INTRODUCTION 



far available is not adequate to decide the question. With 

 brightly illuminated leaves, however, the rate of 'apparent' 

 photosynthesis may be as much as fifty times the rate of res- 

 piration in the dark so that the correction to 'real' photosyn- 

 thesis is here relatively very small. Under field conditions 

 the rate of photosynthesis in normal daylight is 5 to lo times 

 the respiration rate. Under weak illumination respiration 

 may predominate even in the light and the estimate of 'real' 

 photosynthesis will largely depend on the value adopted for 

 the respiration rate. At some intermediate light intensity, the 

 compensating light intensity, photosynthesis and respiration 

 will balance exactly and there will be no net gas exchange. 



The reaction represented by equation (i.i) is in practice 

 not directly reversible by increasing the concentrations of 

 carbon dioxide and water relative to those of oxygen and 

 sugar; under any realizable concentrations of reactants or 

 products very considerable energy is required to make the 

 over-all reaction proceed in the direction of synthesis. 

 During photosynthesis light energy is converted into chemi- 

 cal energy in the form of the ultimate photosynthetic end 

 products. These in turn form the primary reactants in 

 respiration, so that from one point of view the biochemistry 

 of plant respiration and of the metabolic processes with 

 which it is linked is a study of the chemical reactions 

 involved in the distribution of energy originally derived 

 from light. 



In equation (i.i) the product of photosynthesis has been 

 written as organic compounds. If this is simply regarded as 

 carbohydrate the amount of carbon dioxide consumed will 

 equal that of oxygen produced. 



CO2+H2O ^ -(CH20)n+02 (1.2) 



n 



The ratio of the two quantities (COg/Og) called the assimi- 

 latory quotient, is in fact found to be near to unity in a 

 very wide range of plants and under various conditions 

 (Table i.i). In a leaf detached from the plant the product of 

 photosynthesis is frequently stored as starch; in a complete 

 growing plant or culture of micro-organisms, e.g. Chlorella^ 



