake must be measured to show aerobic activity.

    • Failing to control temperature.

      Put it right: Temperature affects enzyme activity, so a water bath must keep it constant.

    • Forgetting to seal the vessel for anaerobic conditions.

      Put it right: Oxygen must be excluded, for example by sealing or layering oil.

    • Recording a single reading.

      Put it right: Repeats and a control are needed to improve reliability.

    Exam tips

    • State clearly what is measured (gas volume) and how rate is calculated (volume ÷ time).
    • Identify the independent, dependent and controlled variables explicitly.
    • Explain how anaerobic conditions are achieved and how a control is set up.

    Try the lesson

    The start of the Respiration: (ii) practical investigations into respiration rates in yeast, under aerobic and anaerobic conditions lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which piece of apparatus is most suitable for measuring the volume of gas produced by respiring yeast over time?

    Listen

    Respiration rate in yeast can be measured by the rate of carbon dioxide production or by the rate of oxygen uptake. Under aerobic conditions yeast respires aerobically, taking up oxygen and releasing carbon dioxide; under anaerobic conditions it ferments, releasing carbon dioxide and ethanol without net oxygen uptake. A common method suspends yeast in a glucose solution and collects gas in a syringe or gas jar, or uses a respirometer to measure oxygen uptake; a dye such as hydrogencarbonate indicator or limewater can detect carbon dioxide. Anaerobic conditions are created by sealing the vessel or layering oil over the culture. Rate is calculated by dividing the volume of gas evolved or taken up by the time taken, and controlled variables include temperature, glucose concentration, yeast mass and pH. A strong answer earns marks for these points. Respiration rate can be measured as the rate of carbon dioxide production or the rate of oxygen uptake over a measured time. A respirometer measures oxygen uptake, whereas gas collection or an indicator measures carbon dioxide release. Anaerobic conditions are produced by sealing the vessel or covering the culture with a layer of oil. Rate is calculated by dividing the gas volume by the time, and controlled variables include temperature, glucose concentration, yeast mass and pH. A common slip: Measuring only carbon dioxide and assuming it proves aerobic respiration. Instead, carbon dioxide is released in both aerobic and anaerobic respiration, so oxygen uptake must be measured to show aerobic activity. Exam tip: State clearly what is measured (gas volume) and how rate is calculated (volume ÷ time).

    See it

    A flowchart starting with yeast in glucose solution, which branches into two paths. The aerobic path leads to oxygen uptake measured by a respirometer and to carbon dioxide release. The anaerobic path, created by sealing the vessel or layering oil, leads to carbon dioxide and ethanol release with no net oxygen uptake. Both paths feed into the calculation of rate as gas volume divided by time, which then leads to the controlled variables: temperature, glucose concentration, yeast mass and pH.
    Shows how yeast respiration is investigated under aerobic and anaerobic conditions and how the rate is calculated from gas volume and time.

    Key words

    Say what each word means before you turn the card.

    Respiration rate

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

    Anaerobic conditions

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

    Respirometer

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

    Controlled variables

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    See every definition
    Respiration rate
    Calculated by dividing the volume of gas evolved or taken up by the time taken.
    Anaerobic conditions
    Created by sealing the vessel or layering oil over the culture, so that oxygen is excluded.
    Respirometer
    Used to measure oxygen uptake, whereas gas collection or an indicator measures carbon dioxide release.
    Controlled variables
    Include temperature, glucose concentration, yeast mass and pH, with a water bath keeping temperature constant because respiration is enzyme-controlled.

    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 flowchart starting with yeast in glucose solution, which branches into two paths. The aerobic path leads to oxygen uptake measured by a respirometer and to carbon dioxide release. The anaerobic path, created by sealing the vessel or layering oil, leads to carbon dioxide and ethanol release with no net oxygen uptake. Both paths feed into the calculation of rate as gas volume divided by time, which then leads to the controlled variables: temperature, glucose concentration, yeast mass and pH.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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