This is a chlorophyll molecule. It contains a porphyrin ring and a long hydrocarbon tail. The hydrophobic tail anchors the molecule into a membrane. The porphyrin ring contains electrons that absorb light energy (photons) and begin the process of photosynthesis. Chlorophyll a has a methyl group (-CH3) at position -R in the porphyrin ring, whereas chlorophyll b contains an aldehyde group (-CHO) in that position.