The structure in a plant cell that is responsible for photosynthesis is the chloroplast. Chloroplasts are specialized organelles that contain chlorophyll, the green pigment that captures light energy from the sun. During photosynthesis, chloroplasts convert light energy into chemical energy by transforming carbon dioxide and water into glucose and oxygen, which are vital for the plant's growth and energy needs. This process is not only crucial for plants but also supports life on Earth by producing oxygen and serving as the foundational source of energy for the food chain.