Function: The chloroplast is a plant organelle that conducts photosynthesis. The photosynthetic pigment chlorophyll captures energy from sunlight and converts it to the energy-storage molecules ATP and NADPH. Structure: The chloroplast has a double outer membrane, with internal stacks of membraneous disks called thylakoids. The thylakoid stacks are attached together by structures called lamellae. The remainder of the internal liquid volume of the chloroplast is called the stroma.