250 FINE-STRUCTURE OF PROTOPLASM II 



rings is shifted to a place between atoms 4 and 5 ; as a result, the C atom 

 marked 6 becomes asymmetrical and the molecule optically active. In the 

 case of y-carotene the six-membered ring is open, the bond between C 

 atoms I and 6 lacking. Small to larger quantities of a- and y-carotene are 

 often present in leaves, as, for example, a-carotene in the leaf of Daucus 

 Carota (Mackinney and Milner, 1933) and y-carotene in Cuscuta salina 

 (Spoehr, 1935, p. 193). To these three carotenes may be added lycopene 

 and others, all of which are distinct from each other by virtue of their melting 

 points and absorption spectra (Smith, 1936). Like ^-carotene, a-carotene 

 and y-carotene are provitamins for the growth factor A, but they produce 

 only half its effect. This is because the two symmetrical halves of ^-carotene 

 have exactly the same chemical constitution as vitamin A, whereas, owing 

 to the slight morphological changes to one of the terminal six-membered 

 rings of a- and y-carotene, only the unchanged half of the structural 

 formula can produce vitamin molecules. With lycopene both the terminal 

 six-membered rings are open, which is why this carotenoid, known chiefly 

 in tomato, has no vitamin A activity at all (Karrer, 1935; Kuhn, 1937). 

 This illustrates most aptly the powerful influence of the special morphology 

 of the molecules upon the specific reactions in the organism. 



There are also numerous yellow xanthophylls C4 3H56-q(OH)jj. Except for 

 the introduction of OH groups at certain places in the structural formula, 

 their molecules are built up in the same way as the orange carotenes. Crypto- 

 xanthin possesses one of these hydroxyl groups at the C atom marked 3, 

 whereas in the zeaxanthin from the grains of maize both six-membered 

 rings are substituted in this way. There are small amounts of both compounds 

 in leaf xanthophyll, though it mainly consists of another xanthophyll with 

 two OH groups viz., lutein, which has been known for some time from egg 

 yolk. It comprises 50-60% of the xanthophyll (Spoehr, 1935) in the leaves 

 of spinach, gourd, sunflower, lettuce, barley and other leaves. The OH 

 groups cause the beginning of light absorption to shift somewhat towards 

 the shorter wavelengths as compared to ^S-carotene. In carotenoids with 

 three and more oxygen atoms, epioxide-bridges have been discovered 

 (Karrer, 1946). 



According to the foregoing considerations, the fundamental principle of 

 the molecular structure of all carotenoids is a relatively short chain of un- 

 saturated hydrocarbon with conjugated double bonds. Minor variations 

 in this type of structure give rise to the numerous carotenoids and hydroxyl 

 substitution produces the various xanthophylls (Smith, 1937). 



As opposed to this variability on the part of the yellow pigments, in 

 higher plants we have the two green pigments, chlorophyll a and h, with 

 their strikingly unvarying constitution. Thanks to this, the percentage of 

 the two chlorophyll pigments contained in leaves can be determined by the 

 quantitative method of spectral analysis (Heierle, 1935 ; Sprecher, Heier- 

 LE and Almasi, 1935). The yellow leaf pigments lend themselves to such 

 analysis only if they are composed of ^-carotene and lutein and nothing 



