PROBLEM 2. The Digestive System Makes Food Usable 



197 



;;j^^;StarGK^;i;V^• 



m 



Protein 



Complex 



|||sugar!|||| 





Insoluble 

 minerals 



Cellulose^ 



+ ptyalin or 



+ pancreatic 



enzyme 



+ pepsin or 



+ pancreatic 



enzyme 



(trypsin) 



or 



+ intestinal 



enzyme 



I 



+ pancreatic 

 enzyme 



IlilUIUIIMJIIll 



.Complex 



II [IP PI H-^. I M I 



III sugar 



+ intestinal 



enzyme 

 (invertase) 



Glucose 



4^Peptones , 



/V/A/.'/y/.-/,/:'./.. 



+ intestinal 

 enzyme 

 (erepsin) 



Amino acids 



+ pancreatic 

 enzyme 

 (lipase) 



+ hydrochloric 

 acid 



Glucose or 



other simple 



sugar 



Fatty acids 

 and glycerin 



Soluble 

 minerals 



^Cellulose 



Fig. 205 This chart shows what happens to the principal jood' compounds in the ali- 

 mentary canal. Which are intermediate products and end products of digestio?i? 



they turn into liquids; that is, they be- 

 come oils. Sometimes you eat fat already 

 in a liquid form as when you eat olive 

 oil. When an oil is mixed with water it 

 soon separates from the water so that a 

 few large drops are formed. Enzymes 

 cannot act quickly on such large drops 

 of oil because they can act only at the 

 surface. The oil must be broken up into 



juice is made active (activated) by the 

 salts. Without this activation the enzyme 

 does not digest fats. 



Absorption of digested food. You have 

 learned that in digestion large molecules 

 are broken up step by step into much 

 smaller ones. Proteins are changed into 

 amino acids, fats into fatty acids and glyc- 

 erin, and starches and sugars into simple 



little droplets before much digestion can sugars (such as grape sugar or glucose). 



go on. 



When bile is thoroughly mixed with 

 an oil it forms a thin film around each 

 droplet of oil so that the droplets can no 

 longer come together. When oil has been 

 broken up into tiny droplets in this way 

 it looks milky. It is in a state of emiilsio7i 

 (ee-mul'shun). Milk is a good example 

 of an emulsion. The fat in milk is in very 

 tiny globules. You can easily make an 

 emulsion in a test tube by doing Exer- 

 cise 17. Could you demonstrate the prin- 

 ciple that by emulsifying fats you 

 increase the digestive surface? See Ex- 

 ercise 18. 



The bile salts help in another way. The 

 fat-digesting enzyme of the pancreatic 



Insoluble minerals are made soluble. As 

 these end products of digestion are pro- 

 duced they may begin to diffuse through 

 the walls of the alimentary canal into the 

 blood. This movement through the walls 

 into the blood is called absorption. 

 Wherever there are digested foods ly- 

 ing close to the lining of the digestive 

 tract for any length of time some ab- 

 sorption takes place. But there is little 

 absorption until the food reaches the 

 small intestine. While food is still in the 

 stomach not much of it is ready for ab- 

 sorption, nor does it have an opportunity 

 to stay in close contact with the mucous 

 membrane since the stomach is a large 

 pouch instead of a narrow tube. 



