EARLY HISTORY OF LIFE 



39 



cules enter freely, uniting with larger mole- 

 cules which suddenly break up into a great 

 many smaller molecules, some of which 

 immediately leave the scene through the 

 plasma membrane. This is the union of 

 oxygen and glucose, producing carbon di- 

 oxide and water. There would be many 

 other kinds of molecules, small and large, 

 apparently floating about but in reality per- 

 forming very specific functions which we 

 do not understand at the present time. 



This mass of molecules in endless motion, 

 constantly changing, adding new mole- 

 cules, losing others, combining and separat- 

 ing, is protoplasm. This dynamic activity 

 constitutes the thing we call life. The mole- 

 cules are no different from those in the in- 

 animate world; they react in the same way. 

 It is only when they come together in the 

 combination to form protoplasm that they 

 exhibit the characteristics we associate with 

 life. 



Although much of the foregoing discus- 

 sion is based on speculation, there is con- 

 siderable evidence to demonstrate that per- 

 haps the speculations are not too far from 

 the truth. What can we learn about proto- 

 plasm with ordinary laboratory equipment? 

 Let us go a little further into its chemical 

 and physical nature. 



CHEMICAL COMPOSITION 

 OF PROTOPLASM 



If a mouse, butterfly, elephant, or a plant 

 were analyzed in the chemist's crucible 

 and the elements named, they would be 

 remarkably similar in all of these widely 

 different forms of life. The four principal 

 elements are carbon (C), hydrogen (H), 

 oxygen (O), and nitrogen (N). Carbon is 

 the core element of the complex molecules 

 found in protoplasm, probably because of 

 its physical nature which was discussed 

 earlier (p. 29). Additional elements regu- 

 larly present in protoplasm are phosphorus 

 (P), calcium (Ca), iron (Fe), potassium 

 (K), sulfur (S), iodine (I), magnesium 



(Mg), sodium (Na), and chlorine (CI). 

 There are many others existing only as 

 traces but nevertheless essential for the nor- 

 mal activities of protoplasm. 



Elements in protoplasm usually are found 

 combined in the form of inorganic and or- 

 ganic compounds. The former go to make 

 up most of our world we live on, whereas 

 the latter are always derived from living 

 things. The inorganic compounds that are 

 important in protoplasm are water, inor- 

 ganic salts, and certain dissolved gases, 

 such as oxygen, nitrogen, and carbon di- 

 oxide. Important organic compounds are 

 lipids (fats and related substances), car- 

 bohydrates, and proteins. These we must 

 consider in more detail. 



Inorganic compounds in protoplasm 



Water. Of all the inorganic compounds 

 in protoplasm the most important is water, 

 the substance in which all other materials 

 are suspended and transported. Life with- 

 out water would be impossible, because 

 there would be no means of mixing and dis- 

 persing the energy-yielding and building 

 materials of protoplasm. Water is important 

 in protoplasm because of its many unique 

 properties. 



Water dissolves more substances than 

 most any other liquid. This property of 

 water makes possible the mixing of a large 

 variety of substances that would not dis- 

 solve in any other liquid. Hence, the great 

 complexity of protoplasm has come about 

 because so many different substances could 

 come together and mix freely in one com- 

 mon medium. Furthermore, interactions oc- 

 cur more readily between substances in a 

 fluid condition where ample freedom of 

 movement of the molecules is permitted. 

 In a dry state, where substances are less 

 free to mix with one another, chemical 

 union would be greatly impeded. 



Water has a high capacity for holding 

 heat. It is reluctant to take on or lose heat, 

 a physical property that is very important 

 in living things. Anyone living near large 



