Biology · Ecosystems · NEET
Energy flow is unidirectional (one direction only). It goes from the Sun to producers, then to consumers, and finally leaves as heat. It never comes back. This is different from nutrients like carbon and nitrogen, which move in a cycle. NEET often confuses students here: nutrients cycle, but energy flows one way and is lost. Statement-type PYQs (2025) test exactly this point.
Step 1: The Sun sends light energy to Earth. Step 2: Less than 50% of this is photosynthetically active radiation (PAR). Step 3: Plants (autotrophs) capture only 2-10% of the PAR and make food. Step 4: This chemical energy passes to herbivores (primary consumers), then to carnivores. Step 5: At each step most energy is lost as heat. So energy keeps decreasing as it moves up.
Because at every trophic level, a large part of the energy is used for life processes and lost as heat during respiration. Once energy becomes heat, it cannot be captured again by organisms. So it leaves the ecosystem and cannot go back to the producers. This is why energy flow is called unidirectional and why the pyramid of energy is always upright.
Yes. Energy flow follows the first law: energy is not created or destroyed, only changed from light to chemical to heat. It also follows the second law: ecosystems are NOT exempt from it, so they always lose usable energy as heat and need a constant supply from the Sun. A 2025 NEET statement question marked 'ecosystems are exempted from the 2nd law' as FALSE.
The Sun is the source of energy for almost all ecosystems on Earth. The only exception is the deep-sea hydro-thermal ecosystem, which uses chemical energy. Plants and photosynthetic bacteria fix the Sun's energy into food, and this small captured energy supports the entire living world.
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. Choose the most appropriate answer.
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). Choose the most appropriate answer.
Which of the following statements are correct? (a) Energy flow from producers to consumers is unidirectional; (b) Energy pyramid can never be inverted; (c) Transfer of energy follows the 1% law.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the movement of energy from the Sun to green plants, then to animals, and finally lost as heat. It always moves in one direction and gets less at each step.
Plants capture only about 2-10% of the photosynthetically active radiation (PAR). This small amount supports the entire living world. Note that less than 50% of incident solar radiation is PAR.
Because energy moves only from the Sun to producers to consumers, and never back. At each level much of it is lost as heat during respiration and cannot be reused.
The deep-sea hydro-thermal ecosystem. There, organisms use chemical energy instead of sunlight. Every other ecosystem depends on solar energy.
No. Energy is only changed from one form to another (light to chemical to heat). Nothing is created or destroyed, so the first law holds. The second law also holds, because usable energy is always lost as heat.
