SUPPLEMENTARY EXPERIMENTS ON THE CHEMICAL 

 COMPONENTS OF CELLS: THE BIOCHEMISTRY 

 OF MILK* 



APPENDIX C 



PART 1 



(Reading: S. T. P., pp., 70-85; 121-123.) 



Milk, the cellular product of a mother, 

 contains everything needed for the cellular 

 growth of a baby: proteins, fats, sugars, mineral 

 salts, and vitamins. 



In this exercise we will work with skimmed 

 milk, since the isolation and properties of the 

 fatty components are already familiar to you in 

 the form of butter. Incidentally, one of the 

 most general properties of a fat is that it makes 

 a nonvolatile grease stain on paper; but you 

 certainly don't need to do that in the laboratory. 



Proteins are the large molecules of which the 

 cell principally builds its structure and ma- 

 chinery. They are composed of chains of amino 

 acids. We shall isolate a protein called casein 

 by neutralizing the negative charges on the 

 casein which cause the molecules to repel each 

 other. This will be done by adding acid, that 

 is, a source of positively charged hydrogen ions 

 (protons). When the hydrogen ions bind to the 

 negatively charged casein molecules, the latter 

 no longer repel one another, and they begin to 

 aggregate, causing precipitation of the protein, 

 and permitting its filtration. 



After the casein is removed, the other proteins 

 will be coagulated by heating and evaporating 

 the solution. Heat causes proteins to lose their 

 delicate normal structure; the normally coiled 

 chains of amino acids unwind. If there are 

 enough molecules in close contact, chains of 



* This material covers two laboratory sessions. 



different molecules intertwine, giving rise to an 

 insoluble coagulum. How does evaporation of 

 the solution enhance this process? (When a 

 protein is thrown out of solution relatively un- 

 changed, so that it can be redissolved, we call 

 that precipitation. In coagulation its structure 

 has been unraveled irreversibly, and it cannot be 

 redissolved.) 



Two important mineral ions of milk are cal- 

 cium (Ca++) and phosphate (P04°). Both of 

 them are essential in the formation of bone, and 

 calcium ions are necessary also for such diverse 

 biological processes as blood clotting and 

 muscle action. On continued evaporation of the 

 milk, calcium and phosphate ions come into 

 closer and closer contact until they reach such 

 a concentration that attractive forces between 

 them cause them to precipitate as the salt, cal- 

 cium phosphate. 



Sugars serve living cells as sources of fuel and 

 material, performing as middlemen which en- 

 able chemical reactions carried out in one cell 

 or organism to support activity in another cell 

 or organism. They are small molecules com- 

 posed of carbon, hydrogen, and oxygen atoms, 

 in the proportion (CH20)n. The sugar present 

 in milk is lactose. It will be isolated by adding 

 the residue from the milk to acetone. Lactose 

 is insoluble in acetone; that is, the lactose mole- 

 cules tend to stick together in an orderly array 

 rather than float about singly in the acetone. 

 Keeping the solution cold hastens crystallization 



143 



