78 - The Cell 



products, liberating the balance of energy 

 tor the useful work of the cell. Moreover, 

 many of these decompositions are oxidative 

 in nature. 



Among the fuels used by cells generally, 

 glucose is very important. Most cells are able 

 to "burn" glucose, and many cells use glu- 

 cose in preference to other fuels. 



The oxidation of glucose proceeds in many 

 steps (p. 148), but the over-all oxidation can 

 be specified as follows: 



C G H 12 O + 6O 2 



1 mol 6 mo! 



glucose oxygen 



decomposition 



synthesis 



6C0 2 + 6H 2 + £ (energy) 



In other words, by uniting with six mole- 

 cules of oxygen, each glucose molecule pro- 

 duces six molecules of carbon dioxide and 

 six molecules of water; and in this oxidation, 

 energy is liberated for the use of the cell. 



Chemical Reactions in Relation to Matter and 

 Energy. The oxidation of glucose exemplifies 

 the nature of chemical reactions generally. 

 In all chemical reactions one or more sub- 

 stances are transformed into one or more 

 otlier substances by a regrouping of the same 

 existing atoms. Accordingly, all chemical re- 

 actions display a consewation of matter — as 

 is plain from two considerations. (1) The 

 total weight of the reacting substances 

 exactly equals the total weight of the result- 

 ing substances, and (2) the total number of 

 each kind of atom remains unchanged when 

 the reaction is completed. In the present 

 case, for example, the glucose and oxygen 

 that are consumed weigh exactly as much as 

 the carbon dioxide and water that are pro- 

 duced. Also, the equation is balanced, since 

 both the initial and the final products are 

 constituted by (J, 12, and 18 atoms respec- 

 tively, of carbon, hydrogen, and oxygen. 



Virtually all chemical reactions involve 

 some kind of energy transaction. On this 

 basis, in fact, chemical reactions are generally 

 subdivided into two groups: (1) exothermic 

 reactions, which discharge energy to the en- 

 vironment; and (2) endothermic reactions, 

 which absorb energy from the environment. 

 Larger and more complex molecules gen- 

 erally possess a greater fund of intramo- 

 lecular energy than a corresponding weight 

 of smaller and simpler molecules. Conse- 

 quently most decomposition reactions — in 

 which larger molecules are fragmented into 

 smaller ones — give forth energy to the en- 

 vironment. But conversely, most synthetic 

 reactions — in which larger molecules are 

 built from smaller units — cannot proceed 

 without absorbing energy pom the environ- 

 ment. 



The foregoing equation shows that the 

 oxidation of glucose is a typical decomposi- 

 tion. It is exothermic to the extent of 4 

 Calories per gram of glucose consumed. How- 

 ever, the same reaction proceeding in the 

 opposite direction — and this occurs when a 

 green plant cell absorbs sunlight and con- 

 verts carbon dioxide and water into glucose 

 and oxygen — is a typical synthesis, namely, 

 photosynthesis. Photosynthesis is an endo- 

 thermic reaction, and can occur only when an 

 equivalent fund of energy, in the form of 

 light, is available for absorption from the 

 environment. 



Glucose is employed more universally than 

 other protoplasmic fuels. The cells of man's 

 body, for example, derive much of their 

 energy, at least indirectly, by oxidizing glu- 

 cose. Man's blood stream carries a verv con- 

 stant supply of glucose, and the "blood sugar 

 level" must be maintained at a minimum of 

 about 0.1 percent if serious collapse is to be 

 avoided (p. 00). 



Synthesis of Other Carbohydrates from Glu- 

 cose. Cells also use glucose as a raw material 

 from which to synthesize other carbohydrates. 

 In this case a number of glucose molecules 

 unite chemically, forming larger types ot 

 molecules. Accordingly, many carbohydrates 



