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:

Gap Junctions (animals)

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!

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    $\begingroup$ I think an answer that explains this question at your level of understanding might be too broad. The things you want to understand are cell membranes, concentration gradients, osmosis, channels for water and other ions, transporters for ions, etc. Someone might find a special example, but typically in biology, besides bulk transport, water is never moved by itself: instead, osmotic gradients pull water into particular spaces. Many plants, for example, collect water by pumping potassium into their roots: water follows this osmotic gradients. $\endgroup$ – Bryan Krause Dec 20 '16 at 22:39
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    $\begingroup$ Gap junctions don't have any "direction" to them: they are basically just open "holes" between cells that allow for diffusion depending on concentration/electrical gradients. $\endgroup$ – Bryan Krause Dec 20 '16 at 22:43
  • $\begingroup$ You've correctly found connexons at gap-junctions work as gated-channels. There are many such of names that doesnot contain its 'categories', for say the term 'trypsin' does not end with 'ase' though it is enzyme. But that is an unrelated issue. You may use the chat because commenting is not considered for extended discussion. $\endgroup$ – Always Confused Dec 21 '16 at 13:50

I don't know any-such continuous-membrane-material that can do so. But yes certain membrane-transport-proteins can do so.

Carrier proteins are normally highly specific about in which-direction to transport which molecule; and in general, they are classified as uniporters, symporters and antiporters. *

wikipedia image

So an uniporter would be an accurate example of situation you are asking about.

Carrier proteins those utilize ATP for pumping (those shows "primary active transport"), are also normally highly specific to direction. For example the Na+-K+pump and the H+ pump.

Channel proteins too, sometimes show directionality, for example certain K+ channels ** such as inward-rectifying potassium channel

Potassium channel image from book by cooper

A potassium channel diagram showing the selectivity filter (diagram from The cell/ Cooper **, slightly edited: Na+ filter-out not shown, and the term 'selectivity filter' is labelled in image ).

KcsA potassium channel Wikipedia

KcsA potassium channel diagram from wikipedia. In its open-conformation out of the following;

Gating mechanic

i.e. the third one, can conduct the K+ ion in a specific direction, because the pore-region is very specific- it allows K+ ion only to enter in the cell.


  1. * Plant biochemistry by Hans Walter Heldt, 3rd edition, Academic Press-Elsevier, Paperback edition.
  2. ** The Cell, a molecular approach/ Cooper/ Sinauer and ASM press.

  3. Harper's illustrated Biochemistry/ Murray, Bender, Botham, Rodwell, Well/ Lange and McGraw Hill.

  4. Wikipedia.

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    $\begingroup$ Clear demonstration, thank you! But i can't transpose this method to able to control fluids. But helped me to go further, check the Update of my question. $\endgroup$ – eapo Dec 20 '16 at 21:24

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