Respiration biological importance

We live under a blanket of the powerful oxidant 02. By cell respiration oxygen is reduced to H20, which is a very poor reductant. Toward the other end of the scale of oxidizing strength lies the very weak oxidant H+, which some bacteria are able to convert to the strong reductant H2. The 02 -H20 and H+ - H2 couples define two biologically important oxidation-reduction (redox) systems. Lying between these two systems are a host of other pairs of metabolically important substances engaged in oxidation-reduction reactions within cells. 

Phenols undergo oxidation to quinones. Quinones are biologically important because their redox properties play a significant role in cellular respiration. 

The biological importance of this enzyme has already been discussed (section II). Its role in producing glutamate, as the first organic amino compound, in bacteria and plants seems reasonably well established. In animals, which do not have the ability to produce all amino compounds from a simple nitrogen source, the enzyme seems to be concerned with the removal of excess amino compounds (equations 2-5) as well as with the production of glutamate for conversion to the acid amide (section V.B) or to take part in transamination reactions to form the non-essential amino compounds. Cellular control of the direction in which reaction occurs may well lie in the ratio of the concentrations of the oxidised and reduced forms of the cofactor. This ratio is not a fixed quantity but depends on the metabolic activity of the cell (NAD and NADP are cofactors for many oxidation-reduction reactions) as well as on the availability of molecular oxygen for the terminal step in respiration, by which the reduced cofactor is reoxidised by the cytochrome system (section IV.A.1). 

Oxygen binds to heme with high affinity, and several biologically important enzymes, especially those involved in mitochondrial respiration, such as cytochromes, and several other oxidases (e.g., NADPH oxidases) contain heme. 

Oxidation-reduction reactions in water are dominated by the biological processes of photosynthesis and organic matter oxidation. A very different set of oxidation reactions occurs within the gas phase of the atmosphere, often a consequence of photochemical production and destruction of ozone (O3). While such reactions are of great importance to chemistry of the atmosphere - e.g., they limit the lifetime in the atmosphere of species like CO and CH4 - the global amount of these reactions is trivial compared to the global O2 production and consumption by photosynthesis and respiration. 

Calorimetry is an important technique in biology as well as in chemistry. The inventor of the calorimeter was Antoine Lavoisier, who is shown in the illustration. Lavoisier was a founder of modem chemistry, but he also carried out calorimetric measurements on biological materials. Lavoisier and Pierre Laplace reported in 1783 that respiration is a very slow form of combustion. Thus, calorimetry has been applied to biology virtually from its invention. 

Obviously the redox poise in biological systems is very important and the movement of selenium through this process has been investigated for denitrifiers such as Paracoccus denitrificans,159 a specialized selenate-respiring bacterium Thauera selenatis which used selenate as the sole electron acceptor,160,161 and phototrophic bacteria which produced different reduced forms of selenium when amended with either selenite or selenate and even added insoluble elemental Se.162 As noted above, Andreesen has commented on the importance of redox active selenocysteines135 and Jacob et al.136 note the importance of the thioredoxin system to redox poise. 

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