Nutrition of Green Plant Cells - 1 59 



mediary stages. Nevertheless it was many 

 years before even this much information be- 

 came available. 



The first inkling that photosynthesis lib- 

 erates free oxygen came in 1772, from the 

 work of Priestley. Priestley showed that a 

 mouse under a sealed bell jar can continue 

 breathing more or less indefinitely, provided 

 that a green plant is also placed under the 

 jar. Within another ten years, Ingenhousz, a 

 Dutch physician, had shown that light must 

 be available if this experiment is to work, 

 and that only the green parts of a plant are 

 effective. In 1804 de Saussure found that the 

 weight gained by a plant growing in a sealed 

 atmosphere was distinctly greater than the 

 weight of the carbon dioxide lost by the at- 

 mosphere. Thus de Saussure concluded that 

 water (as well as carbon dioxide) must be an 

 essential ingredient in the photosynthetic re- 

 action. During the rest of the nineteenth 

 century, the quantitative aspects of the over- 

 all reaction were established by the work of 

 many investigators. 



Chlorophyll and Chloroplasts. One of the 

 most essential enzymes in photosynthesis is 

 a chlorophyll-protein complex, which is 

 often referred to, rather loosely, as chloro- 

 phyll. The nonprotein part, chlorophyll 

 proper, which can be extracted from leaves 

 by ether and similar solvents, proves to 

 be a green pigment with a rather compli- 

 cated molecular structure (Fig. 9-1). Struc- 

 turally, the chlorophyll-protein complex 

 tends to resemble hemoglobin, the red pro- 

 tein pigment of blood, except that a magne- 

 sium, rather than an iron, atom occupies a 

 central position in the pigment part of the 

 molecule; and attached peripherally there is 

 a long-chain alcohol, phytol. In addition to 

 chlorophyll, the cells of higher plants con- 

 tain two other pigments: the deep orange 

 xanthophyll (p. 596) and the bright yellow 

 carotene (p. 350); and the cells of blue-green 

 and red algae (p. 600) possess the accessory 

 pigments phycocyanin and phycoerythrin, 

 respectively. The accessory pigments of the 

 algae probably participate directly in photo- 



CH 2 



II 



CH CH 3 



X— H | 



' \ I \ \/ 



• / \ ,' \ i 

 H;c w , M \ /| H 



C — CH 9 CH^ 



\C- 



\ 



r/ VVY^ ^Cf ' 



phyto 



Fig. 9-1. Molecular structure of a chlorophyll (chlo- 

 rophyll a). The "body" of this molecule (area en- 

 closed by dotted line) is very similar to the heme part 

 of a hemoglobin molecule. In hemoglobin, however, 

 an atom of iron (Fe), rather than of magnesium (Mg), 

 occupies the molecular center. Actually five kinds of 

 chlorophyll, each differing slightly from the others, 

 have been identified. These are specified as chloro- 

 phyll a, b, c, d, and e, respectively. Higher plants 

 generally possess forms a and b; but lower plants 

 often have the other forms. 



synthesis, augmenting the activity of chloro- 

 phyll; but the role of such pigments in higher 

 plants is not well understood. 



The ordinary microscope shows that chlo- 

 rophyll, in the typical plant cell, is not dis- 

 tributed evenly throughout the cytoplasm. 

 Rather it is confined within one or more 

 (usually about 30) rounded bodies, the chloro- 

 plasts. The electron microscope further 

 shows that the chlorophyll is strictly localized 

 even in the chloroplasts. All of it is contained 

 within a number (usually about 50) of 

 smaller bodies, called grana, which lie inside 

 each chloroplast (Fig. 9-2). The chlorophyll 

 is spread out into exceedingly thin, probably 

 monomolecular, layers enclosed within the 

 complexly folded membranes, or lamellae, 



