Chlorella is a photosynthetic, single-celled protoctist that lives in ponds. A group of students decided to investigate the effect of temperature on respiration in Chlorella. The students were provided with: • test-tubes • metal foil • flat-bottomed tubes with lids • glass beakers • hot water supply • thermometers • timer • pH probe and meter • hydrogencarbonate indicator solution in a closed container • a pump connected to an oxygen supply • a pump connected to a carbon dioxide supply • a suspension of Chlorella with a known cell density. Carbon dioxide is an acidic gas so it causes the pH of the hydrogencarbonate indicator solution to change. The indicator solution changes colour as pH changes. [Figure 1.1] shows the range of colours seen in hydrogencarbonate indicator exposed to different carbon dioxide concentrations. atmospheric carbon dioxide (0.04%) pH 8.4 yellow orange red magenta purple increasing carbon dioxide decreasing carbon dioxide concentration in indicator solution concentration in indicator solution Fig. 1.1 The students made a series of colour standards from yellow to purple by bubbling different volumes of carbon dioxide gas into the indicator solution. The pH was measured with a pH meter for each colour and the solutions were sealed in flat-bottomed tubes labelled with their pH. The students used the indicator solution to investigate the effect of temperature on carbon dioxide production during respiration in Chlorella.