Photosynthetic tissues, chlorophylls

F4.2 Extraction of Photosynthetic Tissues Chlorophylls and Carotenoids F4.3 Chlorophylls and Carotenoids Measurement and Characterization by UV-VIS Spectroscopy... 

Carotenoids are also present in animal products such as eggs, lobsters, greyflsh, and various types of hsh. In higher plants, they occur in photosynthetic tissues and choloroplasts where their color is masked by that of the more predominant green chlorophyll. The best known are P-carotene and lycopene but others are also used as food colorants a-carotene, y-carotene, bixin, norbixin, capsanthin, lycopene, and P-apo-8 -carotenal, the ethyl ester of P-apo-8-carotenic acid. These are Upid-soluble compounds, but the chemical industry manufactures water-dispersible preparations by formulating coUoid suspensions by emulsifying the carotenoids or by dispersing them in appropriate colloids. ... 

Both chlorophylls and carotenoids occur in all green leaves, but their color is masked by chlorophyll in photosynthetic tissues. When the chlorophylls break down as leaves senesce (mature), the yellow and orange carotenoids persist and the leaves turn yellow. Carotenoids are responsible for the colors of familiar animals such as lobsters, flamingos, and fish. Often people are unaware of the chemical nature of food colorants. ... 

Chlorophylls a and b and, in smaller amounts, chlorophylls a and b can be found in any photosynthetic tissue obtained from land plants. They may thus be extracted from an almost infinite number of sources [23], although it is easier to work with material containing relatively litde water as this has to be removed at later stages of the workup. Thus, woodier tissues are to be preferred as sources as compared to those containing large volumes of water, e.g., parsley rather than cucumber. 

Very recently it was demonstrated that tocopherol moieties in kerogen are likely precursors of prist-l-ene (Figure 7) (30). This idea was supported by the fact that tocopherols are widely distributed in photosynthetic tissues and that they also occur as such in several recent sediments (31). It is tempting to conclude that during "natural pyrolysis" the generated pristene will be transformed to the well known component, pristane, in ancient sediments and oils. This example nicely illustrates that we have to be very careful when we conclude that acyclic isoprenoid hydrocarbons such as pristane originate from the chlorophyll side chain, phytol, based solely on structural similarities. 

Higher plant chloroplasts typically accumulate lutein, )3-carotene, violaxanthin, and neoxanthin (in order of abundance) as their major carotenoids. In general, carotenoid levels are directly proportional to the amount of chlorophyll in photosynthetic tissues, with lutein and neoxanthin being correlated with Chi b levels and j3-carotene with Chi a (Juhler et al.,... 

In the data for photosynthetic tissue such as the young and old parts of Zea mays L. leaves (Leech et al., 1973), the 2-foId increase in chlorophyll is echoed in the rise of phosphatidylglycerol (PG) which is characteristic of the chloroplast. The phospholipid expressed as a percentage of total lipid declines mainly because of the great increase in galactosyldiglycerides, but in both young and old sections the contribution of phospholipid is very small. 

There are relatively lar amounts of lipoic add and dihydrolipoyl dehydrogenase in photosynthetic tissues. Their presence still lacks a satisfactory explanation in terms of a particular functional role. Proposals implicating the lipoate dithiolane ring system in primary ener trapping or in the transfer and utilization of chlorophyll-trapped energy has not gained any real acceptances ... 

Cyclization of lycopene proceeds only after an d -trans lycopene is formed by the action of carotene isomerase (CRTISO) in nongreen tissue. In the photosynthetic tissues, this conversion is catalyzed by light and chlorophyll (acting as a sensitizer). Oxygenation of the cyclic carotenoids yields xanthophylls. Introduction of hydroxyl groups at positions 3 and 3 in p-carotene produces zeaxanthin. Zeaxan-thin epoxidase (ZEP) and violaxanthin de-epoxidase (VDE) act in tandem to regulate the formation of violaxanthin. Violaxanthin is next converted to 9-cis-neoxanthin, ABA precursor, by neoxanthin synthase. Lutein is mainly present in photosynthetic tissues, biosynthesized from a-carotene via catalysis by p- and E-hydroxylases. 

The carotenoids represent one of the most important and widespread groups of pigments in nature. The structures of some 500 of them are known, and they are responsible for many of the yellow and red colors of flowers, fruits, birds, insects, and other animals. " Carotenoids are synthesized de novo in all organisms except for animals, where the pigments are of dietary origin." Their universal presence in photosynthetic tissues is only noticeable at the onset of leaf senescence, when the chlorophylls disappear. The dramatic changes in color of ripening fruits also reflect the disappearance of chlorophyll and the concomitant, massive increase in carotenoids. 

Further, as clearly indicated by its name, the unique phenomenon in photosynthesis is not the assimilation of carbon dioxide, nor the synthesis of sugars but the utilisation for these processes of the radiant energy of the sun. This means that photosynthesis involves, in contradistinction to the other metabolic processes so far mentioned, certain photochemical reactions, insensitive to temperature and mediated through a photocatalytic system. The chlorophyll pigments organised in special cellular structures called chloroplasts represent this system and give to photosynthetic tissues their green colour. 

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