528 H. N. CHRISTENSEN et al. 
whether these amino acids can be arranged in a single consistent series of increasing 
affinities, whether estimated by relative rates or relative inhibitory actions, as has 
generally been assumed. 
This assumption is not a necessary one. Fig. 1 shows schematically a widely 
known conception of active transport. Several other representations would serve as 
well. At the right-hand limit of the membrane, the transport site or carrier is assumed 
to be removed, in this case by conversion to a less suitable form, and then to be 
regenerated in the reactive form at the left-hand or outer limit. Energy is required 
for this portion of the cycle. The effect of this energy delivery process is to modify 
the transport site to make it more accessible or more attractive on side o of the 
energy-barrier and less so at side 7 so that the amino acid tends to be dissociated 
into the right-hand phase at higher concentrations. Investigators have generally 

Fig. 1. Scheme for active transport. To a standard model we have added, at the bottom of the 
figure, the possibility that the transport carrier or site retains some affinity for amino acids 
after it has been modified or degraded, and can serve for the migration (primarily the exit) of 
some amino acids more than for others. 
assumed that this modification of the transport site causes a complete loss of affinity. 
If so, a single affinity constant should describe all interactions within a class of 
amino acids that use the given site. But if substantial affinities are retained after 
the site modification, so that migration via this site may also occur, as illustrated in 
the lower encircled portion of the figure, then a second affinity, for this wnfavored 
or inferior form of the site becomes relevant. In that event inconsistent sequences 
may be obtained, depending on whether the rate of transport or the steady-state 
asymmetry of distribution is being observed. An amino acid with a high affinity for 
the site in both its initial (preferred) and final (inferior) form may cross the mem- 
brane rapidly in either direction and act as a strong competitive inhibitor to the 
transport of other amino acids, and yet not be greatly concentrated. Another amino 
acid that is very easily pushed off the site by the site modification may eventually be 
strongly concentrated, although at a slow rate if the affinity even for the favored form 
of the site is low. 
If this analysis is correct, the phenomenon under question may be signalled by 
inconsistencies between the order in which the vates and the extents of uptake take 
their place. That is, the extent to which an amino acid is concentrated at the steady 
state will measure a simple transport affinity only if the losses against which active 
References p. 538 
