PROTOPLASM 9 



designated by attaching the suffix ase to the name of their 

 substrate, thus maltase is the enzyme that acts upon malt sugar. 

 Our knowledge of the chemistry of enzymes is not 

 complete, because when attempts are made to isolate them 

 it is frequently found difficult to separate them completely 

 from their substrate. Maltase has been shown not to be of 

 the nature of a protein. 



(4) Essential Nature of Enzymes. — They are colloids (see 

 p. 12), and their colloidal character helps to explain their 

 activity. 



The energy liberated by the enzyme of yeast is used by 

 the protoplasm for grotuth. 



5. Metabolism of Protoplasm — While ordinary proto- 

 plasm gets its energy by breaking down complex molecules 

 and liberating their stored energy, gi^een plants, by the action 

 of their chlorophyll, are able to store the energy of the sun's 

 rays by building up complex molecules such as sugar and 

 starch. It is through these green plants that the energy of 

 the sun is made available for all living things upon the 

 earth. 



Protoplasm is not only growing, it is also constantly 

 breaking down, and, if yeast be kept at a suitable tempera- 

 ture in water without any supply of material for construction, 

 it gives off carbon dioxide and decreases in bulk on account 

 of these disintegrative changes. These are as essential a 

 part of its life as the building-up changes, and it is only 

 when they are in progress that the latter are possible. 



Protoplasm {living matter) is living only in virtue of its 

 constant chemical changes (metabolism), and these changes 

 are on the one hand destructive {kataholic), on the other 

 constructive {anabolic). 



Living matter thus differs from dead matter in tJtis 

 respect, that, side by side with destructive changes, con- 

 structive changes are always going on, ivhereby its amount 

 is maintained or increased, so that it has been able to 

 spread all over the surface of the earth. 



Hence our conception of living matter is not of a definite 

 chemical substance, but of a set of substances constantly 



