124 - The Cell 



to pick up sodium ion (Na+) and to receive 

 energy from the subjacent cytoplasm. Arriv- 

 ing at the external membrane surface, the 

 carrier then discharges the sodium, along 

 with the charge of energy, whereupon the 

 carrier is free to pick up potassium (K + ) 

 and transport it back to the cytoplasm. But 

 even though it is difficult to provide convinc- 

 ing experimental evidence in support of an}' 

 particular ionic transport theory, the fact 

 remains that cells cannot afford to lose their 

 ionic differentials, particularly with refer- 

 ence to potassium and sodium ions. If me- 

 tabolism is damped off, as by anaesthetics or 

 by certain other drugs, the cell membrane 

 potential gradually drops away and the ca- 

 pacity of the cell to respond to stimulation 

 disappears (Chap. 11). 



SUMMARY 



Since a plasma membrane, which is semi- 

 permeable, intervenes between the proto- 

 plasm and surrounding solutions, each t\ pica] 

 cell displays man) 1 of the attributes of an 

 osmotic system. Any substance that can pene- 

 trate the membrane will tend to pass out of 

 the cell if it is more concentrated inside than 

 outside: it will tend to pass into the cell il 

 the outside concentration is greater. The end 

 products of destructive metabolism, such as 

 carbon dioxide, water, and other simple sub- 

 stances, are all able to pass freely through 

 the membrane. Due to their constant produc- 

 tion, the concentrations of these substances 

 inside the cell tend to be higher than outside 

 and consequently the) continuously tend to 

 leave the protoplasm. Substances that are 

 used up in metabolism, such as oxygen or 

 glucose, tend to enter the cell from the sur- 

 rounding solution. The very fact that these 

 substances are consumed or converted into 

 other substances as metabolism proceeds 

 tends to keep their concentrations in the 



protoplasm generally lower than in the out- 

 side medium. The products of constructive 

 metabolism, on the other hand, are mainly 

 colloidal substances (proteins, polysaccha- 

 rides, phospholipids, etc.) that cannot pene- 

 trate the membrane. Consequently these 

 essential components of the protoplasm can- 

 not escape despite their relatively high in- 

 ternal concentrations. 



The water equilibrium between the cell 

 and its environment assumes a very critical 

 importance: first, because water is by far the 

 most abundant substance both inside and 

 outside the cell; and second, because water 

 penetrates the cell membranes much more 

 rapidly than most of the solutes present in 

 the system. Plant cells frequently live in 

 fresh water, but such cells are protected 

 against a rapid and lethal influx of water 

 from the surrounding hypotonic solution by 

 the strength of the cell wall. This permits 

 the development of a turgor pressure high 

 enough to counteract the osmotic force under 

 which the water continues to seek entrance 

 into the cell. Animal cells, in contrast, can- 

 not be maintained in a hypotonic medium 

 unless some special mechanism like the con- 

 tractile vacuole is present to deal with excess 

 water as fast as it enters. In hypertonic solu- 

 tions both plant and animal cells are equally 

 vulnerable. No cell can lose too high a pro- 

 portion of its water without suffering irre- 

 versible deterioration. 



Ion transport mechanisms, by which a cer- 

 tain ion ma\ be forced to move against an 

 adverse diffusional gradient, enable cells to 

 develop and maintain a high content of po- 

 tassium and a low content of sodium ions, 

 relative to surrounding media. Other active 

 transport mechanisms, particularly pinocy- 

 tosis and phagocytosis, permit individual 

 cells to take in macromolecular types of or- 

 ganic substances that are not able to pene- 

 trate through the surface membranes. 



