A chloroplast is a specialized organelle found in the cells of plants and some algae. It is responsible for photosynthesis, the process by which these organisms convert sunlight into energy. Chloroplasts contain a green pigment called chlorophyll, which captures light energy and helps transform carbon dioxide and water into glucose and oxygen. Chloroplasts have a double membrane structure and contain their own DNA, which is similar to that of bacteria. This unique feature supports the endosymbiotic theory, suggesting that chloroplasts originated from free-living bacteria that were engulfed by ancestral plant cells.