MOVEMENTS OF MATERIALS INTO AND OUT OF THE CELL 61 



ity. All of you have experienced the unpleasant reaction that comes 

 when water is drawn up the nose. This is due to the swelling of the 

 cells in the nasal cavity. If you put a little salt in water, however, it 

 can be drawn up the nose without such irritation. In fact, this is some- 

 times done to help clear the phlegm from the nasal cavity during a bad 

 cold. When you go swimming in the ocean and get water in your nose, 

 there will be some irritation because in this case there is too much salt 

 in the water. Sea water is about 3.5 per cent salt and, therefore, causes 

 some cell shrinkage when it contacts delicate tissues of the body. Plain 

 water is said to be hypotonic to the cells of your nasal cavity, whereas 

 sea water is hypertonic. 



Filtration 



Before leaving our discussion of the movement of materials into, 

 out of, and within the cell, we should bring out a method of movement 

 under pressure known as filtration. Whenever pressure becomes great 

 enough, it can force water and dissolved particles through a membrane 

 against the force of osmosis. We have already learned how the force 

 of gravity can do this in a nonliving membrane. The blood is pumped 

 to all parts of the human body by the pressure generated by the heart 

 beat. When this blood reaches the tiny capillaries which have walls 

 only one cell in thickness, some of the water and dissolved substances 

 will be squeezed out by mechanical pressure. This fluid bathes the cells 

 and is a great aid in carrying dissolved food to cells which are not in 

 contact with blood vessels. It will re-enter the blood stream through 

 the lymph vessels so that there is not a continual loss of such large 

 quantities of fluid from the blood. Also, at the kidneys filtration comes 

 into play to aid in the removal of wastes from the blood. Blood 

 within the capillaries is under rather high pressure, and there is a 

 mechanical squeezing of water and dissolved minerals into small tubules 

 within the kidneys. 



Filtration saves many cells from bursting when they are placed 

 in hypotonic solutions. When the pressure within the cell increases 

 to a certain point, filtration forces water out as fast as it comes in and 

 an equilibrium is reached. If the cell membrane can withstand this 

 critical pressure the cell is saved. 



From the discussion in this chapter we can readily see that the 

 movement of materials into and out of cells is a very important part 

 of the activities of all forms of life. 



