42 



OVER-ALL REACTION OF PHOTOSYNTHESIS 



CHAP. 3 



and Morris 1893, Sjoberg 1922) and of invertase, which splits sucrose 

 into fructose and glucose (Robertson, Irvin and Dobson 1909), does not 

 prevent leaves from containing large quantities of these polysaccharides. 

 The relative quantities of different monosaccharides and polysaccharides 

 in living plants must be determined by the rates of their formation and 

 decomposition, which depend on the available quantities of different 

 enzymes and the distribution of the latter in the tissue. 



(d) Polysaccharides 



Among the polysaccharides and their derivatives which occur in very 

 large quantities in plants, some (for example the cellulose of the higher 

 plants, and the alginic acid of algae) are too inert or too far removed from 

 the site of photosynthetic activity, to be suspected of a direct relationship 

 to photosynthesis. Starch is the only polymeric carbohydrate whose 

 association with photosynthesis is evident. The occurrence of starch 

 grains in chloroplasts, i. e., plant organs primarily concerned with photo- 

 synthesis, has been known since 1837, when von Mohl first observed 

 the chloroplasts under the microscope. Boehm (1856) confirmed that 

 these grains consist of starch, by the well-known iodine color test. Gris 

 (1857) noticed their growth during the day and dissolution during the 

 night, and Sachs (1862, 1863, 1864) first postulated their direct associ- 

 ation with photosynthesis. In a famous experiment, Sachs exposed one- 

 half of a leaf to the sun and kept the other covered, and showed that 

 after some time only the exposed half gave the color reaction with iodine. 

 Pfeffer (1873) and Godlewski (1877) completed the proof by showing 

 that no starch is formed by leaves illuminated in absence of carbon 

 dioxide. 



As mentioned above, starch is a high polymeric form of glucose; it 

 contains, in the native state, a small proportion of phosphoric acid (about 

 0.1% P2O5) which is probably important for its enzymatic transforma- 

 tions. As in maltose, the a-glucose molecules in starch are bound to- 

 gether by oxygen bridges : 



-o 



H OH 



Formula S.VI. Starch 



H 



O— 



OH 



A similar chain of jS-glucose molecules forms the basis of the structure 

 of cellulose. 



Most of our knowledge of starch has been derived from the study of 

 storage materials in seeds, tubers and roots; recently the preparation and 



