42 INTRODUCTION TO GENERAL PHYSIOLOGY 



purpose, and the source of this supply is the sun, whose rays are 

 absorbed by the green pigment of those plants which possess it. 

 Other plants, fungi, require sugar or similar substance to be 

 supplied ready made, although a few of them are satisfied with 

 somewhat simpler carbon compounds, so long as these have a 

 higher chemical potential energy than carbon dioxide has. 



Before we proceed further it will be useful to remind ourselves 

 of the reasons why carbon forms such an enormous variety of 

 different compounds, and is, therefore, particularly fitted to be the 

 basis of the chemical changes taking place in living organisms. 

 In the first place, owing to its possession of four valencies, it is 

 able to form a great variety of derivatives of any one compound, 

 one valency combining with a group of one kind, another with a 

 different one, and so on. Secondly, the power that carbon atoms 

 have of combining with one another, gives the possibility of great 

 complexity and size of compounds. Thirdly, carbon is able to 

 combine with elements of opposite characters, owing to its position 

 in the middle of the periodic table. Thus, it can combine with 

 hydrogen or oxygen, with nitrogen or chlorine. It can, therefore, 

 be alternately oxidised and reduced, thus acting as a carrier of 

 energy. The reduced compounds give off energy when oxidised 

 or burned, while the oxidised compounds require the addition of 

 energy in order to reduce them. Fourthly, it alters its character 

 according to the groups with which it is combined. Thus, while 

 NO 2 C=H 2 is usually "negative," that is, has special affinity for 



elements like hydrogen, CH 3 is positive, like hydrogen, and has 

 affinities similar to those which hydrogen has. Fifthly, carbon com- 

 pounds react slowly, or are comparatively stable. Reactions which 

 proceed of themselves with explosive rate are incompatible with 

 vital phenomena. H 2 SO 3 (sulphurous acid) is much more reactive 

 than HCH 3 SO 3 (methyl-sulphonic acid). On the other hand, 

 this same property enables large molecules of high potential 

 energy to be built up, which remain stable when left alone, but 

 decompose with great violence when the powerful shock of a 

 detonator acts upon them. 



Returning now to glucose, we note that there is a class of 

 compounds containing carbon atoms and water molecules in an 

 equal number. For this reason they are called carbohydrates. The 

 actual numbers of the carbon atoms vary from one to six, or more. 

 The most important ones, from our present point of view, are those 

 of six atoms. They are the sugars called hexoses. The five carbon 

 sugars, or pentoses, are of frequent occurrence in plants, and form 

 an important, although not large, constituent of certain com- 

 pounds in the nucleus of the animal or plant cell. It appears, 



