Yeast : Fermentation :: Chlorophyll : ? Complete the analogy.

The first word names the agent that makes the process named second possible: yeast brings about fermentation, and chlorophyll makes photosynthesis possible by trapping sunlight. Respiration goes on in every living cell with no chlorophyll needed, transpiration is water lost through the pores of a leaf, and germination is a seed sprouting — none of the three depends on chlorophyll.