Living systems characteristics

For in vivo studies, animal models are set up and how the target is involved in the disease is analyzed. One such model is the use of knockout or transgenic mice (Exhibit 2.8). It should be borne in mind, however, that there are differences between humans and animals in terms of gene expression, functional characteristics, and biochemical reactions. Nevertheless, animal models are important for the evaluation of drug-target interactions in a living system. 

Living systems are complex, ordered systems. This complexity and order is reflected in the molecules characteristic of life, in their interactions with each other, in the regulatory mechanisms that result from these interactions, and in the complex supramolecular structures characteristic of cells. Organization is also reflected in ordered metabolic and signaling pathways. Such complex, ordered structures and pathways are not characteristic of inanimate objects. 

In this chapter, I focus on two elements that are central for most molecules characteristic of living systems nitrogen and oxygen. 

With each of the C, P and S centers, compounds with several oxidation states are possible, thus multiplying the types of nucleophilic reactions extant. Importantly, the types of compounds cover a variety of classes each with its characteristic behaviors and reactivities, each defining a specific area in chemistry. Since the C, P and S reactive centers are incorporated in the majority of molecules in living systems it follows that the chemistry to be considered in this chapter is closely tied with the chemistry of life, i.e. bioorganic reaction mechanisms. It is known in fact that many organophosphorus and organosulfur compounds are toxic toward mammalian organisms which renders their destruction under mild conditions of critical importance. 

Bioluminescence is the production of light by living systems. The best-known example of this phenomenon is the characteristic glow of the firefly, but other luminous species include bacteria, fungi and other animals such as jellyfish, scale-worms, deep-sea squid, prawn and fish. In animals bioluminescence is used as a diversionary tactic when disturbed, to attract prey and of course as a mating signal during courtship. 

In conclusion, biomacromolecules have evolved to be big because from their size and structural variety arise a number of emergent properties that allow them not only to function with utmost efficiency, but to do so in a manner fully controlled by the higher levels of complexity characteristic of living systems (see Section 2.4). 

Much of the chemistry of the cell is common to all living systems and is directed towards ensuring growth and cell multiplication, or at least the survival of the cell. Organisms also share various structural characteristics. They all contain genetic material (DNA), membranes (the boundary material between the cell and the environment), cytoplasm (small particulate materials, ribosomes and enzyme complexes), and cell walls or surfaces (complex structures external to the membrane). In addition, there are various distinct membrane-bound organelles in eukaryotic organisms which have specialised functions within the cell (Tables S.4, 5.5 and 5.6)(8-, 7). 

Fitness of adaptation to the environment means that biochemical correlates of physical or chemical characteristics of the environment must exist.1 Therefore, adaptive processes are always optimization processes be it that of metabolic sequences or of molecular properties.2-3 To recognize that a living system is adapted to a given environment is simply to recognize its finality4 or teleonomic characteristics.5 Optimization being a consequence of the second law of thermodynamics, it seems to me that what Jacob or Monod call finality or teleonomy is simply the finality of the second law. 

Indications of the condition of living systems in ecology are similar to those in medicine, veterinary science and animal husbandry. The aim of research in this area is to be able to identify characteristics which can define the best possible state of organisms and populations, and can also signal and quantify deviations from it. Such indications should have high resolving power, so as to reveal the situation in deeply hidden processes within the biological systems. 

Epstein (1992) considers that reliability and complexity are as important as sustainability in the basic characteristics of living systems. Supporting the three requires the organism to receive a flow of information as strong as the energy flow. 

Figure 13.4b emphasizes the finite nature and strong irreversibility of an economic system. The stock of energy and resources will eventually run out and so will the absorptive capacity of the environment for waste. An obvious extension of Figure 13.4b, therefore, is the one represented by Figure 13.4c. Just like in nature, waste has to be recycled. In nature, there is no real waste. Every form of waste is a resource for a living system. This living system is very small and called a microbe. Microbes make sure that all matter recycles in nature. Man needs to assume this humble but valuable and important role of microbes in the economic system and make sure that the material cycles get closed. Therefore, energy (or rather work) is required. But obviously this work should not be supplied from a nonrenewable source, like fossil fuels, but rather from a renewable source like the sun. Figure 13.4c therefore seems to be characteristic for a sustainable economic system and agrees remarkably with the definition of sustainability from biological systems A...

Steroids are lipids found in living systems that all have the ring system shown in Figure 3.8 for cholesterol. Steroids occur in bile salts, which are produced by the liver and then secreted into the intestines. Their breakdown products give feces its characteristic color. Bile salts act on fats in the intestine. They suspend very tiny fat droplets in the form of colloidal emulsions. This enables the fats to be broken down chemically and digested. 

Characteristics of biological systems, coupled with the rich chemistry of vanadium in aqueous solutions, make the study of effects of vanadium compounds in living systems difficult. The cell is divided into different organelles and vesicles by mem-... 

Finally, Section 2.4 analyses a simplified model of a bursting pancreatic /3-cell [12]. The purpose of this section is to underline the importance of complex nonlinear dynamic phenomena in biomedical systems. Living systems operate under far-from-equilibrium conditions. This implies that, contrary to the conventional assumption of homeostasis, many regulatory mechanisms are actually unstable and produce self-sustained oscillatory dynamics. The electrophysiological processes of the pancreatic /3-cell display (at least) two interacting oscillatory processes A fast process associated with the K+ dynamics and a much slower process associated with the Ca2+ dynamics. Together these two processes can explain the characteristic bursting dynamics in the membrane potential. 

The caffeine structure is shown below. It is classed as an alkaloid, meaning that with the nitrogen present, the molecule has base characteristics (alkali-like). In addition, the molecule has the purine ring system, a framework which plays an important role in living systems. 

This characteristic energetic relationship between a set of compounds that are intermediates in an evolved metabolism may be a universal biosignature. If an inventory of the small molecules in a suspected living system (on Titan, for example) reveals this characteristic energetic relationship, this may be evidence of Darwinian evolution acting to create an optimal metabolism. 

The different facets of water-micromolecule-macromolecule interactions discussed up to this point involve several of the most important ways in which water has shaped the characteristics of living systems and the ways in which the internal milieu is defended in the face of water stress. Because of water s pervasive influence on the evolution of virtually all properties of organisms, there are many other imprints of water on biological design that remain to be discussed. Below, we present in somewhat abbreviated manner several of these issues. This discussion will help us to understand more clearly how water establishes the boundary conditions for life and dictates many of the engineering principles that are found in the designs of cells. Of particular importance is the issue of packaging how to accommodate tens of thousands of chemical systems in a minute volume of water. 

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