PROTOPLASM AND CELLULAR ORGANIZATION 



23 



volved in cellular respiration; other enzymes 

 in the mitochondria function in chemical 

 reactions which produce and store energy in 

 the cell. By special staining methods the 

 Golgi apparatus is sometimes made visible; 

 its function is not definitely known. Cyto- 

 plasmic inclusions of various sorts that are 

 not considered parts of the living proto- 

 plasm may also be present; these are pig- 

 ment granules, starch granules, fat globules, 

 and other nutritive, secretory, or excretory 

 material. 



Experiments indicate that neither the cy- 

 toplasm nor the nucleus can exist long with- 

 out the other. For example, if the single 

 cell of the protozoan is deprived of its nu- 

 cleus, the remaining cytoplasm may con- 

 tinue to move for a few hours and may 

 ingest food, but all its activities soon cease, 

 and death ensues. Both nucleus and cyto- 

 plasm are necessary for normal cellular ac- 

 tivities, due probably to the exchange of 

 substances between them. 



Passage of materials 

 through cell membrane 



The living animal cells throughout the 

 body are inhabitants of tissue fluid. Tissue 

 fluid is probably of much the same composi- 

 tion as the sea water in which animal life is 

 thought to have originated. All materials en- 

 tering a cell must pass through the fluid 

 surrounding each individual cell before they 

 reach the cell membrane. 



The best-known physical process which 

 enables water and other substances to en- 

 ter the cell is diffusion. Diffusion is defined 

 as the movement of molecules from a re- 

 gion of high concentration to one of lower 

 concentration, brought about by the in- 

 herent heat energy of the molecules. The 

 rate of diffusion depends mainly on the size 

 of the molecule and the temperature. Diffu- 

 sion is fundamental to many biologic phe- 

 nomena, and examples of it in everyday life 

 are familiar to all of us. For example, if a 

 tablespoonful of household ammonia is 



spilled on the floor, the odor will soon be 

 noticed in all parts of the room. The mole- 

 cules of ammonia have become evenly dis- 

 tributed throughout the entire room. 



This same principle holds true if the sub- 

 stance is a solid, such as a small lump of 

 sugar dropped in a jar of water. The sugar 

 dissolves and the individual sugar molecules 

 (solute) diffuse from their original position 

 in the jar of water (solvent) and spread 

 evenly throughout the liquid (Fig. 8A). 

 The individual sugar molecules move in a 

 straight line until they bump into another 

 molecule; then they rebound and move in 

 another direction. 



Diffusion of a solute can be modified oi 

 prevented by the presence of a membrane. 

 A membrane is permeable if it permits water 

 and all solutes to pass through, impermeable 

 if it will permit no substances to pass, and 

 semipermeable ( differentially permeable ) if 

 it will allow some but not all substances to 

 diffuse through. This makes it clear that 

 permeability is a property of the membrane, 

 not the diffusing substance. The dense sur- 

 face film on the outside of an animal cell, 

 the cell membrane, is semipermeable. One 

 of the principle functions of the cell mem- 

 brane is that of regulating the passage of 

 materials into and out of the cell. Certain 

 liquids and dissolved substances can pass 

 through the cell membrane and others can- 

 not. Mineral nutrients dissolved in water 

 pass through the cell membrane by diffu- 

 sion. Water passes through the cell mem- 

 brane by osmosis, a special form of diffusion. 



Osmosis may be defined as diffusion of 

 a solvent through a semipermeable mem- 

 brane. In biological processes the solvent is 

 almost universally water. Osmosis is a kind 

 of one-directional diffusion as explained in 

 Fig. 8B. It plays an important role in the 

 life processes of cells, both plant and ani- 

 mal, because of the indispensable functions 

 of water in a cell. Why does osmosis occur 

 in the living animal cell? It is because the 

 cell contains solutes such as sugars, salts, 

 and others, which reduce the concentration 



