3. CELLULAR ASPECTS OF ACTIVE TRANSPORT
151
sides of the intestine. It is relevant at this point to mention that the
membrane potential will not be expected to exert a strong influence on
the movement of sugar, as we are dealing with an uncharged species.
At the end of a period of incubation the contents of the sac are
recovered and analyzed for sugar. If the concentration has remained
unchanged, no net flux occurred and active transport may be ruled out.
If the concentration increased in one of the solutions, it may be ascertained that sugar has been moved by an active process, providing that
the sugar remains the same free unchanged chemical species on both
sides of the membrane, and that this change in concentration cannot be
accounted for by a change in volume. Using this technique, it has been
shown that D-glucose is actively transported across the intestinal wall
of the following species: rat (59a-61), golden hamster (62), guinea pig
(61), catfish (Ameiurus nebulosus) (63), toad (Bufo bufo) (64), and
turtle (Chrysemis picta)
(65).
Studies of specificity with nonmetabolizable analogs of glucose and
other sugars have also been performed on the golden hamster intestine
(62). The results are shown in Table X.
Sugar transport systems have also been demonstrated in monocellular
preparations as well as across muscle cell membrane. As already pointed
out for inorganic ions and amino acids, when we are dealing with intracellular transport, it is very arduous to define the nature of the forces
responsible for the movements observed, since we have little control,
if any, on the intracellular phase. Direct evidence for the entry of free
monosaccharides into yeast cell was lacking until recently (66, 67).
Previously, yeast cell membrane was found impermeable to these sugars
(68, 69), as well as to fermentable sugars (69-71). Centrifuged yeast is
80% water in volume and 30-33% of the volume is extracellular space
(68). On the basis of this observation it is evident that only a distribution of a substance into a volume greater than 33% of the packed yeast
volume represents entry into the cell. Table XI shows the results obtained
with various nonfermentable sugars (72).
It is evident from these data that the first three sugars listed showed
maximum distribution into the total aqueous space without accumulation, while the presence of the other sugars was limited to the extracellular space. Among the pairs of molecules of equal size listed in
Table XI, xylose entered while arabinose did not; sorbose entered while
galactose did not; a-methyl-D-glucoside entered while a-methyl-D-mannoside did not. Entry was therefore stereospecific and indicated that the
process is more complicated than a simple diffusion.
A carrier-mediated distribution between extra- and intracellular space
could explain the results obtained. This type of transport first proposed
151
sides of the intestine. It is relevant at this point to mention that the
membrane potential will not be expected to exert a strong influence on
the movement of sugar, as we are dealing with an uncharged species.
At the end of a period of incubation the contents of the sac are
recovered and analyzed for sugar. If the concentration has remained
unchanged, no net flux occurred and active transport may be ruled out.
If the concentration increased in one of the solutions, it may be ascertained that sugar has been moved by an active process, providing that
the sugar remains the same free unchanged chemical species on both
sides of the membrane, and that this change in concentration cannot be
accounted for by a change in volume. Using this technique, it has been
shown that D-glucose is actively transported across the intestinal wall
of the following species: rat (59a-61), golden hamster (62), guinea pig
(61), catfish (Ameiurus nebulosus) (63), toad (Bufo bufo) (64), and
turtle (Chrysemis picta)
(65).
Studies of specificity with nonmetabolizable analogs of glucose and
other sugars have also been performed on the golden hamster intestine
(62). The results are shown in Table X.
Sugar transport systems have also been demonstrated in monocellular
preparations as well as across muscle cell membrane. As already pointed
out for inorganic ions and amino acids, when we are dealing with intracellular transport, it is very arduous to define the nature of the forces
responsible for the movements observed, since we have little control,
if any, on the intracellular phase. Direct evidence for the entry of free
monosaccharides into yeast cell was lacking until recently (66, 67).
Previously, yeast cell membrane was found impermeable to these sugars
(68, 69), as well as to fermentable sugars (69-71). Centrifuged yeast is
80% water in volume and 30-33% of the volume is extracellular space
(68). On the basis of this observation it is evident that only a distribution of a substance into a volume greater than 33% of the packed yeast
volume represents entry into the cell. Table XI shows the results obtained
with various nonfermentable sugars (72).
It is evident from these data that the first three sugars listed showed
maximum distribution into the total aqueous space without accumulation, while the presence of the other sugars was limited to the extracellular space. Among the pairs of molecules of equal size listed in
Table XI, xylose entered while arabinose did not; sorbose entered while
galactose did not; a-methyl-D-glucoside entered while a-methyl-D-mannoside did not. Entry was therefore stereospecific and indicated that the
process is more complicated than a simple diffusion.
A carrier-mediated distribution between extra- and intracellular space
could explain the results obtained. This type of transport first proposed