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.
Cilj je ovoga rada povezivanje temeljnih karakteristika tkiva s rezultatima dobivenim na rendgenskoj slici, analizirajući fizikalne principe interakcije koje vode do atenuacije rendgenskih fotona u tkivu. Na slikovitom primjeru dokazujemo da nećemo moći imati superdijagnostički uređaj koji bi mogao zamijeniti sve moderne i buduće dijagnostičke uređaje. Svakase tehnika oslikavanja tkiva temelji na jedinstvenim fizikalnim principima te stoga ona vidi točno određene kakateristike tkiva koja drugi dijagnostički uređaji (tehnike) ne mogu vidjeti. Oslikavanje se rendgenskim uređajem temelji na atenuaciji (slabljenju) snopa rendgenskih zraka nakon prolaska kroz tkivo. Glavni su mehanizmi interakcije rendgenskih fotona i tkiva fotoelektrični efekt (potpuna apsorpcija energije fotona) te Comptonovo raspršenje (djelomična apsorpcija energije fotona). Fotoelektrični efekt dominira na niskim i djelomično srednjim energijama i s povećanjem energije jako slabi, dok na srednjim i višim energijama dominira Comptonovo raspršenje. Analizom oba raspršenja, nakon dekompozicije linearnoga koeficijenta atenuacije na uobičajene fizikalne veličine, nalazimo da u režimu Comptonovoga raspršenja kontrast na slici je određen razlikama u gustoći tkiva, a u režimu fotoelektričnoga efekta ovisi o gustoći tkiva te snažno o rednom broju elemenata od kojih je tkivo sastavljeno. U radu su grafički prikazane vrijednosti kontrasta na rendgenskoj slici za osnovne tvorbe i tkiva i opisane njihove karakteristike, gustoća i redni broj elemenata od kojih su sastavljeni. Kompjuterizirana tomografija (CT) vidi potpuno iste karakteristike tkiva kao i klasična rendgenska tehnika. U radu je na eksplicitnom dvodimenzionalnom primjeru objašnjen princip komjuterizirane tomografije. On se temelji na zamišljenoj podjeli tkiva na male volumene (voksele) i snimanju tkiva iz više pozicija, pri čemu se za svaku poziciju postavljaju jednadžbe za apsorpciju zračenja. U dobivenom sustavu jednadžbi nepoznate vrijednosti su vrijednosti linearnog koeficijenta atenuacije za svaki voksel, koje se odrede numeričkim rješavanjem sustava jednadžbi, i prikažu na prikladnoj skali boja u trodimenzionalnom prikazu.
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