Here's why:

* Photosynthesis: This process uses sunlight, water, and carbon dioxide to produce glucose (sugar) and oxygen. It's how plants and some other organisms make their own food.

* Cellular respiration: This process breaks down glucose (sugar) in the presence of oxygen to release energy in the form of ATP. This energy is used by cells for various functions.

In essence, photosynthesis builds complex molecules (sugars) from simpler ones, while cellular respiration breaks down complex molecules (sugars) to release energy. They are complementary processes that are essential for life on Earth.