METABOLISM 



33 



45. Food. — The food of the organism, in the broadest sense, must 

 inckide all of the compounds which enter into the chemical organization 

 of protoplasm — that is, proteins, fats, carbohydrates, salts, and water. 

 It must also include certain substances termed vitamins which seem to 

 play a necessary part in the carrying on of metabolic activities. It is 

 also essential that this food shall supply energy in such a form as to be 

 available to the organism. 



\. Ingest/on 



10. Eypi ration 

 5 . Inspiration 

 4. Circulation 



*). Excretion of 

 nitrogenous waste 



4. Circulation 

 3. Absorption 



11. Elimination 



In every cell 

 of the body 

 6. Assimilation 

 1. Dissimilation 

 b. Secretion 

 9. Excretion 



2. Digestion 



Q. Secretion of 

 gastric Juice 



Bgesfi 



ion 



Fig. 7. — Diagram to suggest the steps in metabolism as they occur in the human body. 



For comparison with Fig. 6. 



46. Steps in Metabolism. — Metabohsm takes place in the body by 

 a series of very definite steps (Figs. 6 and 7), all of which are necessary 

 in the metabolism of the higher animals, but certain ones of which are 

 simplified or dispensed with in the case of the very simple animals. 

 These steps are referred to in terms that are more or less in popular use 

 with very loose and uncertain meanings. The words excretion, secretion, 

 ehmination, and assimilation are frequently met but are usually used with 

 an uncertain significance. It will be necessary, therefore, for the student 



