Chloroplasts bounded by three rather than the usual two membranes, which has led to suggestions that the chloroplasts in dinoflagellates were originally symbiotic algae. They are large organelles that, like mitochondria, are bounded by a double membrane called the chloroplast envelope. In addition to the inner and outer membranes of the envelope, chloroplasts have a third internal membrane system, called the thylakoid membrane. Within the envelope membranes, in the region called the stroma, there is a system of interconnecting flattened membrane compartments, called the thylakoids. The thylakoid membrane is quite similar in lipid composition to the inner envelope membrane, containing 78% galactolipids, 15.5% phospholipids and 6.5% sulfolipids in spinach chloroplasts. The thylakoid membrane encloses a single, continuous aqueous compartment called the thylakoid lumen. The membranes of the thylakoid contain photosystems I and II which harvest solar energy to excite electrons which travel down the electron transport chain.