The primary gas used by plants during photosynthesis is carbon dioxide (CO2). Plants absorb carbon dioxide from the atmosphere through small openings in their leaves called stomata. In the presence of sunlight, chlorophyll within the plant cells captures light energy, which then facilitates the conversion of carbon dioxide and water into glucose and oxygen. This process occurs primarily in the chloroplasts of plant cells. The glucose produced provides energy and is used for growth, while oxygen is released as a byproduct, contributing to the atmosphere's oxygen supply. Photosynthesis is fundamental for plant life and sustaining the earth's ecosystems.