Chlorophyll a (Chl-a) is the necessary pigment for photosynthesis.
While other pigments (accessory pigments) assist in capturing light energy, chlorophyll a is the only pigment capable of directly converting light energy into chemical energy in the form of glucose. This crucial role makes it essential for the process of photosynthesis and, therefore, for the sustenance of most life on Earth. Without chlorophyll a, plants, algae, and cyanobacteria would be unable to perform photosynthesis.
Think of accessory pigments as antennae that gather light of different wavelengths. They then pass this captured energy to chlorophyll a, which acts as the reaction center, using the energy to drive the photosynthetic process. Chlorophyll a resides within the photosystems (Photosystem I and Photosystem II) of plants and other photosynthetic organisms. It's specifically in the reaction center of these photosystems where the critical light-dependent reactions occur.
Therefore, although accessory pigments like chlorophyll b, carotenoids, and phycobilins broaden the spectrum of light that can be used for photosynthesis and protect against photo-damage, they depend on chlorophyll a to ultimately convert the captured light energy into a usable form of chemical energy.
