Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: The chloroplast is the site of photosynthesis. Part of the photosynthesis reactions occur in an internal membrane within the organelle. The chloroplast contains many of these internal membranes, making photosynthesis very efficient. These internal membranes stack on top of each other, just like a stack of pancakes. Answer: chloroplast
What structure, containing many internal membranes, is the site of photosynthesis?