All the living organisms requires energy to perform vital activities. They obtain energy by different ways and use it in various body functions. The animals obtain energy from plants and other organisms in the form of food.
The flow of energy in the nature is unidirectional. The green plants traps solar radiations and convert them into chemical energy by the process of photosynthesis. Hence the plants are called as producers. The herbivorous animals feed on plants to get energy and are called as primary consumers. From the herbivores, in turn are eaten by carnivores and predators which are called as secondary and tertiary consumers respectively. The decomposers (microorganisms) feed on dead energy which finally lost in the environment. Hence, the solar energy passes through the various levels of the biological system and finally lost in the environment. It never returns to the sun. Hence, it is called as unidirectional flow of energy.
The total energy in a biological system, both usable and decayed energy, is called as Enthalpy. The usable energy which is available to perform the work is called as free energy.
According to the first law of thermodynamics (law of conversion), the energy is neither be created nor destroyed in an isolated system. It can be converted from one form to another form.
According to second law of thermodynamics, the state of entropy of the entire universe will always increase overtime. Further, the changes in the entropy in the universe can never be negative. There is constant flow of energy in biological system.
