Entropy and its significance in transport of ions through the cell membrane

Autor: Selim Pašić, Nato Popara, Antea Klobučar, Denis Cvitković, Marinko Vilić
Jazyk: angličtina
Rok vydání: 2023
Předmět:
Zdroj: Veterinarska stanica
Volume 54
Issue 5 (online first)
ISSN: 1849-1170
0350-7149
Popis: The aim of this paper is to present the concept of entropy in a simple way and to show its key importance in the transport processes of ions through the cell membrane using the latest discoveries in biophysics. Using a real-life example, we show how processes within a system lead to an increase in entropy. We also show how this entropy increase is directly related to the irreversibility of the process, and how it defines the arrow of time (direction of the flow of time). Using an abstract example, we clarify the meaning of the concept of disorder in a system, which is often used in defining entropy by connecting it with the number of microstates that realise a macroscopic state of a system. The importance of entropy in transport processes of ions through the cell membrane is considered. We show that passive transport processes through the cell membrane are the result of an entropy increase in the cell membrane-transported substance system. A model of active ion transport through the cell membrane following Rubi et al. (2017) is presented. The force that transports ions through the channel in the transport protein arises due to the entropy gradient formed along the transport channel, which is a consequence of the funnel shape of the channel. The entropic force is proportional to the ratio of the ion-available cross-sections of the exit and entrance surface of the channel. That means that only a very funnel-shaped channel can produce a sufficiently large force on the ions to overcome the concentration gradient of the substance. We analyse the final result for the force of entropy in the limits of a very wide and very narrow channel and find that the entropic force is proportional to the ratio of the areas of the exit to entrance surfaces of the channel, i.e., when the channel is very wide, while it becomes high as the width of the channel tends to the ion diameter, i.e., when the channel is very narrow. We explicitly explain how the presented model solves several fundamental questions about the active transport of substances: how is energy, a scalar quantity, converted into the directional motion of the ion (a vector quantity), how does energy drive ions considering that the point of release of energy is far from the point of binding of an ion in a transport protein and finally, how does energy, which is released in a very limited space, transport the ions over a very large spatial scale.
Svrha ovoga rada je predstaviti na jednostavan način koncept entropije i njezinu ulogu u transportu iona kroz staničnu membranu koristeći najnovije spoznaje iz biofizike. Na primjeru iz stvarnog života ukazujemo da procesi unutar nekoga sustava vode do porasta entropije, a prikazujemo da je porast entropije direktno povezan s ireverzibilnošću procesa i da određuje „vremensku strijelu“ - smjer protoka vremena. Na apstraktnom smo primjeru razjasnili pojam nereda sustava, koji se vrlo često koristi u definiciji entropije, povezujući ga s brojem mikrostanja koja ostvaruju određeno makroskopsko stanje. Razmatrali smo značenje entropije u transportnim procesima kroz staničnu membranu. Pokazali smo da su pasivni transportni procesi kroz staničnu membranu posljedica povećanja entropije u sustavu stanična membrana - transportna tvar. Detaljno smo prezentirali model aktivnoga transporta iona kroz staničnu membranu slijedeći Rubi i sur. (2017.). Sila koja transportira ione kroz kanal prouzročena je gradijentom entropije koji se prostire uzduž kanala, a posljedica je ljevkastoga oblika kanala. Sila entropije je proporcionalna omjeru ionu raspoloživih poprečnih presjeka izlazne i ulazne plohe kanala. To znači da kanal samo dostatno izraženoga ljevkastoga oblika može transportirati ione suprotno koncentracijskom gradijentu. Analizirali smo i konačni rezultat za entropijsku silu u graničnim uvjetima i izračunali da je za vrlo široki kanal sila proporcionalna omjeru površina ulazne i izlazne plohe, dok je za vrlo uzak kanal, čiji polumjer teži polumjeru iona, sila postaje izuzetno velika. Razumljivo smo objasnili kako prezentirani model rješava nekoliko otvorenih pitanja aktivnoga transporta tvari: kako se energija koja je skalarna veličina pretvara u usmjereno gibanje iona i što je vektorska veličina; kako energija transportira ione s obzirom da je mjesto oslobađanja energije daleko od točke vezivanja iona u transportnom proteinu; kako energija koje se oslobađa u vrlo uskom području može transportirati ione na velikoj skali.
Databáze: OpenAIRE
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