The Basis of Life 35 



cules are too large to pass out of the bag. Since the tendency to equal- 

 ize is so important, more water flows into than out of the sac. In this 

 case, equalization cannot be reached. 



In this experiment, the sugar solution had a much greater con- 

 centration of molecules than did the water surrounding the membrane. 

 The sugar solution is said to be hypertonic to the water, while the 

 water is hypotonic to the sugar solution. If two solutions separated 

 by a membrane have the same molecular concentration, they are con- 

 sidered to be isotonic. 



Both diffusion and osmosis are involved in the movement of ma- 

 terials in and out of cells and within the cells themselves. 



PROTOPLASM 



All living things are composed of a complex material, protoplasm. 

 In this material, the molecules are in all physical states mentioned 

 above, with the colloidal state predominating. Protoplasm varies in its 

 chemical and physical composition between species and even between 

 parts of a single plant or animal body. 



Its physical characteristics may be briefly summarized as follows : 

 it is slightly viscous, but capable of flowing, has a specific gravity 

 slightly greater than that of water due to the many dissolved substances 

 in it, and is semitransparent or translucent. 



Chemical composition.— Chemically, protoplasm is exceedingly 

 complex and is composed of a number of different elements and com- 

 pounds. Except for the heavier metals and rare earths, almost any of 

 the chemical elements may be present in living things and no one 

 chemical element is peculiar to it. In fact, most protoplasm is com- 

 posed of very common elements ; among them are : carbon, hydrogen, 

 oxygen, nitrogen, phosphorus, sulfur, sodium, chlorine, potassmm, cal- 

 cium, iron, iodine, and magnesium. 



The most important compound in protoplasm is water. This com- 

 poses from 60 to 99 per cent. It acts as the medium for solutions and 

 makes possible the many activities of protoplasm. Inorganic salts 

 such as sodium chloride, potassium chloride, magnesium chloride, and 

 sodium phosphate are common. The organic compounds, carbohydrates, 

 lipids, and proteins, are always found. 



Organization of protoplasm.-Protoplasm does not ordinarily 

 exist in an unorganized state. Rather it is organized into definite units 

 known as cells (Fig. 3) which have many forms and shapes. 



