We know that energy and mass are neither created nor destroyed. They are just converted or transformed to other forms and ways. The two most important metabolic processes are the photosynthesis and cellular respiration.
Photosynthesis is the process of converting the light energy, usually coming from the sun, in the form of chemical energy that can be used as energy source of any living organisms. Plants and other organisms capable of doing photosynthesis are often called as the primary producers since they can be able to supply biomass or "food" to other organisms.
In general, we can sum up the overall reaction of photosynthesis as:
`6 CO_2_(g) + 6H_2O _((l)) + light en ergy -> C_(6)H_(12)O_(6) _(s) + 6 O_2 _(g) `
We can see that carbon dioxide and water are converted into glucose and oxygen with the help of light energy. The masses are not destroyed but are transformed into different form that can be beneficial to other organisms.
Cellular respiration is essentially is the "reverse" of the photosynthesis. It is a series of metabolic processes in which an organism is capable of converting the chemical energy into a more usable form of energy that can sustain the growth of the organism. This form of energy is the ATP or adenosine triphosphate. All organisms are capable of performing cellular respiration (aerobic and anaerobic).
The over-all reaction of aerobic cellular respiration can be simplified as:
`C_(6)H_(12)O_(6) _(s) + 6 O_2 _(g) ->6 CO_2 _(g) + 6H_2O _((l)) + en ergy `
We can see that the energy and mass are transformed in other forms via cellular respiration and photosynthesis. Plants breathe carbon dioxide and expel oxygen. On the contrary, humans and animals breathe oxygen and expel carbon dioxide. These are vital processes that sustain the life of the organisms in this planet.