Figure 20.3 Measuring CO2 uptake and release. (a) In a sealed container illuminated by a light, sensors can detect a decline in CO2 in the air as a leaf conducts photosynthesis. This decline occurs because plant photosynthesis consumes CO2 at a faster rate than plant respiration produces CO2. (b) When the light is turned off, sensors detect an increase in CO2 in the air because the leaf only undergoes respiration.