Question

A dog breathes out CO2 and a tree releases O2. How are the dog and the tree connected in the oxygen cycle?

Answer 1

During cellular respiration, the dog breathes in oxygen from the tree and releases carbon dioxide when exhaling. During photosynthesis, the tree takes in the carbon dioxide from the dog, and releases the oxygen as a byproduct. So, both the tree and dog are needed to sustain each other.

Answer 2

A dog breathes in oxygen and then releases carbon dioxide when he exhales. Conversely, a tree gives off oxygen and takes in carbon dioxide. So the breath of the dog sustains the tree while the output of oxygen for the tree sustains the dog.