Explain the flow of energy in an ecosystem. Why is it unidirectional?
How to lay out the answer. Follow the frame, then write it in your own words.
1 · State the principle. Energy flows from the Sun to producers to consumers in one direction, obeying the laws of thermodynamics.
2 · Describe the energy input. Less than 50% of sunlight is PAR; plants capture only 2 to 10% of the PAR.
3 · Describe the transfer. Producers → herbivores → carnivores → top carnivores; about 10% passes to each next level.
4 · Explain the loss. Most energy is used in respiration and lost as heat (second law); dead matter goes to the detritus food chain.
5 · Draw the flow. [Flow diagram: Sun → producers → primary consumers → secondary consumers → tertiary consumers, with a heat-loss arrow from each level]
6 · Say why it is unidirectional. Heat cannot be used again by producers, so energy never cycles back.
7 · Link to real life. Eating at the first trophic level, such as cereals and pulses, uses solar energy most efficiently.
8 · Close in one line. So ecosystems need a constant input of solar energy.
Scoring tip: Quote both numbers: PAR is under 50% of sunlight, and plants fix 2 to 10% of PAR. Then show the 10% law with a short calculation.
