In an ecosystem:
(i) the flow of energy IS unidirectional.
(ii) the flow of materials is unidirectional.
(iii) the flow of materials is cyclic.
(iv) the flow of energy is cyclic.

Question

In an ecosystem: 
(i) the flow of energy IS unidirectional. 
(ii) the flow of materials is unidirectional. 
(iii) the flow of materials is cyclic. 
(iv) the flow of energy is cyclic.

Accepted Answer

In an ecosystem, the flow of energy and materials is fundamental to its functioning. Let's examine why Option 4, which states that (i) the flow of energy is unidirectional and (iii) the flow of materials is cyclic, is the correct answer.

1. **Explanation for Option 4:**
   - **(i) The flow of energy is unidirectional:** In an ecosystem, energy flows from the sun to producers (like plants) through photosynthesis, then to consumers (herbivores, carnivores) and finally to decomposers. At each step, energy is lost as heat due to metabolic processes. This loss means energy cannot be recycled within the ecosystem, making the flow of energy unidirectional.
   - **(iii) The flow of materials is cyclic:** Unlike energy, materials such as nutrients and elements (carbon, nitrogen, water) are recycled within an ecosystem. These materials move through biogeochemical cycles, where they are used by organisms and eventually returned to the environment through processes like decomposition, ensuring a cyclic flow.

2. **Why Other Options are Incorrect:**
   - **Option 1 (i) and (ii):** While (i) is correct, (ii) is incorrect because the flow of materials is not unidirectional. Materials are recycled in ecosystems, as explained above.
   - **Option 2 (ii) and (iii):** (ii) is incorrect for the same reason as mentioned above. Although (iii) is correct, the inclusion of (ii) makes this option incorrect.
   - **Option 3 (i) and (iv):** While (i) is correct, (iv) is incorrect because the flow of energy is not cyclic. Energy is lost as heat and cannot be reused by the ecosystem, making it unidirectional.

In conclusion, Option 4 is the correct answer because it accurately describes the nature of energy and material flows in an ecosystem: energy flows in a unidirectional manner, while materials are cycled.