58 



MOVEMENTS OF MATERIALS INTO AND OUT OF THE CELL 



brane will allow only certain dissolved particles to pass in, we might 

 ask the question, "How did these other materials get into the cell?" 

 To illustrate, let us consider some of the cells in your finger. To trace 

 the source of its protoplasm we must go back to the food which you 

 eat. Food, when it is eaten, is seldom in a state that can diffuse through 

 cell membranes. A few simple sugars require no change, but the ma- 

 jor part of our food must be digested before we can benefit from it. 



Digested food in intestine 



Blood in capillary 



Protoplasm in body cell 



$ffi:$ : Digested food particles 

 — * Direction of diffusion 



Fig. 4.4. Diagram illustrating diffusion of digested food. Note that the concentration 

 of food in the intestine is greater than in the blood, so diffusion takes place through 

 the membranes into the blood within the capillaries. When this blood reaches the 

 body cells it contains more food than the protoplasm of the cells and thus the food dif- 

 fuses into the cells. As cells in contact with the capillaries receive the food, it may 

 then diffuse into surrounding cells through the same principle. 



And what is digestion? It is simply the breaking down of large mole- 

 cules and the even larger particles of colloids and emulsions into simple 

 molecules which can diffuse through cell membranes. Colloidal protein 

 particles, for instance, are broken down into the soluble amino acid 

 molecules which can pass through these membranes. 



After digestion, the dissolved particles within your intestine are 

 more numerous than they are in the blood which comes in close con- 

 tact with the digested food. Hence, diffusion causes the digested food 



