Both cellular respiration and photo synthesis are important biological pathways that have a big impact on our life on earth in which energy have a big role in accomplishing these two processes. Exchanging gases particularly oxygen and carbon dioxide between living organism and the environment is one of main responsibilities of these processes. Photosynthesis is used to convert carbon dioxide and water into sugar and oxygen. This sugar, mostly glucose, s used as food for plants. In the other hand, cellular respiration breaks up the sugar into smaller molecule in a catabolic pathway producing carbon dioxide and ATP, the currency of energy. Briefly, all organism, require a constant input of energy. However, they differ in term of how they get it.