Chap. 2 LIFE IS A CONCERN OF MATTER AND ENERGY 23 



particles of it diffused in the air. Skunks have few enemies because of the 

 diffusion of their scent. The success of the great perfume industry is dependent 

 upon human responses to the diffusion of its products, the various perfumes. 



Osmosis. The diffusion of water or of certain gases through membranes 

 that permit certain simpler molecules to pass, but not the more complex and 

 larger ones, is osmosis. A membrane which does this is said to be semi- 

 permeable. 



Living cells are enclosed by semipermeable membranes containing sub- 

 microscopic pores through which certain molecules can pass and others 

 cannot. The rate of passage varies with the kind of membrane and the 

 material on the two sides of it. Such membranes regulate many functions of 

 the body such as the exchange of oxygen and carbon dioxide, the absorption 

 of food, and the constant come and go between cells and body fluids. Two 

 liquids that contain equal concentrations of dissolved substances are called 

 isotonic. When living mammalian blood cells are examined microscopically 

 they are usually immersed in a solution of 0.9 per cent NaCl in imitation of 

 the body fluids whose salt content is isotonic with the cell content. 



An example of osmotic diffusion or osmosis through an artificial mem- 

 brane illustrates this principle (Fig. 2.12). The membrane is permeable to 

 molecules of sugar as well as water, but so much more so to the latter that 

 equal amounts of sugar and water on each side are never reached. Red blood 

 cells puff out like pillows (called laking of blood) if the salt content of the 

 plasma becomes too much reduced, that is, hypotonic. This is because mole- 

 cules of water enter them, establishing an equal concentration with the too 

 watery plasma (see Chap. 12). If the salt content of the plasma is too high, 

 i.e., hypertonic, the water is drawn out and the cells wrinkle. 



Vacuole 



Vacuole 



Fig. 2.13. Brownian movement occurs in the contents of vacuoles of an ameba 

 (right) and of the green alga Closterium (left). With the high power of a micro- 

 scope the zigzag pathways of the larger particles can be traced. The Brownian 

 movement is due to bombardments of usually invisible particles striking unevenly 

 against the larger ones. 



