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Unicellular Organisms

Organic Compounds

The basic units to build the modern cells involve organic chemicals such as carbohydrates, lipids, nucleotides, and amino acids. The macromolecules for supporting life such as DNA, RNA, and proteins are assembled from these basic units. The organized sum total of these molecules becomes a cell - a living entity. Living organisms use carbohydrates for energy, lipids for body fats, cell walls, hormones, nucleic acids for DNA in cells, and protein for skin, hair, muscles, hemoglobin, antibodies, enzymes.

The advent of organic chemistry is often associated with the discovery in 1828 by the German chemist Friedrich Wohler that the inorganic, or mineral, substance called ammonium cyanate could be converted in the laboratory to urea, an organic substance found in the urine of many animals. Before this discovery, chemists thought that intervention by a so-called life force was necessary for the synthesis of organic substances. Wohler's experiment broke down the barrier between inorganic

and organic substances. Modern chemists consider organic compounds to be those containing carbon and one or more other elements, most often hydrogen, oxygen, nitrogen, sulfur, or the halogens, but sometimes others as well.

The ability to form covalent bonds with other carbon atoms in long chains and rings distinguishes carbon from all other elements. This property of carbon, and the fact that carbon nearly always forms four bonds to other atoms, accounts for the large number of known compounds. At least 80 percent of the 5 million chemical compounds registered as of the early 1980s contain carbon. The affinity of carbon for the most diverse elements does not differ very greatly - so that even the most diverse derivatives need not vary very much in energy content. This ability allows the organic world to exist in a special form of thermodynamic stability.

SP<sup>3</sup> State Tetrahedral Structure The electron configuration of the normal carbon atom has 2 electrons in energy level 2S and 2P respectively. By supplying about 2 ev to a carbon atom, the 4 electrons in the 2S and 2P states are rearranged to the SP3 state (Figure 07a). The four electrons in the SP3 state form the tetrahedral arrangement (Figure 07b) of orbitals

Figure 11-07a SP3 State [view large image]

Figure 11-07b Tetrahedral Structure [view large image]

(probability distribution of electrons), which can form stable covalent bonds with other atoms. This is
the basic reason for pumping energy into biological system to maintain metabolism and cellular structure. Therefore, the biological system is said to be in a non-equilibrium state. The electrical discharge in Stenley Miller's experiment represents the energy input required to move the
Dissipative Process molecular configuration into the non-equilibrium state. The continual free energy input from the environment will lead to a dissipative structure (Figure 11-07c), which is a necessary condition for life. The process is called dissipative because it continuously dissipate free energy into high-entropy energy namely heat. However, not all dissipative structures are living systems, non-life examples include convection, hurricanes, the Solar system, and galaxies, ... Living system can form only when the dissipative structure begins to perform work. As the hybrid orbitals of the tetrahedral configuration do not exist in an isolated atom, but arise while that atom is interacting

Figure 11-07c Dissipative Process
[view large image]

with others to form a molecule, it will dissolve and return to an equilibrium state once the input of free energy ceases causing the removal of the associated constituents. BTW, life is optimized to an near-equilibrium state to make it a highly efficient process in using free energy.

Entropy Beside the infusion of energy to support life, the organization of complexity requires the removal of entropy as shown in Figure 11-07c. Such requirement is fulfilled by re-transmission of lower frequency radiation back to the cold dark space as illustrated in Figure 11-07d. The process re-emits more photons (at lower frequency) back to the space. By Boltzmann's definition of entropy, it has the effect of enlarging the phase space volume and hence returning more entropy to space than receiving from the sun in accordance with the second law of thermodynamics.

Figure 11-07d Entropy and Life
[view large image]

It is known that carbon nuclei are produced in the interior of stars (see more in "Origin of Elements"). It comes about in a two-step process: (1) He2 + He2 Be4, (2) He2 + Be4 C6. One would have expected this two-step process to be extremely improbable, but remarkably the last step happens to be a resonance, which enables it to proceed at a rate far in excess of our naive expectation. The positioning of the resonance levels is determined in a complicated way by the precise numeral values of the constants of physics. Thus, it can be argued that we owe our existence to the fortuitous coincidence of some numbers after all that's been said and done.

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