Biology · Ecosystems · NEET
Almost all, but not every single one. NCERT says: 'Except for the deep sea hydro-thermal ecosystem, sun is the only source of energy for all ecosystems on Earth.' In deep-sea hydrothermal vents there is no sunlight, so bacteria use chemical energy (chemosynthesis) instead. For NEET, remember this one exception. If a statement just says 'sun is the primary/basic source of energy in an ecosystem', that is treated as correct.
Less than 50 percent. Of all the sunlight (incident solar radiation) that falls on Earth, less than half of it is Photosynthetically Active Radiation (PAR) - the part of light that plants can actually use for photosynthesis. This exact NCERT line is a favourite NEET fact, often asked as 'less than 50%'.
Only 2 to 10 percent of the PAR. Even out of the usable PAR, plants and photosynthetic bacteria trap just a small slice - 2 to 10 percent. NCERT stresses that 'this small amount of energy sustains the entire living world', which is why energy flow matters so much.
Total (incident) solar radiation is the whole sunlight reaching Earth, including UV, visible and infrared. PAR is only the part of that light (roughly the visible range) that plants can use to drive photosynthesis. PAR is less than 50% of total solar radiation, and plants then capture only 2-10% of that PAR.
Because producers (autotrophs) fix the Sun's radiant energy to make food from simple inorganic materials. Every other organism - herbivores, carnivores, decomposers - depends on this fixed energy directly or indirectly. So the Sun is where the energy of the whole ecosystem begins, making it the 'primary' source.
Given below are two statements: Statement I: The primary source of energy in an ecosystem is solar energy. Statement II: The rate of production of organic matter during photosynthesis in an ecosystem is called net primary productivity (NPP). In the light of the above statements, choose the most appropriate answer:
Given below are two statements: Statement I: In an ecosystem, there is a unidirectional flow of energy from the sun through producers to consumers. Statement II: Ecosystems are exempted from the 2nd law of thermodynamics. In the light of the above statements, choose the most appropriate answer:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The deep-sea hydrothermal vent ecosystem. There is no sunlight in the deep ocean, so bacteria use chemical energy from vent minerals (chemosynthesis) instead of solar energy.
PAR stands for Photosynthetically Active Radiation - the portion of sunlight that plants can use for photosynthesis. It is less than 50% of the total incident solar radiation.
Producers - green plants and photosynthetic bacteria (autotrophs). They fix the Sun's radiant energy to make food from simple inorganic materials, and all other organisms depend on this food.
Much light is reflected, transmitted, or falls on non-green surfaces, and photosynthesis itself is not fully efficient. So even from the usable PAR, plants trap only 2-10%, yet this tiny amount sustains the entire living world.
Yes. Both the 'less than 50% PAR' and '2-10% capture' figures, plus the 'sun is the primary source of energy' statement, are directly asked (NEET 2025). It is a high-yield, easy-scoring fact in the Ecosystem chapter.
