ENERGY 497 



T AS is often fairly small relative to AF and Af/ and is neglected, be- 

 cause neither A/'' nor AS can be estimated in most cases. l'"or the com- 

 plete reaction 



CeHiaOe + 6O2 -^ 6CO2 + 6H2O 



AF = -686,000 cal./mole and AH = -674,000 cal./mole, leaving 

 T A.S = 12,000 cal./mole, a relatively sTnall factor. Similar situations 

 occur for other overall oxidations but not lor some of the possible in- 

 termediate reactions like 



CHaCOOPO.H- + H2O -^ CH3COO- + H.POr 



In this case AF= -13,500 cal./mole, AH = -7,200 cal./mole, and 

 T AS — 6,300 cal./mole, so T AS can hardly be neglected in this case. 

 Such reactions as this one must be avoided by efficient cells since the 

 loss in the T AS is irretrievable. Metabolic pathways probably do not 

 involve simple hydrolyses of high-energy compounds like acetyl phos- 

 phate. Rather, coupled reactions transfer most of the available free 

 energy to some other useful intermediate. 



Heats of Combustion 



Although the chemical energy and heat content of foods differ, only 

 measurements of the latter are common. They are often made with a 

 bomb calorimeter, a device providing for the oxidation of the organic 

 materials of foods to carbon dioxide, water, and nitrogen. Provision 

 is made for the determination of the heat evolved during the oxida- 

 tion. 



The bomb itself is a heavy steel container in which a weighed 

 sample of the food material is ignited in an atmosphere of oxygen 

 luider pressure. The ignition is achieved electrically by passing a 

 known current for a known length of time between the electrodes of 

 an arc or through a wire filament. Heat from this combustion process 

 raises the temperature of the bomb and an insulated water bath in 

 which the bomb is supported. Since the temperature change produced 

 by a given amount of heat is known, the total heat evolved inside the 

 bomb is readily calculated. This result must be corrected for any 

 losses from the bath to the room and for the heat resulting from the 

 known amount of electricity used in the ignition. The values after 

 adjustment are called heats of combustion and are expressed in calories 

 per gram of material burned. A calorie is the heat needed to raise 

 the temperature of 1 g. of water from 15 to 16°C. Since this is a rather 

 small cjuantity of heat, heats of combustion are often expressed in 

 kilocalories (kcal.) equaling 1,000 small calories. Many books on 



