Here's why:

* Chloroplasts contain chlorophyll, the green pigment that absorbs light energy.

* They have a complex internal membrane system called thylakoids, which are stacked into structures called grana. These are the sites where light-dependent reactions of photosynthesis occur.

* The stroma, the fluid-filled space surrounding the thylakoids, is where the light-independent reactions (Calvin cycle) take place.

Let me know if you have any more questions about photosynthesis or organelles!