The gas that plants absorb during photosynthesis is carbon dioxide (CO2). This process occurs in the chloroplasts of plant cells, where plants convert sunlight, water, and carbon dioxide into glucose and oxygen. Carbon dioxide enters the plant through tiny openings called stomata and is fixed during the Calvin cycle, ultimately contributing to the plant's energy and growth. This process is fundamental to life on Earth, as it forms the base of the food chain and produces the oxygen essential for many organisms.