PROBLEM 2. How Cells Keep Alive 



Sugar molecules 

 (triangles) 



Water molecules 

 (circles) 



Rubber 

 band 



Membrane over end of thistle tube 



Fig. 151 Sugar and water molecules are within 

 the thistle tube. Only water molecules are out- 

 side it. If only water molecides ca?i get through 

 the 7}iembrane, what will happen to the amount 

 of liqtiid ill the tube? What would happen if 

 both kinds could get through the meiiibrane? 



tap water. Where are the molecules of 

 water more concentrated when the ex- 

 periment is set up? Remember more 

 concentrated means closer together, not 

 more numerous. Are the water molecules 

 more concentrated within the thistle tube 

 where you have some water mixed with 

 thick molasses or are they more concen- 

 trated within the tumbler filled with tap 

 water? Since the water molecules tend to 

 diffuse from where they are more con- 

 centrated to where they are less concen- 

 trated water should move from the 

 tumbler into the thistle tube provided 

 it can get through the membrane. If it 

 does get through the membrane how will 

 this become apparent after a short time? 



The same set-up can be used to dis- 

 cover whether sugar diffuses throus^h 

 this membrane. Can you suggest ho\v 

 this could be discovered? 



If conditions are suitable sugar will 

 diffuse through the membrane until the 



123 



concentration of sugar is the same on 

 both sides of the membrane. 



Whatever may be true of other mem- 

 branes and other substances, if you have 

 done the experiment suggested, you now 

 know that water and sugar can diffuse 

 through sausage casing or cellophane 

 membranes. To test your understanding 

 of diffusion do Exercise i i. 



Diffusion through a living cell mem- 

 brane. If you place some living plant 

 cells on each of two slides and add dis- 

 tilled water (water without minerals) to 

 one and a very strong salt solution to 

 the second some interesting results will 

 be obtained. If you observe the cells 

 under the microscope, you will find that 

 those placed in distilled water will swell 

 slightly. What may have happened to 

 make them swell? On the other slide 

 the results will depend somewhat on the 

 strength of the solution used, but you 

 will be able to observe a distinct change. 

 The protoplasm will shrink away from 

 the wall and form a small mass; evidently 

 the water vacuole inside the cell disap- 

 pears. Diffusion of water out of the cell 

 takes place (see Fig. 153). This is good 

 evidence that water diffuses through a 

 living cell membrane. You should now 

 be able to make some practical applica- 

 tions of what you have just read in doing 

 Exercise 12. 



When do substances diffuse through 

 a membrane? To begin with the only 

 substances that diffuse through mem- 

 branes in living things are substances 

 that diffuse or dissolve in water. Not all 

 substances dissolve in water. If shaken up 

 in water, they may appear for a time 

 to do so, but after standing the particles 

 will fall to the bottom. Such substances 



