Investigating photosynthesis – gas exchange

Photosynthesis can be investigated to show gas exchange at different light intensities.

Making oxygen

The production of oxygen by photosynthesis is most easily seen in water plants such as Elodea and Cabomba. The number of bubbles released in a given time can be counted as a measure of the rate of photosynthesis.

The apparatus in the diagram shows how the volume of oxygen produced in a given time may be measured instead.

A lit lamp points at a water-filled beaker containing a thermometer and some pond weed in a test tube.  Rubber tubing connects the test tube to a syringe.

In this experiment, move the syringe to draw the oxygen produced into the capillary tubing. Then measure the length of the oxygen bubble. You can move the lamp towards the pond weed to increase the light intensity.

Carbon dioxide uptake

Hydrogen carbonate indicator is used to show carbon dioxide concentration in solution. It is:

  • yellow in high concentrations of carbon dioxide
  • red in equilibrium with carbon dioxide in the atmosphere
  • purple in low concentrations of carbon dioxide

Place a leaf from a plant in a stoppered boiling tube containing some hydrogen carbonate indicator. You can then investigate the effect of light over a period of a few hours.

The table shows some typical results.

TubeContentsConditionsIndicator turnsConclusion
1LeafLightPurpleThere is an overall absorption of carbon dioxide by a leaf in light
2LeafDarkYellowThere is an overall release of carbon dioxide by a leaf in the dark
3No leafLightRedThis is the control – the two other tubes can be compared with it

Plant cells respire in the light and the dark, releasing carbon dioxide. In the light, photosynthesis can also happen, and carbon dioxide is absorbed from the air. If the light is bright enough, the rate of absorption becomes greater than the rate of release.