Here's why:

* Photosynthesis: Plants use sunlight, water, and carbon dioxide to create glucose (sugar) and oxygen. This process removes carbon dioxide from the atmosphere and incorporates it into organic molecules.

* Cellular Respiration: Organisms (including plants and animals) break down glucose (sugar) in the presence of oxygen to release energy, water, and carbon dioxide. This process releases carbon dioxide back into the atmosphere.

The continuous cycle of photosynthesis and cellular respiration plays a crucial role in maintaining the balance of carbon dioxide and oxygen in the Earth's atmosphere.