Secondary Processes of Photosynthesis

Of the products of noncyclic electron transport, O2, ATP, and NADPH + H+, O2 will not interest us further. On the other hand, ATP and NADPH are the substances that link the primary processes with the secondary processes. They do so in that they arise in the primary processes and can then be utilized in the secondary processes for the fixation and reduction of CO2. 

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In this respect it is important to note, that none of these reactions requires light. Hence, the whole reaction mechanism of the C02-fixation in the stroma region includes only secondary processes of photosynthesis. 

CO2 must be reduced to carbohydrate by means of the hydrogen set free by photolysis. This reduction takes place only after the CO2 has been bound to an organic acceptor. The binding and the reduction of the CO2 are known as the secondary processes of photosynthesis. 

Fig. 34. Experiment to determine which reaction is indirectly light-dependent in the secondary processes of photosynthesis. When it becomes dark the amount of 3-phosphoglyceric acid (PGA) increases, in contrast to that of the other components (modified from Baron 1967).

Fig. 35. Experiment to ascertain the path of carbon in the secondary processes of photosynthesis. C 02 was supplied to algal suspensions. The algae were then killed at specified times and extracts from them were separated in two-dimensional paper chromatography. PGA = 3-phosphoglyceric acid, triose = 3-phosphoglycerinaldehyde + dihydroxyacetone phosphate. The blackened spots on the copies of the chromatograms shown indicate that is present in the respective compounds (modified after Baron 1967).

We can thus state definitely that in the secondary processes of photosynthesis 3-phosphoglyceric acid is converted into 3-phosphoglyceralde-hyde. For this reaction ATP and NADPH + formed in the primary processes, are utilized. Thus, the reaction is indirectly light-dependent, and, by means of it the primary and secondary processes of photosynthesis are linked. 

What holds true for the structural elements of the plastids need not be valid for all of the enzyme systems of photosynthesis. A key enzyme in the secondary processes of photosynthesis is carboxydismutase which fixes CO2 into ribulose-1, 5-diphosphate (page 51). The enzyme is already present before the light-dependent differentiation of the plastid structure, at least in rye seedlings. Nonetheless, illumination induces an... 

The natural cycles of the bioelements carbon, oxygen, hydrogen, nitrogen and sulphur) are subjected to various discrimination effects, such as thermodynamic isotope effects during water evaporation and condensation or isotope equilibration between water and CO2. On the other hand, the processes of photosynthesis and secondary plant metabolism are characterised by kinetic isotope effects, caused by defined enzyme-catalysed reactions [46]. 

The building blocks for secondary metabolites are derived from primary metabolism. In fact, the biosynthesis of secondary metabolites is derived from the fundamental processes of photosynthesis, glycolysis, and the Krebs cycle to afford biosynthetic intermediates, which, ultimately, results in the formation of secondary metabolites also known as natural products. The most important building blocks employed in the biosynthesis of secondary metabolites are those derived from the intermediates acetyl-coenzyme A (acetyl-CoA), shikimic acid, mevalonic acid, and l-deoxyxylulose-5-phosphate (Figure 1.1). 

In the primary processes of photosynthesis ATP and NADPH + H+ are formed (and, in addition, O2 is liberated, a fact which is of no interest in the present context). In which chemical reaction or reactions of the secondary processes are these two substances utilized This question could be approached experimentally in that the reaction concerned must be indirectly light-dependent. Indirect in the sense that light directly excites the chlorophylls in the first and second light reactions which have been discussed. 

According to all the data presently available the primary processes of photosynthesis take place in the membrane structures and the secondary processes at the boundaries of the membranes and in the matrix. Inspite of the hypothetical nature of all statements concerning the relation between ultrastructure and function of chloroplasts two central principles can be sifted out ... 

The phenolic acids of interest here [caffeic acid (3,4-dihydroxycinnamic acid), ferulic acid (4-hydroxy-3-methoxycinnamic acid), p-coumaric acid (p-hydroxycinnamic acid), protocatechuic acid (3,4-dihydroxybenzoic acid), sinapic acid (3,5-dimethoxy-4-hydroxyxinnamic acid), p-hydroxybenzoic acid, syringic acid (4-hydroxy-3,5-methoxybenzoic acid), and vanillic acid (4-hydroxy-3-methoxybenzoic acid)] (Fig. 3.1) all have been identified as potential allelopathic agents.8,32,34 The primary allelopathic effects of these phenolic acids on plant processes are phytotoxic (i.e., inhibitory) they reduce hydraulic conductivity and net nutrient uptake by roots.1 Reduced rates of photosynthesis and carbon allocation to roots, increased abscisic acid levels, and reduced rates of transpiration and leaf expansion appear to be secondary effects. Most of these effects, however, are readily reversible once phenolic acids have been depleted from the rhizosphere and rhizoplane.4,6 Finally, soil solution concentrations of... 

Probably the most important biochemical process on earth is the photosynthesis within the chlorophyl cells in green plants. The photosynthesis process in the chlorophyll complex consists of two parts. In the primary process, light-harvesting molecules with a broad absorption spectrum in the visible absorb photons from sunlight. This leads to molecular electronic excitation. The excited molecules surrounding the central reaction center (Fig. 10.36) can transfer the excited electron in several steps to the molecules in the reaction center, where the secondary process, namely the chemical reaction... 

The addition of exogenous substrates of primary metabolites to organisms in culture (or blocking of particular steps of biosynthetic pathways) can result in overproduction of either primary or secondary compounds. Gross primary production by plants exceeds net primary production. Additionally as much as 38% of carbon fixed by photosynthesis may be lost through photorespiration. The process of cyanide-resistant respiration represents an obvious nonaccumu-lative mechanism by which plants can divide any overflow into carbon dioxide (Waterman and Mole, 1989). 

Calvin and Baretrop suggest that the photochemical reduction of lipoic acid is the primary event in photosynthesis and that reduction of the pyridine nucleotides is a secondary process. 

A. Division of Photosynthesis into Primary and Secondary Processes... 

The natural production of is a secondary effect of cosmic-ray bombardment in the upper atmosphere. Following productiorr, it is oxidized to form " C02. In this form, is distributed throughout the earth s atmosphere. Most of it is absorbed in the oceans, while a small percentage becomes part of the terrestrial biosphere by means of the photosynthesis process and the distribution of carbon compounds through the different pathways of the earth s carbon cycle. Metabolic processes maintain the content of living organisms in equilibrium with atmospheric However, once metabolic processes cease—as at the death of an animal or plant— the amount of will begin to decrease by decay at a rate measured by the half-life. 

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