Which pigments serve as accessory pigments alongside the primary pigment in photosynthesis?

• A. Chlorophyll b and carotenoids.
• B. Chlorophyll a and chlorophyll b.
• C. Hemoglobin and myoglobin.
• D. Phycobilins and xanthophylls.

Answer: (A)

Accessory pigments expand the range of light wavelengths that plants can use and help transfer that energy to the primary pigment in the reaction center. Chlorophyll a is the main pigment that drives the light reactions, but it doesn’t absorb all colors well on its own. Chlorophyll b acts as an additional absorber, picking up blue light and passing that energy efficiently to chlorophyll a. Carotenoids, including various xanthophylls, absorb blue and green light and also protect the photosynthetic apparatus by quenching reactive molecules; they likewise funnel their absorbed energy to chlorophyll a. That combination—chlorophyll b plus carotenoids—provides a broader, more efficient light-harvesting system in photosynthesis.

Hemoglobin and myoglobin aren’t involved in photosynthesis; they’re heme proteins used for oxygen transport in animals. The pairing of chlorophyll a with chlorophyll b would make both primary pigments, not accessory pigments. Phycobilins occur in cyanobacteria and some algae and function in those organisms’ light-harvesting systems, while xanthophylls are carotenoids, but the standard accessory-pigment set alongside the primary pigment in green plants is chlorophyll b plus carotenoids.