Plants primarily absorb carbon dioxide (CO2) from the atmosphere during the process of photosynthesis. This gas is essential for their growth, as it provides the carbon needed to produce glucose, a vital energy source for plants. During this process, plants take in sunlight, which helps convert CO2 and water into glucose and oxygen. The absorbed CO2 is stored as carbohydrates, which are used for energy and growth. This conversion process not only supports plant life but also plays a critical role in regulating atmospheric CO2 levels, contributing to the overall health of the planet's ecosystems.