Assertion: Energy flow is unidirectional.
Reason: First, plants capture solar energy and then, food is transferred from the producers to decomposers.

Both assertion and reason are true and reason is the correct explanation of assertion .

Assertion: Energy flow is unidirectional. This is true. Energy flow in an ecosystem is unidirectional, meaning it flows in one direction only. It enters the ecosystem from an external source (usually the sun in terrestrial ecosystems) and then passes through various trophic levels (producers, consumers, and decomposers) before ultimately leaving the ecosystem in the form of heat. Once energy is used by an organism, it is no longer available for transfer to another organism in the same direction. Energy cannot be recycled within an ecosystem.

Reason: First, plants capture solar energy, and then, food is transferred from the producers to decomposers. This reason is also correct and complements the assertion. In most ecosystems, plants (producers) are the primary organisms that capture solar energy through photosynthesis. They convert this solar energy into chemical energy stored in organic compounds, such as glucose.

Next, this energy is transferred from producers to consumers in a series of trophic levels. Primary consumers (herbivores) eat plants to obtain energy, and then secondary consumers eat primary consumers, and so on. This transfer of energy from one trophic level to another occurs as organisms are consumed and utilized for energy.

Ultimately, the energy reaches the decomposers (such as bacteria and fungi) in the ecosystem. Decomposers break down the organic matter of dead plants and animals, releasing energy in the process. This energy is dissipated as heat and eventually leaves the ecosystem, making the energy flow unidirectional.

In summary, the energy flow in an ecosystem is indeed unidirectional, as solar energy captured by plants is transferred through the food chain from producers to consumers and finally to decomposers, and then exits the ecosystem in the form of heat.