How does chlorophyll a differ functionally from chlorophyll b?

Chlorophyll a is the main pigment that initiates photosynthesis by directly participating in the light reactions. Chlorophyll b, on the other hand, acts as an accessory pigment that broadens the range of light a plant can use. 

Both have similar structures but slightly different chemical groups - chlorophyll a has a methyl group (-CH₃), while chlorophyll b has a formyl group (-CHO). This small difference shifts chlorophyll b’s absorption spectrum toward blue light (≈455 nm), allowing plants to capture light in a broader range of wavelengths. 

Chlorophyll b passes its absorbed energy to chlorophyll a in the reaction center, making light harvesting more efficient. Without chlorophyll b, plants would absorb less light, especially under shade conditions. Thus, chlorophyll a is the “engine” of photosynthesis, while chlorophyll b acts like a “booster antenna” to maximize light capture.