Heterogeneous systems biological functions

From an energetic viewpoint, these reactions fuel a good number of biological functions which are responsible for biochemical reactions crucial to life . In common chemical processes, these reactions require the use of heterogeneous catalytic systems and drastic reaction conditions (pressures of 100 atm and temperatures around 400°C), whereas, at a cellular level, they proceed smoothly under ambient conditions (room temperature and 1 atm pressure). 

Water behaves differently in different environments. Properties of water in heterogenous systems such as living cells or food remain a field of debate. Water molecules may interact with macromolecular components and supramolecular structures of biological systems through hydrogen bonds and electrostatic interactions. Solvation of biomolecules such as lipids, proteins, nucleic acids, or saccharides resulting from these interactions determines their molecular structure and function. 

Several homogeneous gas- and liquid-phase reactions are now also known to exhibit self oscillations and it is clear that many living organisms depend on coupled oscillatory reactions catalysed by enzymes to control biological functions.However, only heterogeneous oxidation reactions catalysed by noble metals are reviewed here. Experimental studies are first described, followed by a discussion of kinetic analyses which have been put forward to account for them. Particular attention is given to the most extensively studied system to date, the oxidation of CO over Pt catalysts. 

Liquid-liquid-solid reactors are commonly used for biphasic reactions catalyzed by immobilized phase-transfer catalysts (which form the third, solid phase). Certain basic aspects of such reactors were considered in Chapter 19. Three-phase reactions of this type are also encountered in biological reactions, for example, the enzymatic synthesis of amino acid esters in polyphasic media (Vidaluc et al., 1983), and the production of L-phenylalanine utilizing enzymatic resolution in the presence of an organic solvent (Dahod and Empie, 1986). Predictably, the performance of these reactors is influenced by the usual kinetic and mass transfer aspects of heterogeneous systems (see Lilly, 1982 Chen et al., 1982 Woodley et al., 1991a,b). Additionally, performance is also influenced by the complex hydrodynamics associated with the flow of two liquids past a bed of solids (Mitarai and Kawakami, 1994 Huneke and Flaschel, 1998). It is noteworthy, for instance, that phase distribution within the reactor is different from that in the feed and is also a function of position within the reactor and within the voids of each pellet in the bed. More intensive research is needed before these reactors can be rationally designed. 

Measurements of spin-lattice relaxation rate as a function of the magnetic field/resonance frequency are commonly referred to as nuclear magnetic relaxation dispersion, NMRD. Measurements of this kind, when performed over a broad range, are an invaluable source of information on frequency-resolved molecular motions in complex biological or colloidal systems. Several years ago, Halle proposed a theory for relaxation of quadrupolar spins in dynamically heterogeneous systems... 

Disturbances that increase water scarcity promote the physical uniformity of river systems and the decrease of biological diversity in streams and rivers. The structure and functioning of heavily impacted river systems become mutually and strikingly similar, irrespective of the river s origin and the climate. The more intense and persistent the disturbance, the greater is the resemblance. On the other hand, river organisms use resources most efficiently in spatially heterogeneous chaimels, and under moderate disturbance frequencies, rather than in steady conditions, to which they are not adapted. 

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