120 THE GREEN PLANT CELL 



the photosynthesis of sugar becomes negUgibly smalb 

 Since a certain amount of sugar is always being 

 removed from the cell through the veins, the concentra- 

 tion of sugar in the mesophyll cells falls, and the starch 

 grains in the chloroplasts are gradually reconverted 

 into sugar during' th«-«ight, and this sugar is likewise 

 conveyed away by the veins. Thus both during the 

 day and during the night there is a constant stream of 

 sugar away from the leaf to other parts of the plant 

 where it is used in respiration, in the ■f onnation of new 

 protoplasm and new cell walls, or is stored as starch, or 

 sometimes remains as sugar (beetroot, sweet fruits, 

 etc.), or as some other carbohydrate such as inulin 

 (Jerusalem artichoke, etc.), or is converted into fats. 



The formation of glucose from starch and of starch 

 from glucose may be represented by the general 

 reversible equation : — 



condensation 



hydrolysis 



The reaction is catalysed by the enzymes diastase 

 and maltase, probably in either direction (see p. 45 

 for the intermediate substances formed). 



Photosynthesis in a Green Cell living in Water. — In 

 a green cell living in water the processes of sugar and 

 starch formation are essentially the same as in the 

 mesophyll cell of a leaf. The only difference is that 

 the carbon dioxide is derived directly from that which 

 is dissolved in the water instead of being absorbed 

 from the air, and that the sugar does not leave the cell. 

 Starch (or in some species fat) is formed when the 

 concentration of sugar passes a certain point. The sugar 

 is used for respiration, to form the cellulose for cell 

 wall substance, and also (together with the elements of 



