Active transport, in biological systems

It should be mentioned here that, in living systems the transport of mass sometimes takes place apparently against the concentration gradient. Such uphill mass transport, which usually occurs in biological membranes with the consumption of biochemical energy, is called active transport, and should be distinguished from passive transport, which is the ordinary downhill mass transport as discussed in this chapter. Active transport in biological systems is beyond the scope of this book. 

There exist a large number of phenomenological laws for example, Fick s law relates to the flow of a substance and its concentration gradient, and the mass action law explores the reaction rate and chemical concentrations or affinities. When two or more of these phenomena occur simultaneously in a system, they may couple and induce new effects, such as facilitated and active transport in biological systems. In active transport, a substrate can flow against the direction imposed by its thermodynamic force. Without the coupling, such uphill transport would be in violation of the second law of thermodynamics. Therefore, dissipation due to either diffusion or chemical reaction can be negative only if these two processes couple and produce a positive total entropy production. 

In the first chapter, Uosaki and Kita review various theoretical models that have been presented to describe the phenomena that occur at an electrolyte/semiconductor interface under illumination. In the second chapter, Orazem and Newman discuss the same phenomena from a different point of view. In Chapter 3, Boguslavsky presents state-of-the-art considerations of transmembrane potentials and other aspects of active transport in biological systems. Next, Burke and Lyons present a survey of both the theoretical and the experimental work that has been done on hydrous oxide films on several metals. 

Impedance spectroscopy (IS) is a measurement of the conductive and dielectric properties of electroactive systems over a wide range of frequencies. Its popularity and applicability has increased dramatically over the past 25 years with the advent of fast-response potentiostats and frequency response analyzers. Impedance spectroscopy has been applied extensively in electrochemistry, especially in battery and sensor research, and it has been used to study active transport in biological membranes. Skin impedance has also been investigated with IS, but many of these studies attempted to correlate impedance with hydration and provided no insight into the mechanism of charge transport. More recent studies have used IS to elucidate the pathways of ion transport through skin, with special emphasis on understanding the mechanism... 

Therefore, it appears reasonable to assume the existence of the tunneling mechanism of electron transfer between the electrode and the enzyme active center. The possibility of a sub-barrier electron transfer at large distances (10-20 A) has been shown, at least theoretically, for electrochemical systems and is widely used to discuss electron transport in biological systems. ... 

In biological systems, therefore, the behavior of Li+ is predicted to be similar to that of Na+ and K+ in some cases, and to that of Mg2+ and Ca2+ in others [12]. Indeed, research has demonstrated numerous systems in which one or more of these cations is normally intrinsically involved, including ion transport pathways and enzyme activities, in which Li+ has mimicked the actions of these cations, sometimes producing inhibitory or stimulatory effects. For example, Li+ can replace Na+ in the ATP-dependent system which controls the transport of Na+ through the endoplasmic reticulum Li+ inhibits the activity of some Mg2+-dependent enzymes in vitro, such as pyruvate kinase and inositol monophosphate phosphatase Li+ affects the activity of some Ca2+-dependent enzymes— it increases the levels of activated Ca2+-ATPase in human erythrocyte membranes ex vivo and inhibits tryptophan hydroxylase. 

In biological systems, one often observes membrane structures with nonzero spontaneous curvatures, e.g. in mitochondria. This type of bilayer structure is also essential in various transport related processes such as endo- and exocy-tosis (see Chapter 8 of this volume). These curved membrane systems may be stabilised by protein aggregation in the bilayer, or may be the result of the fact that biological membranes are constantly kept off-equilibrium by lipid transport and/or by (active) transport processes across the bilayer. These interesting... 

The most fundamental process dealing with the activation of C02 involves the hydration of C02 to produce bicarbonate and the reverse dehydration of bicarbonate to produce C02. These processes are of biological and environmental significance since they control the transport and equilibrium behavior of C02. The spontaneous hydration of C02 and dehydration of HCO3 are processes that are too slow and must therefore be catalyzed by metal complexes in order to expedite the overall conversion rate. In biological systems, a series of enzymes, the carbonic anhydrases, are the efficient catalysts and can accelerate the reactions by up to 7 orders of magnitude. The mechanism of this... 

The design of biocatalytic electrodes for activity toward gaseous substrates, such as dioxygen or hydrogen, requires special consideration. An optimal electrode must balance transport in three different phases, namely, the gaseous phase (the source of substrate), the aqueous phase (where the product water is released and ionic transport takes place), and the solid phase (where electronic transport occurs). Whereas the selectivity of biocatalysts facilitates membraneless cells for implementation in biological systems that provide an ambient electrolyte, gas-diffusion biofuel cells require an electro-... 

Such coupling is possible in anisotropic media, such as in active transport at membranes in biological systems. 

Macromolecules have large molecular weights and various random shapes that may be coil-like, rod-like, or globular (spheres or ellipsoids). They form true solutions. Their sizes and shapes affect their diffusion in solutions. Besides that, interactions of large molecules with the small solvent and/or solute molecules affect the diffusion of macromolecules and smaller molecules. Sometimes, reaction-diffusion systems may lead to facilitated and active transport of solutes and ions in biological systems. These types of transport will be discussed in Chapter 9. 

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