PROBLEM 2. The Digestive System Makes Food Usable 20^ 



14. What in the gastric juice digests protein? x\re you convinced that 

 it was the pepsin that digested the white of t^g? Can you prove that it 

 was not the water? Or the hydrochloric acid? Try each of the three sub- 

 stances alone. What happens? Now plan an experiment to discover what 

 is really necessary for the digestion of protein. Include in your report of 

 this experiment a statement of the control experiments that were used. 

 Tell why they were necessary. 



15. What is the effect of hydrochloric acid on the minerals in the food? 

 Test such minerals as table salt and calcium phosphate. Use dilute acid 

 and very small amounts of mineral. Describe what happens. 



16. How does the mechanical breaking up of food affect digestion in 

 the stomach? Can you devise an experiment to answ^er this question? 

 (Hint: Use hard boiled white of to^g and gastric juice. Use two tubes. 

 How must the white of t^^ in the two tubes differ when you set up the 

 experiment? ) 



17. How can oil be emulsified? Place about one-half teaspoonful of 

 olive oil in a test tube half filled with water. Shake the contents and allow 

 to stand for several minutes. What happens? If it is possible to obtain 

 the gall bladder of a chicken, add this bile to the oil and water in the test 

 tube. If not, use a weak alkaline solution obtained in the laboratory. Hold 

 your thumb over the mouth of the test tube and shake the tube thor- 

 oughly. You now have an emulsion. What is its color? Examine after 

 several minutes. Does the oil again rise to the top? Examine a drop of the 

 emulsion under low power of the microscope. What do you see? Can you 

 explain the emulsion? Explain how emulsification prepares oils for diges- 

 tion. 



18. Does emulsification appreciably increase the surface to be acted on 

 by digestive juices? Fasten together several board erasers or books with 

 an elastic. Measure their total surface area. Separate them and measure 

 the surface of each one. Add together the measurements of the separate 

 objects. Compare this sum with your first measurement. Can you ex- 

 plain? A cube whose side is one inch has a total surface area of six square 

 inches. If this cube is cut up into small cubes whose sides measure 0.0 1 of 

 an inch there will be one million such cubes. What will be the total sur- 

 face area of all of the small cubes? How does it compare with the area of 



the original cube? 



Further Activities in Biology 



1. Devise an experiment to determine whether the enzyme in saliva 

 really acts like a catalyst; that is, whether a small amount can be used over 

 and over again without being used up in the process. 



2. If saliva is swallowed with starch, can its work continue after it 

 reaches the stomach? You must expose saliva and starch in a test tube to 

 the surroundings they would have in the stomach. What will you do? Ask 

 to have your experiment tried in class. 



