MOVEMENTS OF MATERIALS INTO AND OUT OF THE CELL 63 



If their diameter were doubled, the same volume of blood cells could 

 carry only one half as much oxygen as they now can. If their diam- 

 eter were quadrupled, they could not carry enough oxygen to keep 

 us alive, even though in this enlarged condition they would still have 

 a diameter of only about one thousandth of an inch. 



When a portion of a human lung is viewed under the microscope, 

 it can be seen that the tissue is composed of many very tiny air sacs. 

 This gives great exposed surface which allows ready diffusion of 

 oxygen and carbon dioxide. If the lungs were hollow like a balloon 

 there would be so little surface exposed to the air that it would be 

 impossible to obtain sufficient oxygen to support life. 



The surface-volume ratio of the entire body is also very important 

 in determining many of the physiological processes and life habits of 

 animals, as we shall see as we proceed with our study. 



REVIEW QUESTIONS 



1. Why is the plasma membrane of such importance in the physiology of the 

 cell? 



2. Explain how diffusion through a dead membrane in the laboratory differs 

 from diffusion through the living plasma membrane. 



3. If a lump of sugar is dropped in a glass of water, the water will eventually 

 taste sweet at the top of the glass even though the water is not stirred. Explain why 

 this is so. 



4. Diffusion proceeds more rapidly in a cup of hot coffee than in a glass of 

 iced tea. Explain. 



5. Why is it that the cells cannot utilize most foods if they are injected into 

 the blood stream rather than taken into the body by the mouth? 



6. A little fertilizer on a lawn will make it grow better, but a large amount 

 may kill the grass. Explain. 



7. How does filtration differ from diffusion? 



8. If you slice potatoes and place them in plain water they become crisp, but 

 when placed in rather strong salt water they become very limp. Explain. 



9. When medicines are injected into the body they are usually dissolved in a 

 1 per cent salt solution. Why? 



10. From your knowledge of diffusion, work out a logical explanation of why 

 carbon dioxide is absorbed by the blood from the body cells and given off from the 

 blood into the air spaces of the lungs. 



11. A bacterial cell may double its size in as short a time as twenty minutes. 

 Such rapid growth is unthinkable in the larger animals. Explain fully. 



