516 PROTOPLASM 



ments, notably carotene. (While carotene has no vitamin-A 

 content, it is a precursor of it and is converted into vitamin A by 

 the liver.) The formula of vitamin A is as follows: 



CH3 CH3 



CHsv /CHs 

 C 



HaC^^C— CH=CH— C=CH— CH=CH— C=CH— CH2OH 



I II 



H2C C — CH3 



\ / 

 CH2 



The molecular structure of vitamin D is similar to that of the 

 sterols, which are converted into it through activation by ultra- 

 violet light. 



PIGMENTS 



Pigments have played but a small part in physiological studies 

 in the past. Only recently, with the finding of the striking 

 relationship between carotene and vitamin A, has an at least 

 partial realization of the nutritional value of pigments been 

 attained. Equally fundamental has been the discovery of 

 Keilin; by spectroscopic observations he proved the existence of 

 three pigments, collectively known as cytochrome, in most living 

 cells. Cytochrome is now recognized as a respiratory pigment of 

 prime importance. 



To what extent other pigments are of nutritional or enzymatic 

 value is not yet fully known. For carotene (from carrots and 

 other plants, e.g., from the oil of palms), the problem seems to be 

 solved; its physiological value lies in its capacity for conversion 

 into vitamin A. It is given the formula C40H56 and is thought to 

 consist of two isomeres called alpha and beta carotene. Karrer 

 finds the structural formula to be that of a carbon chain attached 

 to a ring at both ends. Xanthophyll, C40H56O2, is associated 

 with carotene and is regarded as a glycol of it. Other pigments 

 that have a biological value are the anthocyanins (the basic sub- 

 stance of the red and blue flower and fruit pigments). 



The foremost of plant pigments is chlorophyll. It has long 

 been thought to be of dietary value. Its close relationship, 

 chemically and phylogenetically, to hemoglobin has often been 



