Re: Does active transport ONLY occur against a concentration gradient?

NO.  Active transport is generally independent of the concentration 
gradient.  It can occur with a favorable gradient or without.  The reason 
is that "active transport" means that energy is input from OUTSIDE the 
transport system itself (e.g., ATP or a proton gradient).  The transporter 
couples that energy to the transport process.  As long as there is energy 
available to the transporter it will transport.

Now, the concentration gradient (either direction) will influence the 
transporter but usually not directly.  That is, there are feedback 
mechanisms of many different kinds that will "regulate" the transporter. 
The transporter can be regulated (either direction) by something as simple 
as phosphorylation of the transporter caused directly or indirectly by the 
substrate. 

Second, the concentration gradient will affect transport directly in the 
sense of simple mass action kinetics.  Take for example, the Na+,K+-
ATPase, the sodium pump.  It normally will exchange Na+ and K+, 3 Na+ out 
and 2 K+ in per cycle.  If you raise the EXTERNAL K+ concentration thus 
diminishing the K+ gradient, you will inhibit the pump.  But this is 
largely due to simple competition of external K+ for the K+ binding sites 
on the transporter.  On average, the K+ occupies the sites for a longer 
period of time as the concentration increases.  This means the pump cannot 
cycle back to a state where it can bind Na+ and ATP and go through another 
reaction cycle.  But this isn't really an effect of the K+ gradient, just 
simple mass action binding of K+ to a binding site.

I've greatly simplified this of course and a biophysics guru would raise 
their eyebrows I'm sure at my explanations, but for initial practical 
purposes, the concentration gradient does not have a major effect on the 
instantaneous activity of active transport systems.