Attached to the porphyrin is a long, insoluble carbon-hydrogen chain which interacts with the proteins of the thylakoids and serves to anchor the molecule in the internal membranes of the chloroplast. Chlorophyll a is the pigment that participates directly in the light requiring reactions of photosynthesis. Chlorophyll b differs from chlorophyll a only in one of the functional groups bonded to the porphyrin (a -CHO group in place of a -CH3 group). It is an accessory pigment and acts indirectly in photosynthesis by transferring the light it absorbs to chlorophyll a. Alternating single and double bonds, known as conjugated bonds, such as those in the porphyrin ring of chlorophylls, are common among pigments, and are responsible for the absorption of visible light by these substances. Both chlorophylls a and b primarily absorb red and blue light, the colors most effective in photosynthesis. They reflect or transmit green light, which is why leaves appear green. The ratio of chlorophyll a to chlorophyll b in the chloroplast is 3:1.