Here's why:

* Cellular respiration breaks down glucose to produce ATP (energy) and releases carbon dioxide as a byproduct. It occurs in the mitochondria of eukaryotic cells.

* The Calvin cycle is part of photosynthesis, where light energy is used to convert carbon dioxide into glucose. This process takes place in the chloroplasts of plant cells.

Therefore, while both processes involve carbon dioxide and glucose, they are separate and have opposite functions:

* Cellular Respiration: Glucose --> Energy (ATP) + CO2

* Photosynthesis (Calvin cycle): CO2 + Energy (light) --> Glucose

Think of them as two sides of the same coin. Cellular respiration uses the glucose produced by the Calvin cycle to fuel cellular processes, while photosynthesis uses the CO2 released by cellular respiration to create glucose.