18 THE STORAGE OF ENERGY 



From (i) we see that a gram-molecule of formaldehyde has an 

 energy-content approximating 112,300 gram calories. This store 

 of energy is derived from the constituents H 2 O (117 gram cals.), 

 CO 2 (92-4 gram cals.), and from absorbed sunlight (112,090-6 

 gram cals.). 



With the formation of formaldehyde, practically all the energy 

 necessary for the formation of carbohydrates has been absorbed. 

 As we shall learn later (Chap. V.), osmotic energy is a function 

 of concentration. Therefore, when six molecules of formalde- 

 hyde are condensed to one molecule of glucose, a corresponding 

 amount of osmotic energy is liberated, and this may be utilised 

 in part in endowing the glucose with the slightly higher content 

 of chemical energy which it possesses over that of the formalde- 

 hyde. Sunlight here acts as a catalyst (Chap. IX.). 



As we have seen, all the light falling on the leaf is not utilised 

 even all the light absorbed is not stored. Some energy is 

 required for direct domestic use, e.g. transpiration. It has been 

 calculated that about 10 per cent, of the incident light is absorbed 

 by the chloroplast pigments. In an experiment by Brown and 

 Escombe it was found that a total amount of incident light, 

 which, if converted into heat units would correspond to 0-041 

 cal. per sq.cm. per minute, caused the decomposition of 0-00034 

 c.cm. of CO 2 per sq.cm. per minute. In the conversion of 1 c.cm. 

 of CO 2 into glucose 5-02 gram cals. are stored. Therefore, in 

 building 0-00034 c.cm. of CO 2 into sugar the amount of energy 

 rendered potential would be 



0-00034 x 5 -02 =0-0017 gram cals. per cm. per minute. 



As that is about 4 per cent, of the total incident radiation, 

 the efficiency of the chloroplast under maximal conditions is 

 somewhere about 40 per cent. When the process is reversed 

 and carbohydrate split up with the assimilation of oxygen and 

 the evolution of carbon-dioxide, this energy is again set free. 

 It may be freed in such a way that a certain proportion of it 

 appears as light. This light has, according to Trautz, the same 

 wave length as the originally absorbed light. Of course, in 

 general, the energy will be evolved in a form more suitable for 

 utilisation than this (see Chaps, on Osmosis, Surface Tension, etc.). 



Fats are stored up also in the plant. Very little research has 

 been done on the synthesis of fats in the plants. Whether the 

 plant can form these compounds directly or whether they are 

 only synthesised from carbohydrate is not known. That they 



