382 CHLOROPLASTS AND CHROMOPLASTS CHAP. 14 



C. Pigments in the Chloroplasts * 

 1. Association of Pigments with Proteins and Lipoids in the Cell 



The composition of the chloroplast pigment system and the properties 

 of the single pigments will be discussed in chapters 15 to 20 and in 

 chapters 21 to 24 of volume 11. In the present chapter, we shall deal 

 only with the state of the pigments in the chloroplasts and their relation 

 to proteins and lipoids. 



A chloroplast is perhaps one-half water (no exact data on water 

 content appear to be available; c/. Menke 1938), the other half consisting 

 of 50% protein, 30% "lipoids" (i.e., ether- or ether-alcohol soluble 

 compounds) and up to 10% pigments. The concentration of matter in 

 the chloroplasts is so high that each pigment molecule is continually in 

 interaction with several neighbors. The problem arises whether this 

 interaction leads to associations of a more or less permanent nature and, 

 if so, whether these associations occur in stoichiometric proportions, or 

 are more in the nature of "adsorptions" — a variable number of smaller 

 molecules being bound by one larger molecule or attached to a colloidal 

 interface. 



Chlorophyll molecules are similar to tadpoles, with a flat, slightly 

 hydrophihc head (porphine nucleus) and a hydrophobic tail (phytol 

 chain). The "head" may have the tendency to associate itself with 

 protein molecules; the "tail" may have an affinity for other paraffin 

 chains and thus tend to associate itself with other hydrophobic "lipoid" 

 molecules. The carotenoid molecules are all tail and no head; they are 

 either completely nonpolar (carotenes) or slightly polar (carotenols). 

 Their strongest tendency is for association with lipides, which is why 

 they are often designated as "lipochromes" — although the carotenols are 

 occasionally also found in association with proteins. The phycobilin 

 molecules are all head and no tail; their tendency for association with 

 proteins can therefore assert itself without interference by the lipides; 

 they belong to the class of "chromoproteids." The result of these 

 gradations in polarity is that of all plastidic pigments only the phycobilins 

 can be dissolved directly from the cells by water, forming a colloidal 

 pigment-protein solution, and are not extracted from the latter by or- 

 ganic solvents. 



The behavior of chlorophyll in the living cell is much more complex. 

 Grinding of leaves with pure water produces a green suspension, consisting 

 of broken cells, chloroplasts, or single grana, which is more or less stable 

 depending on the grinding procedure and the species, but does not 

 represent a true colloidal solution. The particles in the suspension are 

 comparatively large and nonuniform and contain proteins, lipoids, and 



* Bibliography, page 397. 



