Distinguish between energy flow and matter cycling in ecosystems.

• A. Energy flows in one direction and is lost as heat, while matter cycles within the ecosystem.
• B. Energy flows in one direction, and matter also flows in one direction.
• C. Energy cycles within the ecosystem, and energy also cycles.
• D. Energy is conserved and does not dissipate; matter cycles.

Answer: (D)

The main idea is that energy and matter behave differently in ecosystems: energy flows through the system and eventually dissipates as heat, while matter is recycled and reused.

Energy enters as sunlight and moves from producers to consumers, but with each transfer some is transformed into heat due to metabolism. Because new solar energy continually enters, energy moves in one direction and isn’t recycled for future use at the same level. Matter, however, cycles through living organisms and the environment—carbon, nitrogen, water, and other nutrients move among air, soil, water, and biomass and are reused again and again through processes like photosynthesis, respiration, decomposition, and mineralization.

That’s why the description that best fits is: energy flows in one direction and is lost as heat, while matter cycles within the ecosystem. The other ideas conflict with how energy and matter actually behave—matter does not run in a single loop, energy is not continually recycled in usable form, and energy isn’t simply conserved without dissipation.