How a Complex Ajiimal Uses Food unit iv 



190 



made, it is still a physical change. The 

 nature of the crumbs is still the same. 

 But if the starch in the bread is con- 

 verted to some other substance the 

 change is chemical. The nature of the 

 starch has been changed. 



End products of digestion. An interest- 

 ing and important fact about digestion 

 is that it occurs in steps. A food sub- 

 stance is first changed into smaller mole- 

 cules; then these molecules are changed 

 into still smaller molecules. These are 

 called mtermediate products of digestion. 

 Finally the substance is changed into 

 molecules small enough to diffuse into 

 cells and to be usable by them. These 

 last products of digestion are called efid 

 products. 



There are other interesting facts about 

 digestion. You may eat proteins from a 

 hundred different kinds of animals and 

 plants. The proteins are all slightly dif- The starch is changed to a sugar called 

 ferent from one another. And all of them maltose. Exercise 2 will show this, 

 are so different from your body proteins Saliva has been studied carefully and 



that they could not be changed directly found to contain a small amount of a 

 into your body proteins even if they substance that can change starch to malt- 

 could diffuse into your cells. In digestion ose. The substance in saliva that changes 

 all of these many different kinds of pro- starch to maltose is called ptyalin (ty'a- 

 teins from various organisms are broken lin). 



up into the same few compounds. These What is an enzyme? An enzyme is a 



simpler chemical compounds which are substance made by a living cell. It acts 

 the end products of protein digestion are on another substance in such a way as 



are broken up or digested in the body. 

 You see then that they arc all made out 

 of the same twenty odd building stones 

 or amino acids. 



In a similar way carbohydrates and 

 fats, too, are broken up into a relatively 

 small number of end products. No matter 

 what kinds of carbohydrates are broken 

 down or where they come from, the 

 same few end products are always pro- 

 duced. All fats, too, from every kind of 

 animal or plant are broken down into the 

 same few end products. And the end 

 products are the same whether the proc- 

 ess occurs in us or in a dog or in any 

 other kind of animal. 



Digestion shown in a test tube. If you 

 mix some saliva with a little cooked 

 starch in a test tube and warm the tube 

 in vour hand for four or five minutes, 

 digestion of almost all the starch occurs. 



called amino acids. Do you remember 

 that plants in building up proteins first 

 make amino acids? It may help you to 

 think of amino acids as "building stones" 

 of proteins. A large variety of buildings 

 may be built out of the same kinds of 

 stones. It should not surprise you then 



to cause a chemical change in it. The 

 enzyme itself is not changed. Thus, given 

 time, a small amount of an enzyme can 

 change a \'crv large amount of a sub- 

 stance, since throughout, it remains just 

 as it was originally. Chemists are familiar 

 with substances that behave in this way 



that a large variety of proteins can be in the laboratory. There are many of 

 built out of the same few amino acids, them. They call all of them catalysts 

 This is shown when the various proteins (cat'a-lists), and reserve the name en- 



