RESPIRATION AND PHOTOSYNTHESIS 121 



mineral salts entering the cell from the surrounding water), 

 to make the proteins which form the basis of the new 

 protoplasm produced as the cell grows and divides. 



Respiration and Photosynthesis. — It will be noted 

 that the general equation representing respiration is 

 the exact reverse of that representing photosynthesis. 

 The two equations may be combined thus : — 



RESPIRATION 

 potential energy — i> liberation of energy — > kinetic energy 



(oxidation) 



CeHiaOe + 6O2 :?=^ 6CO2 + eHgO 



(reduction) 



potential energy < — locking up of energy < — kinetic energy 

 PHOTOSYNTHESIS 



In the mesophyll cell of a well-lighted leaf the photo- 

 synthetic process very greatly exceeds respiration, 

 so that carbon dioxide and water are continuously 

 used up, sugar and free oxygen produced. When 

 illumination falls to a certain low level of intensity 

 the two processes will exactly balance, and the system 

 represented by the green cell will be in equilibrium in 

 respect of these processes. In still weaker light or 

 in the dark photosynthesis stops altogether and sugar 

 and oxygen are continuouly used up, carbon dioxide 

 and water produced. Thus it is true that green plants 

 give off carbon dioxide in the dark. But the popular 

 notion that it is " unhealthy " to keep plants in a bed- 

 room at night is nevertheless unfounded — the amount 

 of the gas given off as a result of the respiration of 

 the plant cells is far too small to raise appreciably 

 the carbon dioxide content of the air. 



Synthesis of Proteins. — ^The formation of sugars in 

 the green cells, their condensation into starch, and the 



