I have read in my biochemistry textbook, that some membrane transporters transport only in one direction, moreover, they don't require ATP energy. I wonder, why these proteins do not act as Smoluchowski's Trapdoor and break second law of thermodynamics. Smoluchowski's Trapdoor

It seems that concentration of substance on the "A" side somehow affects protein's ability to pump molecules, even though on that side protein doesn't bind with former (on the A side protein has conformation with low affinity to the molecule, thus complex disassociates). So I don't understand how this form of Maxwell's demon is countered, or how concentration on the A side affects protein's function.

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    $\begingroup$ Can you quote from your textbook where it says "transport only in one direction"? $\endgroup$
    – Bryan Krause
    Feb 15, 2022 at 17:33
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    $\begingroup$ the transport is only in one direction but only as long as a gradient exists, it is no different than a ball rolling downhill and not back uphill. the gate just stops the gradient working in the opposite direction. think of one way seals $\endgroup$
    – John
    Feb 15, 2022 at 21:20
  • $\begingroup$ Thank you for your attention. Bryan Krause, my textbook is not in English, so I translated it. @John you confirmed that these kind of proteins exist, thank you. I will soon post my question's improved version and I hope for your help there as well. $\endgroup$ Feb 16, 2022 at 9:05
  • $\begingroup$ I am afraid that this is a biology site, and I, for one, have never heard of Smoluchowski's Trapdoor. You have provided no links to this, and the obscurity and complexity of the thermodynamic ideas are evident from a paper in the journal Entropy from which your diagram appears to be taken: “Skordos and Zurek [1], in their deliberations on Smoluchowski’s trapdoor, fall into the same syllogistic error as does Feynman [2,3] in his discussions of a ratchet and pawl within the micro- domain.” If Feynman got it wrong, do you expect us to get it right? $\endgroup$
    – David
    Feb 18, 2022 at 18:18


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