76 - The Cell 



Organic Components of Protoplasm. Ex- 

 cept when synthesized artificially, organic 

 compounds occur only in living bodies, or 

 in their products and remains. Organic mole- 

 cules tend to be relatively large and com- 

 plex, and usually organic molecules lack any 

 strong tendency to ionize. However, the most 

 important distinction between organic and 

 inorganic compounds lies in the intrinsic 

 composition. The element carbon (C), which 

 rarely appears in inorganic chemistry, is pres- 

 ent in all organic compounds. In other words, 

 orgariic chemistry is the chemistry of carbon 

 compounds. In fact, the commonest test to 

 determine whether an unknown substance is 

 organic or inorganic is the charring test. Vir- 

 tually every organic compound decomposes 

 and yields carbon if it is heated drastically in 

 a dry condition. 



The unique capacity of carbon to form a 

 great variety of compounds — which are segre- 

 gated as a separate branch of chemistry — 

 derives from the structure and behavior of 

 the carbon atom. Carbon differs from most 

 other atoms in that it is intermediate be- 

 tween the electropositive atoms, which lose 

 electrons, and the electronegative atoms, 

 which gain electrons, when they unite to 

 form their respective compounds. Carbon 

 can combine with either type of atom, and 

 this capacity enriches the variety of carbon 

 compounds. Furthermore, the valence of car- 

 bon is relatively high. By gaining, losing, or 

 sharing the four valence electrons, carbon 

 can unite with four other univalent atoms or 

 atomic groups. 



An even more important property of car- 

 bon is its capacity to unite with other carbon 

 atoms. This property enables carbon to form 

 a wide variety of chainlike and ringlike 

 "molecular skeletons," and in this way car- 

 bon is able to unite with many other atoms, 

 forming very large complex molecules. 



Table 4-5 shows a simple series of organic 

 compounds composed entirely of carbon and 

 hydrogen. Here it should be noted that the 

 different molecules are constructed in similar 

 fashion, in that: (1) every carbon atom pos- 



sesses four combining points; (2) each mole- 

 cule is formed by a series of carbon to carbon 

 linkages; (3) the linkage between any two 

 consecutive carbon atoms mutually pre-empts 

 one valence from each carbon; and (4) each 

 hydrogen atom occupies just one valence 

 point of any given carbon atom. 



Kinds of Organic Substances. Living cells 

 contain a very great variety of organic com- 

 pounds. These carbon compounds are con- 

 tinually changing by interaction with each 

 other, but certain kinds tend to be most 

 abundant in the protoplasm. The most 

 abundant organic compounds are divided 

 into three main classes — the carbohydrates, 

 the lipids, and the proteins — which are dis- 

 tinguishable on the basis of their physical 

 and chemical characteristics, and on the basis 

 of their functions in the cell. There are still 

 other organic compounds (p. 89), which can- 

 not be put into any of the three major classes, 

 although they may be very important, 

 metabolically. 



Carbohydrates. The most familiar carbo- 

 hydrate compounds are the sugars, starches, 

 glycogens, and celluloses. Chemically, all car- 

 bohydrates have much in common, although 

 different carbohydrates may have different 

 functions in the cell. 



The chemical structure of carbohydrates 

 can be exemplified by two specific com- 

 pounds: (1) glucose (C 6 H 12 O e ), a white crys- 

 talline sugar, present in practically everv 

 cell; and (2) sucrose (C^.H^On), the familiar 

 sugar of the dining table. The formulas of 

 these compounds show that (1) the only con- 

 stituents of a carbohydrate are carbon, hydro- 

 gen, and oxygen; (2) the hydrogen and oxy- 

 gen display a ratio of 2:1 (as in water); and 

 (3) six (or a multiple of six) carbon atoms are 

 present. The first two points are true for all 

 carbohydrates, and the third is true for a 

 majority. Accordingly, typical carbohydrates 

 are compounds solely of carbon, hydrogen, 

 and oxygen, in which frequently carbon 

 atoms are present to the extent of six (or a 

 multiple of six) and the hydrogen and 

 oxygen are in a two to one ratio. 



