18 THE STORAGE OF ENERGY 



during formation, we may limit our problem to the total energy 

 change according to the two equations given above, viz. (i) and (iii). 

 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 HgO (117 gram-cals.), 

 CO2 (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 particular concentration. Therefore, when six molecules of 

 formaldehyde are condensed to one molecule of glucose, a corre- 

 sponding 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 

 formaldehyde. Sunlight here acts as a catalyst (Chap. X.), 

 Moore and Webster have synthesised formaldehyde by exposing 

 an aqueous solution of COg to ultra-violet light (Chap. XIII.) in 

 the presence of inorganic colloids (Chap. VIII.). 



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 CO2 per sq.cm. per minute. In the conversion of 1 c.cm. 

 of CO2 into glucose 5-02 gram cals. are stored. Therefore, in 

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

 rendered potential would be 0-00034 x 5-02 = 0-0017 gram-cals. 



Total incident light per sq.cm. per minute = 0-041 cals. 



Total volume of CO2 per sq.cm. per minute = 0-00034 c.cm. 



Energy rendered potential per sq.cm. per minute = 0-0017 cals. 



That is, 17 cals. out of every 410 that fall on the leaf are stored 

 in starch. But as only 41 cals. (10 per cent, of the total incident 

 light) are absorbed, we may consider that 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 pro])ortion 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 



