Why we breathe oxygen while plants breathe carbon dioxide?

By using the energy of sunlight, plantscan convert carbon dioxide and water into carbohydrates and oxygen in a process called photosynthesis. As photosynthesis requires sunlight, this process only happens during the day.We often like to think of this as plants`breathing in carbon dioxide and `breathing out oxygen

Basically when air fills our alveoli, by the process of diffusion, only oxygen in the air is taken into the blood stream while the other gases along with the waste CO2 is exhaled. So you do breathe in nitrogen, but it is exhaled as it is by the body.