Energy Flow is Unidirectional in Ecosystems

Steps:

• Recall that energy enters ecosystems mainly from the sun via producers like plants.
• Understand that energy transfers through trophic levels (producers to consumers to decomposers) but is lost at each step as heat.
• Note that this loss follows the second law of thermodynamics, making energy flow one-way.
• Conclude that energy cannot recycle back to the start, unlike nutrients.

Why C is correct:

• Energy dissipates as heat during transfers, per the second law of thermodynamics, preventing complete return to the original source like sunlight.

Why the others are wrong:

• A: Energy changes form (e.g., light to chemical) and is not passed unchanged.
• B: Energy passes to all trophic levels, not just larger organisms.
• D: Energy transforms within organisms (e.g., glucose to ATP).

Final answer: C