BIOLOGICAL TRANSPORT 



Because of this behavior, observations of the concentrative uptake 

 of amino acids by intestinal tissue are conceded considerable validity 

 from which to predict concentration across the mucosa and are 

 often substituted experimentally for the latter. Parallel behavior 

 for monosaccharides permits use of the same technique with them 

 (Crane and Mandelstam, 1960). Similarly, the ability of the kidney 

 tubule to reabsorb galactose has been related to the ability of kidney 

 slices to accumulate this sugar (Krane and Crane, 1959). In a num- 

 ber of other cases, solutes have been shown to be accumulated into 

 a secreting tissue prior to or in association with their secretion. 

 The over-all conclusion is that the secretory process probably utilizes 

 the transport capabilities attributed to the plasma membrane of most 

 cells. 



Pinocytosis and membrane flow 



Warren Lewis discovered in 1931 that cells in tissue culture 

 form invaginations along their periphery and that these invaginations 

 are frequently pinched off to become vacuoles. He called the be- 

 havior "pinocytosis" and likened it to a drinking of extracellular 

 fluid by the cells. This possible mode of transport has received in- 

 creased interest from the observation that insulin has been reported 

 to increase the frequency of invagination of the cells of the rat 

 epididymal fat pad, in association with the increased rate of uptake 

 of glucose (Barrnett and Ball, 1960). 



Prohibitive difficulties appear, however, to stand in the way of 

 accounting for many of the rapid, mediated transports of small 

 solute molecules by pinocytosis. Such a nonspecific inclusion of 

 extracellular fluid would appear to produce more transport prob- 

 lems than it solves, since huge volumes would need to be engulfed 

 to account for the amounts of some solutes taken up. In addition, 

 almost all other solute molecules, and the water, would then need 

 to be discharged from the cell. Moreover, the specificity of transport 

 would be unexplained and the kinetic observations entirely un- 

 accounted for. The possibility that extracellular fluid enters cisternae 

 formed by endoplasmic reticulum has already been mentioned; this 

 arrangement, if it exists, might serve to increase the area available 

 for transport into and out of cells. 



A distinct yet related concept supposes that the cell membrane 

 flows into a sink, so that portions steadily enter the interior of the 



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