14 THE INDIVIDUAL ORGANISM 



Brownian movement, surface tension, adsorption, and an increasing 

 number of chemical reactions came to be included in the growing list of 

 characteristic nonvital properties of protoplasm. 



The modern attack upon the problems of the structure and nature of 

 protoplasm is made possible by the great advances that have been made 

 in chemistry and physics. The application of the data and methods of 

 these sciences to the problems of biology has created the new sciences of 

 biophysics and biochemistry, both of which are concerned primarily 

 with what exists and goes on within the cell. Here we can only attempt 

 to summarize briefly some of the more elementary and basic findings 

 which throw light upon the nature and activities of protoplasm. 



Chemical Composition. Protoplasm is made up of nearly a score of 

 common elements and contains none that are not also found in nonliving 

 substances. They include carbon, oxygen, hydrogen, nitrogen, calcium, 

 sodium, potassium, sulfur, phosphorus, iron, copper, and chlorine, with 

 traces of boron, bromine and a few other elements. The great bulk of 

 these occur in the form of compounds — water (composing about three- 

 fourths of the total volume of protoplasm), a variety of salts in solution, 

 and the characteristic organic compounds classed as proteins, fats, and 

 carbohydrates. Few if any of these compounds are present in fixed 

 amounts, but vary within certain limits and are ordinarily in a state of 

 change. Protoplasm takes in selected substances and gives off others in a 

 continual interchange with the surrounding medium. 



Physical State. Protoplasm exists in what is known as the colloidal 

 state. This means that many of its component substances are dispersed 

 as minute separate particles suspended in a fluid or semifluid. This fluid 

 is a watery solution of salts, gases, and soluble organic substances; the 

 particles comprise notably the proteins, fats, and other nonsoluble 

 organic substances. Most of the particles are too small to be seen under 

 the microscope and yet are larger than the dispersed molecules of a true 

 solution. 



The colloid state, in which minute discontinuous particles are dispersed 

 throughout a continuous medium, is not peculiar to protoplasm. Familiar 

 examples of nonliving colloids are seen in liquid glue (the name colloid 

 comes from the Greek kollos, "glue"), jelly, and gelatin. The simplest 

 colloids consist of only two substances — a dispersed phase and a con- 

 tinuous phase. The colloid of gold in water is a beautiful red or blue fluid 

 in which the dispersed phase is a solid ; cream is a colloid in which both the 

 dispersed and continuous phases are fluid. Protoplasm contains both 

 solid and fluid particles. 



Even the simplest nonliving colloids show many complex and charac- 

 teristic properties. One of the most conspicuous is the capacity of many 

 colloids to change from semifluid to semisolid and back again with rela- 



