RESPIRATION 19 



accomplish the whole result. * The union of oxygen and the 

 substances to be oxidized is accomplished by the living cell. 

 How this is done is not known, though conjectures are not 

 lacking. Whether the combustible substances and the oxy- 

 gen are divided into such small particles and so intimately 

 associated in the living protoplasm that union takes place 

 spontaneously, or whether the oxidation is accomplished by 

 enzyms, or whether more readily or spontaneously oxidiza- 

 ble substances are first formed from sugar ( perhaps by the 

 action of an enzym), is not known, though there is a cer- 

 tain amount of evidence in favor of each hypothesis. All 

 that is known is that sugar, or some similar substance, and 

 oxygen, unite, forming as end-products mainly carbon- 

 dioxide and water. The following reaction, without indicat- 

 ing what intermediate stages there may be, if there are any, 

 shows the material results : 



0„ H.., 0, + 6 0, (+Aq) = 6 CO., + 6 H, (+Aq) 



( Aq. represents the water in which the sugar is dissolved 

 in the cell : it does not enter the reaction. The 6 H^ may 

 unite with this or these molecules may pass off as vapor, as 

 in ordinary combustion, being kept in a state of vapor by 

 the heat liberated.) 



Since other substances than sugar are also oxidized physi- 

 ologically in the cell, there are also other products of sorts 

 and in quantities varying with these. The commoner of 

 these minor products are oxalic, malic, and citric acids, 

 which accumulate in considerable quantities in certain 

 plants (e. g. in the leaves of Oxalis acetocella and in the 

 Crassulacese, in apples, etc., and in the citrous fruits — 

 lemons, limes, oranges, etc. ) , or are converted into salts 

 {e. ff. calcic oxalate, crystallizing out of the solutions in 

 which it is formed in the cell), or undergo other changes 

 ( e. g: further oxidation ) . 



In all organisms the oxidation of nitrogenous as well as 

 non-nitrogenous compounds occurs in normal respiration. 

 The proportional amounts of these two groups of com- 

 pounds physiologically oxidized varies with different organ- 



* Pfe£fer,W. Oxydationsvorgange. Physiol, of Plants, Vol. I., pp. 54-56. 



