48 CELL-CONTENTS AND CELL-WALLS. 



55. Experiments with Pepsin. Boil an egg hard, and chop 

 the clotted white into small pieces. Label six test-tubes A, B, C, 

 .Z), E, F. Half fill each tube with water, and drop in some of the 

 chopped albumin. To A add some pepsin extract or some pepsin 

 powder, with a pinch of bicarbonate of soda to make the liquid dis- 

 tinctly alkaline ; to B and C add some pepsin and a few drops of 

 dilute hydrochloric acid ; to D add a few drops of acid, but no 

 pepsin ; to E, some acid together with pepsin extract (or dissolved 

 pepsin powder) which has been boiled ; and leave F with nothing 

 added to the albumin. 



Set all the tubes, except C, in a beaker of warm water, and keep 

 at 40 C. on a bath for an hour. Put C in a freezing mixture, or ice 

 and water, for the same period. Note that in A, C, E, and ^ 7 tho 

 albumin is unchanged ; in B it has disappeared, having become 

 swollen up and clear. 



Now apply to a few drops of liquid from each tube the xantho- 

 proteic and the biuret tests. Peptone is present in B, but not in 

 any of the others. In E the pepsin has been destroyed by the boil- 

 ing. In A the action of the pepsin has been prevented by the 

 alkaline medium ; on adding acid to the liquid and keeping the 

 tube at 40 C. again digestion takes place. In C the action has been 

 prevented by the cold ; on transferring the tube to the bath at 

 40 C. digestion takes place. In Z>, the weak acid used, without 

 pepsin, has only changed the albumin into acid-albumin, but not 

 into peptone. 



56. Products of Peptic Digestion. Repeat the preceding 

 experiment on a larger scale, so as to get more material to test for 

 the products of pepsin action. This time place in a flask some 

 pieces of albumin, dilute hydrochloric acid (add 4 c.c. of strong 

 acid to 300 c.c. of water), and some pepsin extract or powder. Keep 

 at 40 C. for an hour ; if the liquid is cloudy, filter it. 



(A) To the liquid, or nitrate, add dilute caustic soda solution 

 until it becomes neutral a precipitate is given, consisting of acid- 

 albumin ; filter off this precipitate, dissolve it in dilute acid, and 

 note that the acid solution gives protein reactions and does not 

 coagulate on boiling. 



(B) Test part of the filtrate from A for proteose. It gives the 

 protein reactions. On adding nitric acid and common salt, a pre- 

 cipitate is formed, which is re-dissolved on heating but reappears 

 on cooling. It is precipitated by (a) acetic acid and potassium 

 ferrocyanide, and by (b) acetic acid and saturated sodium sulphate 

 solution, neither of which precipitates peptones. It gives the same 

 biuret reaction (rosy pink) as peptones and, like them, is soluble in 

 water. 



(C) Saturate another portion of the filtrate from B with ammonium 

 sulphate crystals, or the powdered salt ; this precipitates the pro- 

 teoses, while the peptones remain in solution test with biuret, 

 using a large amount of soda. 



