rn Brief 



In plants and in animals, starches, proteins, and other nutrients are con- 

 verted into soluble crystalloids by the action of various enzymes. 



Excess sugar produced in leaves is converted into starch by the action of 

 an enzyme — a process just the reverse of digestion. At night starch is con- 

 verted into sugar by the digestive enzyme diastase, and the sugar is trans- 

 ported to other parts of the plant through the phloem tubes. 



Digestion takes places in plant cells which make or store food. Single- 

 celled animals digest food within their bodies. Bacteria give out enzymes 

 which digest food in the surrounding medium. Higher animals carry on 

 digestion in specialized organs. 



Food entering the mouth passes successively through the pharynx, gullet, 

 stomach, small intestine and large intestine. Food is moved along through 

 the alimentary canal by peristalsis. Undigested portions are discharged from 

 the body through the rectum. 



Digestive juices are produced in special glands and delivered by ducts 

 into the food canal. Other products of glands with ducts are lubricants, 

 cooling secretions, excretory substances, and food. 



Starch is changed to sugar by digestive enzymes present in the saliva and 

 in the pancreatic juice. Complex sugars are changed to simple sugars by 

 several specific enzymes present in the intestinal juice. 



Proteins are split into amino-acids by enzymes in the gastric, pancreatic 

 and intestinal juices. 



Fats are split into fatty acids and glycerin by an enzyme secreted in the 

 pancreatic juice. This digestion requires an alkaline medium, which is 

 furnished by the bile. 



Digested food is absorbed and transformed by the Villi, specialized ab- 

 sorbing organs that project into the cavity of the small intestine. 



Plants and animals accumulate surplus food in their tissues, and then use 

 it when new supplies are scarce. 



EXPLORATIONS AND PROJECTS 



1 To determine which food substances diffuse through osmotic membranes, 

 place dilute starch paste, corn sirup, olive oil, and raw egg white in four wide- 

 mouthed bottles, tie bladder membranes tightly over the tops, and suspend in jars 



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