Here's why:

* Chloroplasts are the sites of photosynthesis in plants and algae. They contain chlorophyll, the green pigment that absorbs sunlight energy for photosynthesis.

* Not all plants and algae have chloroplasts. Some plants are parasitic and don't need to photosynthesize, and some algae are heterotrophic.

* Other photosynthetic organisms exist that are not plants or algae. These include some bacteria which have different structures for photosynthesis.

Let me know if you'd like more information about chloroplasts!