Living system

Living systems are characterized by a high structural order and ordered behaviour in space and time. There appears to be a contradiction between the increasing complexity in the course of biological development and the second law of thermodynamics. However, it may be noted that living system is an open system whose entropy may increase or decrease. 

Biological systems dominated by a point attractor may be disturbed by the action of a drug but they come to their original state as soon as the drug is eliminated or excreted from the body, or they reach a new steady state when the drug concentration is kept constant. 

Living systems involve quite a complicated set-up, such as involving 

As an example, we may consider interconnections of cells, tissues and body organs in the human body as indicated in Fig. 15.1. 

The thermodynamic equilibrium is disturbed because of the changing character of membrane transport involving ion-channels, ion-pumps and constant flux of lipids in cellular transport. 

The development of novel titanium carbene complexes by Grubbs has opened up a route to living polymer systems, using coordinating polymerizations as opposed to those derived from ionic initiators, which can be used to form block copolymers or produce chains with a functionalized end group. The initiating species are formed by the reaction of norbomene with a titanocyclobutane derived from 3,3-dimethyl cycloprene 

The stable living polymer chain derived from norbomene is 

Heating (III) in the presence of another monomer leads to diblock copolymer formation 

The chains can be terminated by heating with a reagent that will react with the carbene, and aldehydes or ketones will undeigo a Wittig-type reaction to form a terminal olefin unit. 

The use of convaitioiial anionic polymerization methods to produce Uving polymers from acrylic or methacryhc monomers has not met with much success. In 1983, however, the discovery of group trarrsfo polymerization by workers at DuPont rectified the situation. The method developed is a new type of reaction leading to living polymers, which can be rrsed for polar monomers, particularly derivatives of acrylic and methacrylic acids. 

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C2.14.4 Kineticsit has already been emphasized (section C2.14.1, section C2.14.2.2 and section C2.14.3.1) tliat kinetics are of paramount importance in describing living systems [76]. The root of tliis may ultimately he in tire fact tliat whereas inanimate matter has endless time in which to undergo its transfonnations, mortal, animate matter is constantly racing against tire clock. 

Most chemically reacting systems tliat we encounter are not tliennodynamically controlled since reactions are often carried out under non-equilibrium conditions where flows of matter or energy prevent tire system from relaxing to equilibrium. Almost all biochemical reactions in living systems are of tliis type as are industrial processes carried out in open chemical reactors. In addition, tire transient dynamics of closed systems may occur on long time scales and resemble tire sustained behaviour of systems in non-equilibrium conditions. A reacting system may behave in unusual ways tliere may be more tlian one stable steady state, tire system may oscillate, sometimes witli a complicated pattern of oscillations, or even show chaotic variations of chemical concentrations. 

Chemists are satisfied how atoms of the different elements could form from the initial enormous energy of the big bang explosion, without, however, the need to concern themselves with the reason for its origin. Atoms subsequently can combine into molecules, which in turn build increasingly complex systems and materials, including those of the living systems. This is the area of interest for chemists. 

Section 7 8 Both enantiomers of the same substance are identical m most of then-physical properties The most prominent differences are biological ones such as taste and odor m which the substance interacts with a chiral receptor site m a living system Enantiomers also have important conse quences m medicine m which the two enantiomeric forms of a drug can have much different effects on a patient... 

Nucleophilic substitution is one of a variety of mechanisms by which living systems detoxify halogenated organic compounds introduced into the environment Enzymes that catalyze these reactions are known as haloalkane dehalogenases The hydrolysis of 1 2 dichloroethane to 2 chloroethanol for example is a biological nude ophilic substitution catalyzed by a dehalogenase... 

The reverse reaction also occurs m living systems NADH reduces acetaldehyde to ethanol m the presence of alcohol dehydrogenase In this process NADH serves as a hydride donor and is oxidized to NAD" while acetaldehyde is reduced... 

The major classes of organic compounds common to living systems are lipids pro terns nucleic acids and carbohydrates Carbohydrates are very familiar to us— we call many of them sugars They make up a substantial portion of the food we eat and provide most of the energy that keeps the human engine running Carbohy drates are structural components of the walls of plant cells and the wood of trees Genetic information is stored and transferred by way of nucleic acids specialized derivatives of carbohydrates which we 11 examine m more detail m Chapter 28... 

We 11 see numerous examples of both reaction types m the following sections Keep m mind that m vivo reactions (reactions m living systems) are enzyme catalyzed and occur at far greater rates than those for the same transformations carried out m vitro ( m glass ) m the absence of enzymes In spite of the rapidity with which enzyme catalyzed reactions take place the nature of these transformations is essentially the same as the fundamental processes of organic chemistry described throughout this text... 

How living systems convert acetate to fats is an exceedingly complex story one that IS well understood m broad outline and becoming increasingly clear m detail as well We will examine several aspects of this topic m the next few sections focusing mostly on Its structural and chemical features... 

Living systems contain thousands of different enzymes As we have seen all are structurally quite complex and no sweeping generalizations can be made to include all aspects of enzymic catalysis The case of carboxypeptidase A illustrates one mode of enzyme action the bringing together of reactants and catalytically active functions at the active site... 

The reactions that amino acids undergo in living systems include transamination and decarboxylation... 

Dou ble hel ix (Section 28 8) The form in which DNA normally occurs in living systems Two complementary strands of DNA are associated with each other by hydrogen bonds be tween their base pairs and each DNA strand adopts a helical shape... 

Enzyme (Section 27 20) A protein that catalyzes a chemical reaction in a living system... 

These chemical effects become important in medicine because living systems operate mostly through the reactions of enzymes, which catalyze all sorts of metabolic reactions but are very sensitive to small changes in their environment. Such sensitivity can lead to preferential absorption of some deleterious isotopes in place of the more normal, beneficial ones. One example in metabolic systems can be found in the incorporation of a radioactive strontium isotope in place of calcium. 

Although scientists have known since the time of Louis Pasteur (1) that optical isomers can behave differentiy in a chiral environment (eg, in the presence of polarized light), it has only been since about 1980 that there has been a growing awareness of the implications arising from the fact that many dmgs are chiral and that living systems constitute chiral environments. Hence, the optical isomers of chiral dmgs may exhibit different bioactivities and/or biotoxicities. 

Biologists are quite sure that reagents are not homogeneously mixed in living systems. Chemical physicists striving to understand the ultrafast, primary processes of chemical change know that uniform concentration is too cmde an approximation for their purposes. Nonetheless, the assumption of a weU-stirred mixture is so pervasive that kineticists rarely point it out in their reports. 

Proteins, ubiquitous to all living systems, are biopolymers (qv) built up of various combinations of 20 different naturally occurring amino acids (qv). The number of proteins in an organism may be as small as half a do2en, as in the case of the simple bacterial vims M13, or as large as 50,000, as in the human system. Proteins are encoded by the deoxyribonucleic acid (DNA) that is present in all living cells. 

In general, the higher the foaming power, the lower the toxicity (39). Sulfonates that are absorbed into a living system are readily distributed and excreted. 

Biopolymers are the naturally occurring macromolecular materials that are the components of all living systems. There are three principal categories of biopolymers, each of which is the topic of a separate article in the Eniyclopedia proteins (qv) nucleic acids (qv) and polysaccharides (see Carbohydrates Microbial polysaccharides). Biopolymers are formed through condensation of monomeric units ie, the corresponding monomers are amino acids (qv), nucleotides, and monosaccharides, for proteins, nucleic acids, and polysaccharides, respectively. The term biopolymers is also used to describe synthetic polymers prepared from the same or similar monomer units as are the natural molecules. 

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