PHOTOSYNTHETIC PIGMENTS 27 



in a concentration as low as io~^ M. entirely prevents the 

 bleaching. Carbon dioxide has little effect, whilst formic 

 acid greatly increases it. So far no reversible changes in the 

 spectrum of chlorophyll under biological conditions have 

 been observed. Thus there is no evidence yet as to whether 

 reversal changes in the pigment observed in solution 

 have any bearing upon the function of chlorophyll in the 

 cell. 



6. Irreversible bleaching. Solutions of chlorophyll also 

 bleach irreversibly in the light. In chloroplast preparations 

 and in the living cell the pigment appears to be protected in 

 some way and is only bleached under extreme conditions. 



Distribution of chlorophylls 



The content of chlorophyll in green leaves varies between 

 about 0-05 to 0-20% fresh weight, generally consisting of 

 about 2*5 parts of chlorophyll a to one part of chlorophyll b. 

 Leaves of yellow-green varieties may have a far greater rela- 

 tive amount of chlorophyll a, the total amount of chloro- 

 phyll often being less than one-third that in green varieties. 

 A greater proportion of chlorophyll a is frequently found in 

 'shade' plants compared to 'sun' plants. 



Great variation in chlorophyll content occurs in the dif- 

 ferent classes of algae. In Chlorella the chlorophyll content 

 may be as much as three times that in leaves (see Appendix 

 I), which means that the concentration in the chloroplasts 

 is of the order of o-o5M. and perhaps therefore even o-2M. 

 in the grana. In Chlorella the ratio of chlorophyll atob may 

 be as great as 3 : i, whereas in many other green algae it is 

 only 1-4 : i. Chlorophyll h is absent from tht brown algae, 

 red algae, diatoms and the blue-green algae. Thus chloro- 

 phyll a is the most abundant in all plants and with the 

 exception of the bacteria is common to all types of auto- 

 trophic plants. In the brown algae, diatoms, and the flagel- 

 lates, from which chlorophyll b is absent, a different pigment 

 chlorophyll c is present in an amount about one-tenth the 

 total chlorophyll content. In the red algae chlorophyll a is 

 accompanied by chlorophyll d, the rnain absorption band of 

 which is further in the red (6,860 A in ethyl ether). 



