Required practical 9: Investigation into the effect of a named variable on…

The idea

The organism is usually yeast, and the named variable might be temperature, substrate concentration or the type of sugar supplied. Rate can be measured by the volume of carbon dioxide produced, collected in a gas syringe, or by the time taken for a redox indicator such as methylene blue or TTC to change colour as it is reduced by hydrogen from the respiring cells. In a respirometer, carbon dioxide is absorbed by potassium hydroxide or soda lime, so the liquid movement in the manometer measures oxygen uptake alone, not carbon dioxide release. A control tube containing boiled yeast corrects for pressure changes caused by temperature. Controlled variables include the volume and concentration of the yeast suspension, the volume of substrate, pH, and the time allowed for equilibration in the water bath before readings begin.

What earns the marks

• A valid measure of rate, such as volume of gas produced per unit time or the reciprocal of the time taken to decolourise the indicator.
• Naming controlled variables, including yeast concentration, substrate volume, pH and temperature where it is not the independent variable.
• Using a water bath and allowing the apparatus to equilibrate before readings are taken.
• A control such as boiled or dead yeast, to show the change is caused by living cells.
• Repeating readings and calculating a mean.
• Stating that in a respirometer, carbon dioxide is absorbed by potassium hydroxide or soda lime so that liquid movement measures oxygen uptake alone.

Common mistakes and how to put them right

• Taking a single reading with no repeats, so that no mean can be calculated.

  Put it right: Repeat each condition and calculate a mean.

• Starting the timer before the apparatus has reached the temperature of the water bath.

  Put it right: Allow equilibration in the water bath before taking readings.

• Leaving potassium hydroxide out of a respirometer, so that carbon dioxide released masks the oxygen taken up.

  Put it right: Include potassium hydroxide or soda lime to absorb carbon dioxide so liquid movement measures oxygen uptake alone.

• Judging a colour change by eye with no colour standard, then treating the end point as precise.

  Put it right: Use a colorimeter or a colour standard to make the end point objective.

• Treating the decolourisation of methylene blue as proof of aerobic respiration specifically, when it shows dehydrogenase activity.

  Put it right: Describe it as a measure of dehydrogenase activity, not proof of aerobic respiration.

Exam tips

• Say how you will calculate the rate before you describe the method, because the units tell the examiner what you measured.
• Name the control and say what it controls for; a boiled yeast tube is the usual answer.
• If asked to improve the investigation, offer more repeats, narrower intervals between values of the independent variable, and a colorimeter in place of judging colour by eye.