30 



FUNDAMENTALS OF CYTOLOGY 



and water react in such a manner as to yield a sugar. The energy for 

 the reaction is obtained from visible light. The green pigment chloro- 

 phyll absorbs this energy and transfers it to the reacting substances, and 

 oxygen is liberated as a by-product. This process of photosynthesis is 

 the primary source of the world's organic food supply. The reaction 

 resulting in the production of a gram molecule of hexose may be repre- 

 sented in a convenient though misleadingly simple manner as follows: 

 6CO2 + 6H2O + 673 kg. cal. = CeHisOe + 6O2. The energy of sun- 

 light, after being thus captured and stored, is released to the organism 

 through the process of respiration, which may be represented as follows: 

 CeHioOe + 6O2 = 6CO2 + 6H2O + 673 kg. cal. 



Chlorophyll is not the only pigment in the chloroplast. Chlorophyll 

 itself exists in two slightly different forms designated as a and h. Present 



Gelativona Sheath 



Cell Wall 

 -_J- 



Nucleus 



Dense Cytoplasm. 



Chloroplast 



Pyrenoid 



Central Vacuole 



Fig. 19. — A cell of a green alga (Spriogyra). {From Smith, Overton et al., A Textbook of 

 General Botany, Ath ed.. New Yoric, The Macmillan Company.) 



with these are the yellow carotenoid pigments xanthophyll and carotene, 

 whose color is evident after the chlorophyll disintegrates in drying autumn 

 leaves. Chemical analysis of tobacco leaves has revealed the following 

 amounts of these pigments in milligrams per square meter of leaf surface : 

 chlorophyll a, 29.30; chlorophyll b, 10.38; xanthophyll, 10.63; carotene, 

 3.52. Four of the many known carotenoids show a pronounced vitamin 

 A activity, although their value to the plant is obscure. There is also 

 evidence that vitamin C (ascorbic acid) is present in the chloroplast. 

 In the algae other pigments accompany the chlorophyll. Familiar 

 examples are fucoxanthin, a carotenoid, in the brown algae and phy- 

 coerythrin, a chromoprotein, in the red algae. Other carotenoids such as 

 riboviolascin occur in the purple bacteria. Bearing directly upon the 

 problem of the early stages of organic evolution is the striking chemical 

 resemblance of the complex chlorophyll molecule to that of the red 

 hemin in animal hemaglobin. The place of the magnesium atom in 

 chlorophyll is occupied by iron in hemin, thereby rendering hemoglobin 

 an efficient oxygen carrier. 



