Here's why:

* Photosynthesis: This process uses sunlight, water, and carbon dioxide to create sugar (glucose) and oxygen. It's how plants and some other organisms capture energy from the sun.

* Cellular respiration: This process breaks down sugar (glucose) in the presence of oxygen to release energy in the form of ATP. This energy is used to power all the cell's activities.

Think of it this way:

* Photosynthesis: Like a plant factory that uses sunlight to make food (sugar).

* Cellular respiration: Like the cell's power plant that burns the food (sugar) to produce energy.