(l) practical investigations into the effect of factors such as…

The idea

This practical criterion asks you to design and carry out investigations measuring how the rate of respiration responds to changing factors. Respiration rate can be followed by the rate of oxygen uptake (for example using a respirometer with organisms such as germinating seeds or invertebrates) or by the rate of carbon dioxide release (for example using a dye indicator or a CO₂ sensor). You vary one factor at a time: temperature (using a water bath and allowing equilibration), substrate concentration (for example glucose solutions of different molarity supplied to yeast), or the identity of the respiratory substrate (for example glucose versus a lipid or protein source, or different sugars). Control variables include mass or number of organisms, volume, pH and time. Record readings at intervals, calculate a rate (change divided by time), and repeat to reduce random error.

What earns the marks

• Identifies a valid dependent variable for respiration rate, such as oxygen uptake per unit time or carbon dioxide release per unit time, and states how it is measured.
• Describes a controlled experimental design that changes one named factor (temperature, substrate concentration or respiratory substrate) while holding others constant.
• Explains how rate is calculated from raw readings, for example volume change divided by time, and how repeats improve reliability.
• Interprets a trend or graph linking the changed factor to respiration rate, using biological reasoning such as enzyme activity or substrate availability.
• Evaluates at least one source of error or limitation and suggests an improvement, such as equilibrating temperature or sealing apparatus to prevent leaks.

Common mistakes and how to put them right

• Recording only a total volume of gas rather than a rate.

  Put it right: Divide the change by the time interval so the dependent variable is a rate.

• Changing several factors at once, such as temperature and substrate concentration together.

  Put it right: Vary only one independent variable and control the rest.

• Assuming respiration rate always rises with temperature without limit.

  Put it right: Recognise that enzyme denaturation causes a fall above an optimum.

Exam tips

• State clearly which variable is independent, which is dependent and which are controlled before describing the method.
• Show the rate calculation explicitly, for example dividing the volume change by the time taken, so the numerical reasoning is visible.
• Link any trend to respiration biochemistry, such as enzyme activity or substrate availability, rather than describing the graph alone.