THE LIVING CELL 34 
Protoplasm cannot be analyzed chemically. Dead proto- 
plasm, which is the material examined when protoplasm is 
subjected to reagents in the procedure of chemical analysis, 
consists of a variety of different chemical compounds including 
proteins, carbohydrates, fats, salts of sodium, potassium, mag- 
nesium, calcium and sometimes iron, etc., also gases like oxygen 
and carbon dioxide and water. 
PROPERTIES OF PRoropLasM.—The peculiar properties which 
distinguish protoplasm from non-living matter are as follows: 
1. SrructuRE.—Protoplasm, as it occurs in cells of plants 
and animals, invariably exhibits structure. No region of it, 
however small, has been found to be homogeneous. Each 
advance in microscopical technique reveals new complexities. 
The protoplasm of a single cell, far from being a single unit, 
must rather be looked upon as a microcosm. 
2. METABOLISM.—Perhaps the most significant peculiarity of 
living matter is found in its instability and the chemical changes 
which continually go on withinit. It is constantly wasting away, 
and as constantly being built up. These losses and gains are not 
upon the exterior surface but throughout its mass. Its growth 
and renewal are by intussusception, or the taking in of new particles 
and storing them between those already present. Inorganic 
‘substances, such as crystals on the other hand, grow by accretion, 
e.g. through the addition of similar material in layers on their 
outer surfaces. A bit of protoplasm may retain its identity 
while all the matter of which it is composed is changed over and 
over. It is like a whirlpool or wave in a river which remains the 
same while the water of which it is composed changes continually. 
Metabolism has been aptly defined by Huxley as the whirlpool 
character of the organism. It may also be defined as the series 
of processes concerned with the building up and breaking down 
of living matter. To those processes concerned with the building 
up of protoplasm, such as absorption, photosynthesis, digestion 
and assimilation, the term anabolism is applied, Another term 
for anabolism is nutrition. To those processes which involve the 
breaking down of protoplasm, such as respiration, excretion 
and secretion, the term katabolism has been given. 
3. IrRITABILITY.—This is the property possessed by proto- 
plasm of responding to a stimulus. All living matter responds 
