Energy flows in a one-way direction i.e. unidirectional flow
Explanation:
Matter and energy both pass through an ecosystem as organisms feed on one another. However, as rightly emphasized in this question, matter in form of elements such as water, nitrogen, phosphorus etc. moves in different directions and is recycled within and between ecosystems through closed loops called biogeochemical cycles.
Contrarily to how matter is recycled in an ecosystem i.e. within and between, ENERGY only flows in one direction. Energy enters an ecosystem in form of light (sun) and is passed down from one trophic level to another starting from the producers. This unidirectional pattern of energy flow differentiates energy flow from matter recycling.
Not sure if I'm correct but my best guess is the cell membrane seeing as it surrounds the cell itself and is responsible for allowing certain molecules to diffuse in or out of the cell. Good Luck! :)
