Here's why:

* Outer membrane: A smooth outer membrane that surrounds the entire organelle.

* Inner membrane: A folded inner membrane called the thylakoid membrane.

* Thylakoid membrane: This membrane forms flattened sacs called thylakoids, which stack into structures called grana. Within the thylakoid membrane, the light-dependent reactions of photosynthesis occur.

* Stroma: The fluid-filled space between the inner membrane and the thylakoid membrane, where the Calvin cycle (light-independent reactions) of photosynthesis takes place.

Let me know if you have any other questions about organelles!