Photosynthesis is the process in which carbon dioxide and water are used (generally by green plants that contain chlorophyll) in presence of sun light and we obtain sugar molecules and oxygen. This is the process, primarily responsible, for generating food for higher life forms like animals and human beings. Respiration, on the other hand, is the process that makes use of the food that we (or other life forms) consume and converts it into energy (in the form of ATP or adenosine triphosphate molecules) and other useful metabolites. In the process of respiration, oxygen is consumed (as the primary electron acceptor) and carbon dioxide is generated. Photosynthesis requires the sun light and hence cannot take place in absence of sun. In other words, photosynthesis cannot take place at night or in dark places. Respiration, on the other hand, can take place anytime and anywhere (preferably in the presence of oxygen, but can also work without its presence as well, although anaerobic respiration has very low efficiency).
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