STARCH SYNTHESIS 323 



Measurements upon a number of species of plants within the temperature 

 range 10-35° C., while subjected to such conditions, resulted in values for 

 the photosynthetic ratio which seldom varied by more than ± 0.02 from unity. 



Starch Synthesis. — That starch is actually synthesized in the chloro- 

 plasts can easily be demonstrated. A potted plant of suitable species is kept 

 in a dark room until the mesophyll cells are starch free. Upon transference 

 to bright light the appearance of starch can be detected in the leaves by means 

 of the familiar iodine test within a relatively short time, in many species in 

 less than an hour. ]\Iicroscopic examination of the leaves will show that the 

 starch grains are located within the chloroplasts. 



However, the presence of starch in the cells of a tissue is not necessarily 

 proof that photosynthesis has taken place in those cells. Starch is insoluble 

 in water and cannot diffuse from cell to cell. The presence of starch grains 

 in a cell must mean, therefore, that the grains have been formed in that cell. 

 Starch grains often occur abundantly in the cells of non-green tissues or in 

 the tissues of roots or other plant organs which are never exposed to light. 

 Obviously photosynthesis cannot take place in such cells and the starch must 

 have been synthesized from sugars translocated to these cells which ultimately 

 came from the green parts of the plant. 



Photosynthesis and starch synthesis are therefore two distinct processes. 

 In the higher plants the former occurs only in the chloroplasts and in the 

 presence of light; the latter may occur in the chloroplasts, but also takes 

 place in many non-green cells and in the complete absence of light, providing 

 there is a suitable concentration of sugar in the cells, and other necessary 

 internal physiological conditions prevail. In non-green cells starch is synthe- 

 sized in the leucoplasts. 



The synthesis of starch from glucose may be represented by the following 

 equation : 



n CsHizOe -^ (C6Hio05)„ + n H2O 



Glucose % 



As this equation shows one molecule of starch is built up from a large 

 number {n) of ^-glucose molecules, with the elimination of 71 molecules of 

 water. The exact magnitude of n is unknown. Reactions of this type in 

 which a relatively large number of molecules are combined in the synthesis 

 of one larger molecule with the elimination of water are called condensation 

 reactions and are of common occurrence in living organisms. 



The starch formed in the chloroplasts of green plant organs is therefore 

 the product of a secondary reaction — starch synthesis — which can proceed 

 only when glucose is present in the cell. The critical concentration of simple 



