The Photosynthetic Apparatus

The photosynthetic apparatus in green plants and algae is located in the chloroplast, which is a flattened, double-membraned structure about ISO-200 A thick. The two flat membranes lie one above the other and are united at their peripheries. These double-membraned structures have been termed thylakoids (from the Greek sacklike ). Each membrane of the thylakoid consists of a water-insoluble lipoprotein complex which contains the light-absorbing chlorophyll and other pigments utilized in photosynthesis. 

Thus far we have used the word chlorophyll as if this term related to a unique chemical species. In actuality there are a number of structurally related molecules present in photosynthetic organisms which are collectively referred to as chlorophyll. The general chlorophyll structure is  

The various chlorophyll types are listed in Table 12.1. An alternate resonance form is available for all the chlorophylls listed in Table 12.1 except for bacteriochlorophyll. Chlorophyll a is found in all green plants. In most land plants and algae one also finds chlorophyll b or d. Thus a more correct statement than the one made previously would be that chlorophyll a is the photochemically active pigment in all green plants, as shown by experimenta-tion.  

A second major group of compounds associated with photosynthetic organisms are the carotenoids. These compounds are practically all derivatives of the same linear skeleton composed of eight isoprenoid (C5) units combined such that the two methyl groups at the molecular center are 1 6 to each other and other methyl groups are in 1 5 positions  

The carotenoids are subdivided into two main groups, the carotenes (hydrocarbon carotenoids) and the xanthophylls (oxygenated carotenoids). The structure of j8-carotene is as follows  

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Bjorkman, O. Holmgren, P. (1963). Adaptability of the photosynthetic apparatus to light intensity in ecotypes from exposed and shaded habitats. Physiologia Plantarum, 16, 889-914. 

In purple bacteria investigated thus far, the development of the photosynthetic apparatus which is controlled by the oxygen partial pressure is not influenced by light. However, the prevalent light intensity modifies the morphogenetic process of formation of intracytoplasmic membranes. Light intensity also controls the total bacteriochlorophyll concentration per cell and per membrane protein, as well as regulating... 

Unlike the photosynthetic apparatus of photosynthetic bacteria, that of cyanobacteria consits of two photosystems, PS I and II, connected by an electron transport chain. The only chlorophyll present is chlorophyll a, and, therefore, chlorophylls b—d are not of interest in this article. Chlorophyll a is the principal constituent of PS I. Twenty per cent of isolated pigment-protein complexes contain one P700 per 20—30 chlorophyll a molecules the other 80% contain only chlorophyll a20). The physical and chemical properties of chlorophyll a and its role in photosynthesis have recently been described by Meeks77), Mauzerall75), Hoch60), Butler10), and other authors of the Encyclopedia of Plant Physiology NS Vol. 5. 

These experiments show that, as in the case of chlorophyll a and the carotenoids, the energy absorbed by the phycobiliproteins is utilized via the photosynthetic apparatus furthermore, they provide evidence that photophobic responses in blue-green algae are caused by sudden changes in the steady state of the photosynthetic electron transport, especially the non-cyclic one. 

The effect of zearalenone on crop development may be connected to its influence on the status and functioning of the photosynthetic apparatus (Koscielniak et al. 2008). The after-effects of zearalenone on the growth of soybean and wheat plants, net photosynthesis and transpiration rates, stomatal conductance, photochemical efficiency of photosystem 2 and on final seeds yield were determined. Modifications in leaf area were more pronounced in soybean than in wheat, and this tendency increases in successive developmental phases. The net photosynthesis was stimulated during the juvenile phase and during that of the final one by about 13.6% (average) in soybean plants. Stimulation of transpiration was also observed after... 

Pigments of the photosynthetic apparatus can also be destroyed after UV-exposure, with the phycobilins (main pigments of red algae and cyanobacteria) being the most sensitive, and carotenoids generally being less affected than chlorophylls (Teramura 1983). 

As mentioned in Sect. 13.3.6, carotenoids serve mainly as accessory pigments in the light harvesting complexes of the photosynthetic apparatus, and additionally as cellular protection against ROS. Whether carotenoids also act as passive UV-sunscreens is... 

Vass I (1997) Adverse effects of UV-B light on the structure and function of the photosynthetic apparatus. In Pessarakli M (ed) Handbook of photosynthesis. Marcel Dekker Inc., New York, pp 931-949... 

L. Bogorad I. K. Vasil (eds.) (1991) The Photosynthetic Apparatus, Academic Press, Orlando. 

FIGURE 2. Simphfied Z-scheme of the photosynthetic apparatus in higher plants... 

The photosynthetic apparatus of modem cyanobacteria, algae, and vascular plants is more complex than the one-center bacterial systems, and it appears to have evolved through the combination of two simpler bacterial photocenters. The thylakoid membranes of chloroplasts... 

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