Cells organic pathways 152

To build their structures and to carry out the myriad biochemical reactions that take place within their cells, organisms need a source of energy. The needed energy is obtained via biochemical pathways driven either by sunlight or by energy contained in reduced chemical compounds. 

Fig. 4.7. A scheme for the organic pathways in the cytoplasm of all cells illustrating their interconnected nature (after Kauffman see Further Reading), (a) Glycolytic (b) Krebs cycle pathways.

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. 

Animal cells add a significant layer of complexity to the system due to their dependence on cell-matrix and cell-cell signaling pathways. Unicellular species such as bacteria and yeasts tend to grow and proliferate as fast as nutrients can be supplied. In fact, their growth rates are typically proportional to the amounts of nutrients available. By contrast, however, cells from multicellular organisms must develop mechanisms that include both nutrient supplies and signal pathways that control cell division. Thus, while nutrients are necessary for an animal cell to proliferate, the cell must receive stimulatory chemical signals from other, usually adjacent, cells. 

Biologists have not only organized the cell into pathways and modules but also classified these pathways into various types. Each of the main types has a different computational representation in pathway databases. Bader and Enright (2005) discussed three biochemical- and biophysical-based pathways that are modeled ... 

Sulfite reductases contain siroheme and iron-sulfur centers. Siroheme, also present in some nitrite reductases, is an iron tetrahydroporphyrin of the isobacteriochlorin type with eight carboxylic acid side-chains (Fig. 1). Siroheme isolated from Desulfovibrio species was found to be a monoamide, heptamethyl ester derivative, rather than the usual octamethyl ester derivative, which suggests that in these organisms an amidated form of the siroheme may be the physiologically active prosthetic group [93]. Sulfite reductases are divided into two classes, the assimilatory and the dissimilatory enzymes. The assimilatory sulfite reductases produce sulfide for use in the cell biosynthetic pathways. The dissimilatory enzymes are present in the sulfate-reducing organisms, and reduce sulfite as a respiratory substrate in a process coupled to ATP formation. 

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