ROUND TABLE DISCUSSION 705 
This observation is similar to that described by Sistrom, which Dr. CHRISTENSEN mentioned, and 
also resembles the observation of Dr. HoLpEN on the osmotic effects of glutamate uptake into 
bacteria with weakened cell walls (Fig. 1). 
By assuming that swelling of protoplasts suspended in a solute at high osmotic pressure re- 
presents the osmotic effects of a penetration of extracellular solute, the permeability of the 
membrane to a variety of compounds, including amino acids, can be tested (ABRAMS, YAMASAKI, 
REDMAN AND MACKENZIE®). The rate of swelling becomes a measure of rate of penetration. An 
illustration of this type of investigation with amino acids is shown in Fig. 2. The experiment 
described in Fig. 2 is designed to test the permeability of L- and p-alanine and L- and D-serine 
in the absence of glycolysis. The curves in Fig. 2 indicate that L-alanine and L-serine penetrate 
rapidly and spontaneously, the latter with a long lag period. On the other hand, the D-isomers 
penetrate the non-glycolyzing protoplasts very slowly if at all. However, on the addition of glucose, 
the p-isomers also penetrate very rapidly. It appears that the membrane barrier distinguishes 
sharply between the optical isomers of alanine and serine with respect to their spontaneous down- 



0.8 
perp 
Control 
ABSORBANCY 
ALANINE 
0.2 

25 50 25 
MINUTES MINUTES 
Fig. 2. The effect of glycolysis on protoplasts stabilized with the optical isomers of serine and 
alanine. Protoplasts from S. faecalis were suspended in solutions of L- and b-alanine (0.4 V7) and 
L- and p-serine (0.4 17) containing K phosphate (0.075 MW, pH 7.2). Glucose (0.01 M) was added 
as indicated. The rate of swelling, photometrically observed, indicates the rate of penetration of 
the amino acids. 
hill penetration, and during glycolysis the membrane seems to open up to allow a far more rapid 
penetration of both L- and pD-isomers. 
Similar experiments have been carried out with many of the common amino acids with results 
which indicate a more or less unique behavior for each amino acid. Thus, both L- and D-valine 
penetrate spontaneously very rapidly, whole both L- and p-threonine are practically impermeable. 
Howeve1, the addition of glucose causes a very rapid penetration of L- and p-threonine. In the 
light of these findings it appears that the structure of the membrane is constituted in such a way 
as to discriminate between the different amino acids with regard to their spontaneous penetration 
into the cell. Moreover, the membrane seems to change its permeability under the influence of 
glycolysis allowing, in most cases, more rapid penetration. 
CuEN: In this connection I should like to mention the recent work of J. E. TREHERNE® on the 
absorption of amino acids in the gut of the locust Schistocera gregaria. He injected into the ali- 
mentary canal an experimental solution which had about the same concentration and osmotic 
pressure as the hemolymph. Using “C-labeled glycine and serine he found that these amino acids 
are rapidly absorbed in the mid-gut region, especially from the caeca. An important finding was 
the significant increase in concentrations of glycine and serine above those in the body fluid, 
obviously due to removal of water, after injection of the experimental solution into the gut. 
He concluded that the transfer of these amino acids across the gut wall is mainly due toa diffusion 
gradient thus established. 
I think his experiment demonstrates that here the movement of water plays an important role, 
and there is, at least in insects, no evidence of active transport of amino acids against concentration 
gradients. 
BritTTEN: I don’t think anybody has denied that an amino acid can be in solution, particularly 
in water, and that indeed the presence of different concentrations in different cellular fluids is a 
References p. 777 
