Biological activity requires utilization of energy, which comes from the sun. Solar energy is converted into chemical energy by the process of photosynthesis. This chemical energy is then transformed into mechanical and heat forms during metabolic activities. The flow of energy in the biological world takes place from the sun to the plants and then to the heterotrophic organisms (animals, humans, microorganism) and so on.

A few generalizations can be made in the flow of energy and inorganic nutrients through the ecosystem.

1.       For most of the ecosystems the vital source of energy is the sun.

2.       The eventual outcome of energy in ecosystems is for it to be lost as heat.

3.       Through the food chain energy and nutrients are passed from one organism to another.  

4.       The decomposers remove the last energy from the remains of organisms.

5.       There is cycling of inorganic nutrients but energy does not recycle

(Energy flow-diagram)