24 



of water molecules to a point lower than 

 that of the tissue fluid in which the cell is 

 immersed. Hence, in accordance with the 

 principle of osmosis, the water moves from 

 the region of higher concentration (tissue 

 fluid) to the region of lower concentration 

 (cell protoplasm). Cell membranes are im- 

 permeable to many substances that we eat, 

 such as starch, because they must be di- 

 gested, that is, made soluble, before they can 

 be absorbed into the cells. For definitions 



COLLEGE ZOOLOGY 



of the terms isotonic, hypertonic, and hypo- 

 tonic see the Glossary. 



Contrary to a common misconception, ex- 

 change of foods, wastes, and respiratory gases 

 between cells and fluids in animal bodies is 

 not by osmosis. The chief factor in the 

 transport of these substances is ordinary 

 diffusion. When water molecules move 

 either inward or outward by osmosis they do 

 not carry other molecules along with 

 them. 



Semipermeable membrane 

 (permeable to water 

 molecules, impermeable 

 to sugar molecules) 



Membrane permeable 

 to all substances 



Difference in levels of 

 iquids when chambers 

 separated by semiperme- 

 able membrane measures 

 osmotic pressure 



Water molecule:.. 

 Sugar molecule: tv 



chamber a, sugar solution placed in chamber b 



Figure 8. Diagram to illustrate ordinary diffusion and osmosis. A, ordinary diffusion. The 

 battery jar is divided into two chambers, a and b, by a permeable membrane which offers prac- 

 tically no hindrance to the diffusion of both water and sugar molecules (particles). In ordinary 

 diffusion, any kind of molecule tends to diffuse (move) from where it is more abundant, per 

 volume of space, to where the molecule is less abundant. Diffusion through a permeable mem- 

 brane continues until every component reaches equal concentration; therefore in diagram A, 

 the water and sugar molecules are of equal concentration on both sides of the permeable mem- 

 brane. B, osmosis. The battery jar is divided into two chambers, a and h, by a semipermeable 

 membrane, that is, one that is permeable to water but hinders the passage of sugar molecules. 

 Under these conditions, water molecules will diffuse through the membrane more rapidly into 

 chamber b than into chamber a. In accordance with the law of diffusion, the water molecules 

 move in greater numbers from the place of higher water molecule concentration (higher water 

 diffusion pressure) to the region of lower water molecule concentration (lower water diffusion 

 pressure). Diffusion through a semipermeable membrane is known as osmosis. Osmosis in living 

 things has almost always to do with the movement of water through a semipermeable membrane. 



Cell division 



Reproduction is a fundamental property 

 of protoplasm, and cell division is a type of 

 reproduction. For many years after cells were 

 discovered, division of the nucleus, which 

 precedes cell division, was supposed to take 



place by a process which we call aniitosis tc 

 distinguish it from mitosis. 



Amitosis means a sort of mass division ol 

 the nucleus. This type of nuclear division h 

 rare and of little importance. As a rule, the 

 protoplasm in a cell grows until the cell 

 reaches a certain size; then the cell divides 



