604 R. J. BRITTEN AND F. T. MCCLURE 
termed the “carrier”. The carrier molecule is assumed to be large enough to form a 
stereospecific complex with the amino acid, but still small enough to diffuse within 
the cell with some freedom. The mobility of the carrier is necessary since there are 
few carriers to transfer amino acids to the many pool-holding sites. 
Soe eee 
Ve He 
ve it 
E 
External amino acid = A—:———> 2 
AR 
ky Energy coupled 
A+EZAE Y 
ko oh ks ; 
IMD Sp IN Sas NIN Se le 
ky 
Fig. 8. Carrier model. Properties of the carrier model: (a) The cell contains a small quantity of 
mobile stereospecific carriers which freely form complexes (AE) with amino acids, without 
participation of energy donors. (b) The cell also contains a relatively large quantity of non-mobile 
groups (the sites) which form complexes (AR) with the amino acids. (c) The site complex AR 
can only be formed by a reaction with the carrier-complex AE and this reaction is coupled to an 
energy donor. (d) Exchange may occur between the site-associated amino acids and those 
associated with carriers, without coupling to energy donors. (e) Exchange also occurs between 
free amino acids and carrier associated amino acids, but mot between free amino acids and site- 
associated amino acids. (f) There are several classes of sites, some stereospecific and some non- 
specific. (g) There may be an osmotic barrier near the surface of the cell and “free” amino acid 
may not diffuse through the protoplasm at the same rate as in water, but the formation of the 
carrier complex nevertheless occurs at a sufficient rate, without the participation of an energy 
donor. 
A schematic diagram of the carrier model is shown in Fig. 8, along with equations 
indicating the reactions that are proposed. The listed properties of the model have 
been specified quite sharply so that the deductions may be analyzed quantitatively. 
It cannot be ruled out that certain “forbidden” processes, such as exchange between 
free amino acid and site-associated amino acids, proceed at slow rates. Further, the 
evidence is insufficient to specify which of the two processes is actually coupled to an 
energy donor. 
According to this model, the pool is formed in the following way. An external 
amino acid diffuses into the cell and collides with an unoccupied carrier. A complex 
with the stereospecific carrier is formed (AE); the complex diffuses through the 
cell and collides with an unoccupied site. In a reaction coupled to an energy donor, 
the amino acid is transferred from the carrier to the site. In turn an unoccupied 
carrier may collide with an occupied site and remove the amino acid. The evidence 
at the moment does not specify whether or not the reverse reaction is also energy- 
requiring. 
References p. 609 
