46 Practical Plant Biology. 



crystalloidal chlorophyll. Chlorophyll is in the colloidal state in 

 the chloroplast and does not exhibit fluorescence. 



The green colouring matter, as it is found in the chloroplasts, 

 is a mixture of four pigments, two of which are green and are 

 called chlorophyll a and b respectively. Chemically they are both 

 esters of methyl (CH 3 OH) and phytyl (C 2 oH 3 gOH) alcohols. It 

 has been shown that they both contain magnesium. This element 

 is supposed to play a similar part in the chlorophyll molecule to 

 that of iron in the molecule of haemoglobin, the pigment of the 

 blood. Chlorophyll b contains more oxygen than chlorophyll a. 

 They are represented chemically as follows : 



Chlorophyll a, C 55 H 72 O 5 N 4 Mg ; Chlorophyll b, C 55 H 70 O ti N 4 Mg. 



Besides the two chlorophylls there are two yellow pigments 

 present in the chloroplasts called respectively carotin and xantho- 

 phyll. They appear to bear a relation to one another similar to 

 that borne by the chlorophylls to one another. Xanthophyll 

 (C 40 H 56 O 2 ) contains oxygen, while carotin, the formula of which is 

 C 4 oH 56 , has none. These four pigments have been found in the 

 chloroplasts of all the higher plants so far examined. They are 

 present in about the proportions of 14 parts of chlorophyll a, 5 

 parts of chlorophyll b, i part of carotin and 2 parts of xanthophyll. 



The exact relation of the green colouring matter to the chloro- 

 plast is doubtful. It sometimes appears as a film spread over 

 the surface of the chloroplast while some observers believe that 

 the green colloidal substance interpenetrates the protoplasm of 

 the chloroplast occupying cavities within it. 



As we have seen, the substances formed during photosynthesis, 

 viz. carbohydrates, possess a large amount of energy. They may 

 be oxidised and show a large heat of combustion. The substances 

 from which they are built up, viz. carbon dioxide and water, are 

 fully oxidised and can supply no energy. Hence it is evident 

 that to effect such synthesis of substances containing a large 

 amount of energy from ones which, from this point of view, are 

 without any, a supply of energy is needed. This supply is 

 evidently drawn from the energy of sunlight, absorbed by the 

 pigments of the chloroplast, which we know is necessary for 

 photosynthesis. In fact, we may regard the chloroplast with its 

 pigments as a molecular engine for converting raw simple sub- 

 stances with a small energy content into more complex ones con- 

 taining a large amount of energy. The energy for driving this 

 engine is supplied by the sun. How these engines effect this 

 transference of energy into the products of photosynthesis is a 



