Question

Main source of energy in ecosystem is
a. Sun
b. Green plants
c. Bacteria
d. Animals

Answer 1

Hint: Energy flow is a sequential process of the movement of energy in the ecosystem. Sun is the only source of energy for all ecosystems on earth. All consumers depend directly or indirectly on green plants for nutrition and solar energy is trapped in green plants.

Complete answer:
The energy coming from the sun known as solar radiation. Of the incident solar radiation less than 50% of it is photosynthetically active radiation (PAR). Plants capture only 2-10% of PAR for the synthesis of organic matter. Energy does not remain trapped permanent in any organisms. It is either passed on to the higher trophic level or becomes available to detritivores or decomposers after the organism dies. Herbivores feed on producers. Part of energy is wasted in digestion and assimilation. Herbivores are eaten by primary carnivores, latter by secondary carnivores and so on. So the energy flow in an ecosystem is always unidirectional.

Solar radiation (sun) $\to$ Producers $\to$ Herbivores $\to$ Carnivores.
The energy which passes from autotrophic plants to the herbivores does not pass back to the autotrophs.

Energy flow follows laws of thermodynamics. Expenditure of energy in the ecosystem is based on the two basic laws of thermodynamics:
- First law of thermodynamics: Energy is neither created nor destroyed, but can be transformed from one state to another.
- Second law of thermodynamics: There is no transfer of energy unless and until it is accompanied by degradation of energy.

Hence, the correct answer is option (A).

Note: Sun is the only source of energy in the ecosystem on earth but there is an exception, in deep sea hydrothermal ecosystem sun is not the source of the energy. Energy flow in an ecosystem is always unidirectional.