Chlorophyll a is not an accessory pigment
Accessory pigments are pigments that assist chlorophyll in capturing light energy for photosynthesis. They are able to absorb light wavelengths that chlorophyll cannot absorb, thereby expanding the range of light that can be utilized for photosynthesis.

Explanation:

Chlorophyll a:
- Chlorophyll a is the primary pigment involved in photosynthesis.
- It is directly involved in converting light energy into chemical energy during the light-dependent reactions of photosynthesis.
- Chlorophyll a absorbs mainly blue and red light and reflects green light, which is why plants appear green.

Accessory pigments:
- Accessory pigments include molecules like chlorophyll b, phycobilins, and carotenoids.
- These pigments are able to absorb light at different wavelengths compared to chlorophyll a, allowing plants to capture a wider range of light energy for photosynthesis.
- They pass the captured energy to chlorophyll a for the conversion into chemical energy.

Significance:
- Accessory pigments play a crucial role in optimizing the efficiency of photosynthesis by expanding the range of light that can be utilized.
- They help plants to adapt to different light conditions and enhance their overall photosynthetic capacity.
In conclusion, while chlorophyll a is the primary pigment in photosynthesis, accessory pigments like chlorophyll b, phycobilins, and carotenoids act as assistants in capturing light energy. Chlorophyll a is not considered an accessory pigment because of its direct involvement in the primary light-absorption process in photosynthesis.