EARLY HISTORY OF LIFE 



41 



In the leaf of the plant, CO2 and HoO 

 unite under the influence of the sun's rays 

 and in the presence of a green plant pig- 

 ment called chlorophyll, to form glucose in 

 this manner: 



6 CO2 + 6 H2O -^ CeHisOe + 6 O2 

 This chemical reaction is known as photo- 

 synthesis. It is the starting point in the for- 

 mation of all food substances. Glucose is 

 known as a monosaccharide. By the simple 

 subtraction of water (dehydration) from 

 two glucose molecules a disaccharide, su- 

 crose, is formed, thusly: 



2 (CeH.oOe) - H2O 

 glucose 



C/12H22O11 

 sucrose (cane sugar) 



If this is continued, large molecules can 

 be built up. Such compounds are called 

 polysaccharides, and are illustrated by 

 starch in plants and glycogen in animals. 

 Starch and glycogen do not dissolve readily 

 in water and consequently are ideal storage 

 materials. 



The reverse process is simple also, for by 

 merely adding HoO (hydrolysis), the large 

 molecule falls apart into many molecules 

 of glucose, the number depending on the 

 original carbohydrate molecule. This is 

 what happens during digestion in the ani- 

 mal body, a process we are to discuss in 

 some detail later. 



Glucose is the most important carbohy- 

 drate in protoplasm because it is the sub- 

 stance that unites with oxygen to release 

 energy that is essential in the business of 



H H H H H H H 



living. We think of carbohydrates as energy 

 foods. 



Lipids. These include all of the fats, oils, 

 and fat-like substances identified by a 

 greasy texture. They are relatively insoluble 

 in water, but soluble in hot alcohol, ether, 

 and chloroform. With a few exceptions they 

 all contain C, H, and O like carbohydrates, 

 the chief difference being their low oxygen 

 content. This is easilv illustrated bv exam- 

 ining the formula of a common fat taken 

 from beef tallow, C57H110O0. Note that 

 there are only 6 oxygen atoms as compared 

 to 57 carbon and 110 hydrogen atoms. This 

 means that when fats burn, they require 

 more oxygen than does glucose. It also 

 means that the molecule can release more 

 energy per gram when burned. Their in- 

 solubility makes fats desirable material for 

 the storing of energy. 



Like the starch molecule, a fat molecule 

 is composed of simpler components that 

 can be separated by hydrolysis. These are 

 glycerol (CsHsOs) and fatty acids. A com- 

 mon fatty acid is stearic acid with the 

 formula (CH3(CHo)io • COOH). This is 

 called an acid because the carboxyl group 

 (GOOH) can liberate one hydrogen ion 

 when dissolved in water. It can be written 

 as in Equation (A) below. 



Going back to the plant leaf again, fatty 

 acids are joined with glycerol to form fats 

 of different kinds. This is done by the loss 

 of water, as shown in Equation ( B ) below. 



HHHHHHHHHH 



(A) 



HO— C— C— C— C— C— C— C— C— C— C— C— C— C— C— C— C— C— C— H 

 OHHHHHHHHHHHHHHHHH 



stearic acid (from Vjeef fat) 



H H 



