Photosynthesis is endergonic (needing energy) because in the process OF photosynthesis, these plants use sunlight and carbon dioxide and water to make glucose which is sugar. It's taking substances to make the energy it needs.
Photosynthesis is an endergonic reaction because the plants are absorbing light energy. It's not heat energy but it's light energy. So they're absorbing that energy to make new stuff. Photosynthesis cannot happen without adding energy.
