Protoplasm, pigments

Although the pigmented cells are not directly related to the role of vitamin A in vision, these cells are of interest because they contain pigment granules that move under the influence of light from the body of the cell into the thin protoplasmic expansions. This pigmented protoplasmic film prevents diffusion of light from one photoreceptor to another. 

The above outline of the evolution of the plant kingdom during coal-forming eras has been presented largely in the language of botanical anatomy. However, the alert chemist will note that the anatomical differences imply considerable quantitative and spatial differences in the distribution of the principal plant constituents [cellulose, lignin, cutin, suberin and other waxes, contents of protoplasmic cells, pigments, resins, sporopollenin. 

Pigments.— These are substances which give color to various plant parts in which they are found. They occur either in special protoplasmic structures, as chloroplasts, chromoplasts or chroma-tophores, or dissolved in tjie cell sap. Of the pigments named the following will be considered Chlorophyll, Xanthophyll, Chromophyll, Etiolin, Anthocyanin, Phycocyanin, Phycophxin, and Phycoerythrin. 

Chromoplas tid.— A protoplasmic body in the cells of certain parts of plants containing a pigment other than chlorophyll. 

Fig. 9. Piece of the upper epidermis of a petal of Dendrobium nobile plasmolyzed and treated with iodine in potassium iodide. The heavy shading indicates alkaloid precipitated in the cell vacuoles, surrounded by the protoplasm (lighter shading) containing the pigmented chromoplasts. After Wildeman (8).

The two major pigments of protoplasm, heme and chlorophyll, originate from the same biosynthetic chain. The first product of the chain which is committed to the pathway (Fig. 1) is -aminolevulinic acid. The main branch point is the product protoporphyrin. When iron is inserted into the protoporphyrin ring the metalloporphyrin, iron protoporphyrin or heme, is formed in both plants and animals. When magnesium is inserted into the protoporphyrin ring the metalloporphyrin, Mg protoporphyrin, is formed which is converted in a series of steps to chlorophyll and to bacteriochlorophyll this occurs only in photosynthetic organisms. 

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