Pre AP Biology students used encapsulated algae beads to simultaneously study photosynthesis and cellular respiration. Algae beads were placed in a pH indicator solution sensitive to changes in CO₂ concentrations. The beads were then exposed to either light or dark conditions. Over time the students observed shifts in pH as indicated by a color change in the solution. The goal for the student was to apply prior knowledge to explain that photosynthesis fixes the carbon from dissolved CO₂ in the solution resulting in higher pH values. Inversely, cellular respiration releases CO₂ lowering the pH of the indicator solution as the CO₂ combines with the water to form an acid.