4.7 Core practical: rate of respiration

The aim

To measure the rate of respiration of small organisms, such as woodlice, maggots or germinating seeds, and see how temperature affects it. This is Edexcel core practical 8.11 and part of OCR PAG B6.

How it works

1. The organisms take in oxygen for aerobic respiration.
2. They give out carbon dioxide, but the soda lime absorbs it.
3. So the volume of gas in the tube decreases, and the pressure falls.
4. The coloured liquid in the capillary tube is pulled towards the organisms.

The distance the liquid moves in a set time shows how much oxygen was used.

Method

1. Put soda lime in the bottom of a boiling tube and a gauze above it, so the organisms do not touch it (it is corrosive).
2. Put a known mass of woodlice or seeds on the gauze and seal the tube with a bung and capillary tube.
3. Set up a control tube with glass beads of the same mass instead of organisms.
4. Put both tubes in a water bath at a set temperature and leave them to adjust for a few minutes.
5. Add a drop of coloured liquid to the capillary tube and record how far it moves in, say, 5 minutes.
6. Repeat at different temperatures.

To find the volume of oxygen used, multiply the distance by the cross-sectional area of the tube (π r²).

Ethics and safety

Treat the animals carefully: do not let them touch the soda lime, keep the experiment short, and return them to their habitat. Wear eye protection and gloves when handling soda lime.