Here's why:

* Photosynthesis is the process where plants use sunlight, water, and carbon dioxide to create their own food (glucose) and oxygen.

* Sunlight is the primary source of energy for this process.

* The energy from sunlight is absorbed by chlorophyll (the green pigment in plants) and used to convert carbon dioxide and water into glucose and oxygen.

So, in essence, photosynthesis is an endergonic reaction, meaning it requires energy input (from sunlight) to occur.