Rates of carbon dioxide fixation

Under certain conditions, the green particulate material is capable of rates of energy conversion and oxidation-reduction reactions comparable to the rates exhibited by whole cells. However, in the combined system, the rates of carbon dioxide fixation and reduction are, at best, only 1 to 5% of the rates of the intact organism on a unit chlorophyll basis. It is important that this difference be kept in mind when one considers the probable organization and relationships of the component machinery of photosynthesis as it is performed in vivo (Bassham, 1963). 

Todd and Todd and Probst also measured the effects of ozone (at 4 ppm for 40 min) on photosynthesis and found that development of symptoms was associated with inhibition of carbon dioxide fixation. This effect was also confirmed by Macdowall, who reported that the inhibition of photosynthesis was greater than that which could be accounted for by chlorophyll destruction. Hill and Littlefield associated decreased net photosynthesis caused by ozone (at 0.06 ppm for 1 h) with both stomatal opening and rates of transpiration. These studies have generally shown that net photosynthesis can decrease without visible injury. 

It has already been mentioned that utilization of the permethylated ligand (L19)2 in place of (L23)2 drastically alters the ease of substitution reactions of the [M2(L19)(C1)]+ complexes (Section III.D). Further studies revealed a remarkable influence of the hydrophobic pocket on the rate and course of several substrate transformations, as for instance the fixation of carbon dioxide (239) (Scheme 8), the cis-bromination of a,/)-unsaturated carboxylate ligands (256), and some Diels-Alder reactions (215). Of these the latter two reactions will now be discussed. 

Ozone causes both quantitative and qualitative changes in carbon dioxide fixation patterns. Wilkinson and Bames, using carbon dioxide-found a reduction in radioactivity in soluble sugars and increases in free amino acids and sugar phosphates in white pine after a 10-min exposure to ozone at 0.10 ppm. Miller observed a decrease in carbon dioxide-fixation in ponderosa pines that correlated with loss of chlorophyll, after exposure to ozone at 0.30-0.35 ppm. The Hill reaction rates of chloroplasts isolated from healthy and ozone-injured ponderosa pine indicated that both light and dark reactions of the chloroplasts from ozone-injured plants were depressed. Barnes found depressed photosynthesis and stimulated respiration in seedlings of four pine species of the southeastern United States after exposure to ozone at 0.15 ppm. 

Fig. 15.22. Electrochemical behaviors of p-type photoex-cited semiconductors and Mg electrodes in C02-satu-rated acetonitrile solution with 50 mM of benzyl chloride. (Reprinted from H. Ueda, K. Nakabayashi, Z. Ushizaki, and K. Uosaki, A Photoelectrochemical Fixation of Carbon Dioxide. Spontaneous Up Quality Conversion of Organic Compound, Cherrt. Lett. 190 1748, Fig. 2, 1993. Reproduced with permission of The Chemical Society of Japan.)...

In vivo, most electrons from reduced ferredoxin are passed onto nicotinamide adenine dinucleotide phosphate cation (NADP ), via ferredoxin-NADP reductase, to generate the NADPH needed to drive carbon dioxide fixation by the Calvin cycle. Thus electrons from photosystem I can pass through at least three routes (Figure 1), of which route C is preferred (II). However, if the supply of NADP were limited, for example, because of a poor supply of carbon dioxide causing a slow turnover of the Calvin cycle, the electron flow rate along pathway C would be expected to be decreased and more 02" should be made by route B and, to a lesser extent, by route A (15-17), Some oxygen reduction takes place even when carbon dioxide is present in ample amounts (18). 

An average 60-day-old tobacco plant contains about 250 mg of nicotine. Based on the net carbon dioxide fixation and the rate of turnover of nicotine in the plant, maintenance of this amount of nicotine may represent as much as 10% of the plant s total metabolism (Robinson, 1974). The half-life of nicotine in tobacco plants is about 22 h (Waller and No-wacki, 1978). The alkaloid content of plants of Nicotiana sylvestris undergoes a fourfold increase following damage to the leaves (Hartmann, 1991). The production of nicotine and related pyridine alkaloids such as anabasine (45), anatabine (46), and nornicotine (47) in plant tissue, cell, and hairy root cultures has been reviewed (Verpoorte et al., 1991). 

The methanol molecule is smaller than carbon dioxide and penetrates most plant tissues quickly for rapid metabolism. As a plant source of carbon, methanol is a liquid concentrate 1 cc methanol provides the equivalent fixed-carbon substrate of over 2,000,000 cc of ambient air. Methanol absorbed by foliage is metabolized to carbon dioxide, amino acids, sugars, and other structural components. Two major paths of methanol metabolism are the internal production of carbon dioxide that is then utilized in photosynthesis and the incorporation of methanol as a fixed source of carbon. Briefly stated in field terms, methanol treatments are a means of placing carbon directly into the foliage. Hi li t intensity is necessary to drive photosynthesis at the rates necessary to process the high internal levels of carbon dioxide presented by methanol. Serine formation and carbon dioxide fixation by photosynthesis may lead to the production of su. Increases of su concentration in the presence of moisture lead to increased turgidity. 

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