LIFE AND PROTOPLASM 



i:{3 



is called a colloid. Xow these terms are not api)lie(l t(j fixed sub- 

 stances but to states of matt(M-. Gelatin passes from a li{|uid to ii 

 solid state on being heated or cooled. A study of the diap;ram shows 

 how this might be possible. In the left-hand diagram the solid i)ar- 

 ticles are floating freely in the fluid of the medium ; in the middle 

 diagram the solid portion is becoming a loose mesh; while in the 

 right-hand diagram the mesh has become a solid mass, including the 

 liquid within it . The protoplasm within the cells of plants and animals 

 probably behaves in a similar manner, under some conditions assum- 

 ing the "sol," and at others the "gel" state. Remembering that 

 protoplasm is not a single protein substance, but rather a mi.xture 

 of proteins, fats, carbohydrates, and sometimes even other sub- 

 stances, it is clear why there are many slightly different protoplasms 

 depending on the part of the animal or plant examined. This fact 

 may help us to see why the living matter of a muscle, the blood, or 

 the brain differs visibly in structure. For one thing, the water con- 

 tent differs greatly. Living bone is said to be 25 per cent water, 

 muscles about 75 per cent, the jellyfish almost 99 per cent, and some 

 fruits as high as 98 per cent water. 



Diffusion 



We have spoken of the work of the enzymes in making food sub- 

 stances soluble. Let us now see why .'^olul)ility is necessary for the 

 life processes of cells. The physical phe- 

 nomenon of diffusion is easily demonstrated 

 by the slow spread of red ink when a droj) 

 is put into a glass of water. Brownian 

 movement of dancing particles visible under 

 the high power of the microscope is a mani- 

 festation of molecular kinetic energy caused 

 by the water molecules bombarding these 

 particles. It is a similar movement of 

 molecules that occurs when diffusion takes 

 place. Molecules of any substance are 

 always in motion. If this substance is 

 soluble (the solute) in another substance 

 (the solvent), there is always a tendency 

 for these molecules to move from the 

 place of their greatest concentration to i^laces where they are not 

 so highly concentrated, until an equilibrium is reached and there 



Lonfjit udiiial sect ion 

 through a tumbler of watt-r 

 containing soluble crystal. 

 sliowiiif; by arrows tlic direc- 

 tion of (lillusion. ami by 

 (lotted circles the lines of 

 equal concentration. 



