Photosynthesis is the process of production of food. Green plants (containing chlorophyll) use this process to convert sunlight energy into chemical energy in the form of glucose. Some other organisms are also capable of photosynthesis, but plants are the major species involved in the process. The process can be represented by the following chemical reaction:
`6 CO_2 + 6 H_2O + light -> C_6H_12O_6 + 6O_2`
This process is the major source of food for most of the organisms and makes green plants the primary producers.
Cellular respiration is, in simplest terms, the process of conversion of food into energy. Cells break down food and generate energy in the form of ATP molecules, as per the following chemical reaction:
`C_6H_12O_6 + 6O_2 -> 6 CO_2 + 6 H_2O + ATP`
In essence, cellular respiration and photosynthesis are complementary reactions to each other. The reactants of one process are the products of the other and hence these processes feed each other.
The two processes take place within cell, photosynthesis within the chloroplast and cellular respiration within mitochondria. Both the processes generate energy, although in different forms: photosynthesis generates food (Which is source of energy and its breakdown releases energy) and cellular respiration generates ATP molecules, whose breakdown releases energy. Both the processes uses gases and generate gases.
There are a number of differences between the two processes. Photosynthesis requires sunlight and hence takes place during daytime, while respiration does not need sunlight and hence takes place continuously. Photosynthesis generates oxygen gas, a necessary gas for our survival and also consumes carbon dioxide, thus decreasing the greenhouse effect. Respiration consumes oxygen and generates carbon dioxide, thus resulting in increased greenhouse effect (which is causing global warming). Photosynthesis produces food, while respiration consumes food. Photosynthesis takes place only in green plants (containing chlorophyll) and some other species. Respiration, on the other hand, takes place in all organisms.
Hope this helps.