Here's why:

* Photosynthesis: Plants and some other organisms use sunlight, water, and carbon dioxide to produce glucose (sugar) and oxygen. This process stores energy from sunlight in the chemical bonds of glucose.

* Cellular Respiration: All living organisms (including plants) break down glucose (sugar) in the presence of oxygen to release energy in the form of ATP (adenosine triphosphate). This energy is then used for various cellular processes like growth, movement, and repair.

In essence, photosynthesis creates fuel (glucose), and cellular respiration burns that fuel to release energy.

They are complementary processes that together form the basis of the energy flow in ecosystems.