Chloroplasts are cell structures larger than any other organelle except the nucleus; in fact, their ancestors billions of years ago may have been free-living bacteria. Today, chloroplasts are the structural and functional units where photosynthesis takes place— the process whereby green plants use light energy for the synthesis of organic molecules from carbon dioxide and water, with oxygen released as a by-product. They contain the green pigment chlorophyll, which traps light energy for photosynthesis. Overall, unicellular algae may only have one large chloroplast, whereas a plant leaf cell may have between twenty and one hundred chloroplasts.