Respirometers and redox indicators

Last time

Before we start: last time, in ATP, coenzymes and glycolysis, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Measuring a breath you cannot see

You cannot watch respiration happen. But you can measure what it uses and what it changes. A respirometer measures the oxygen used. A redox indicator changes colour as hydrogen is released. Both are standard practicals, and both give exam marks for method as well as calculation.

The respirometer

Living organisms, such as woodlice or germinating peas, are placed in a sealed tube. A small tube of soda lime, or potassium hydroxide solution, absorbs the carbon dioxide they produce. As the organisms use oxygen, and the carbon dioxide is absorbed, the volume of gas in the tube falls. The pressure drops, and a drop of coloured liquid in the capillary moves towards the tube. The distance moved shows the volume of oxygen taken in. Divide by time, and you have the rate of respiration.

Making it valid

A control tube contains inert glass beads of the same volume as the organisms, with the soda lime. It shows changes from temperature or pressure in the room. Keep both tubes in a water bath so that temperature is constant. Allow a few minutes for the apparatus to equilibrate before starting. Check that the joints are airtight, use the same mass of organism each time, and repeat at least three times to find a mean.

A past-paper example

May June 2025, Paper 41, question 2. A respirometer is used to investigate how temperature affects respiration in woodlice. Explain how the set up allows the rate to be found. The woodlice use oxygen and produce carbon dioxide. The carbon dioxide is absorbed, so the gas volume falls and the liquid moves. The distance moved over a measured time gives the rate. A rise in temperature increases the rate, up to the point where enzymes denature.

A worked calculation

The liquid in the capillary moves thirty millimetres in ten minutes. The capillary has a radius of zero point five millimetres. Volume is pi times the radius squared times the distance: pi times zero point two five times thirty is twenty-three point six cubic millimetres. Divide by ten minutes: two point four cubic millimetres of oxygen per minute.