Chloroplasts play a primary role in plant cells by conducting photosynthesis, the process through which plants convert sunlight into chemical energy. They contain chlorophyll, the green pigment responsible for capturing light energy, which is then used to convert carbon dioxide and water into glucose and oxygen. This process not only fuels plant growth but also contributes significantly to the Earth's atmosphere by producing oxygen and acting as a carbon sink, thus playing a crucial role in regulating climate and sustaining life on the planet.