Structure of the Chloroplast Chloroplasts are organelles found in the green leaves of plants and some algae. They are double-membraned organelles with a smooth outer membrane and an inner membrane, which scientists believed formed when prokaryotes engulfed one another to form eukaryotic cells (endosymbiotic theory). There are rmore membranes folded into disc-shaped sacs called thylakoids, which contain the green pigment chlorophyll responsible for absorbing sunlight. Thylakoids are found in stacks called granum (grana, plural). Grana are surrounded by a gel-like material called stroma. le 1. In what types of organisms are chloroplasts found? 2. How many membranes surround a chloroplast and why? 3. What is chlorophyll and in what structure is it found?