I am searching for an example in the nature, where a layer of something (skin, coat, film etc) can allow fluid to flow trough (not absorb) from one direction and prevent/repel from other direction. I am not talking about vein valves, prefer more like a nano-structure of valves as a layer on/around something. Update: I found something similar what i am searching for, but its not clear:Directly connects the cytoplasm of two cells, allowing various ions and molecules to pass freely between the cells. And there is a reason why this channel is hydrophilic, someone can explain this? About me I have no deep knowledge about biology, please be patient and help me to understand things on basic level. Thank You!