[nj_bartel]

In our biochem class, we just learned what each is.  Are there advantages/disadvantages to each?

[renge ishyo]

Well, Primary active transporters require ATP to perform their function. For example, they can be used to pump ions out against a concentration gradient to establish a new electrochemical gradient across a membrane. The secondary active transporters do not require ATP to function, BUT they rely on a previously established electrochemical gradient to function. So indirectly, a secondary active transporter needs a primary active transporter to establish its gradient for it to work later on down the road. This is why the terms are "primary" and "secondary" to begin with. Secondary active transporters operate by allowing a molecule to travel into a cell "up" against its concentration gradient by coupling it to another molecule which is traveling "down" the electrochemical gradient that you established earlier with the primary active transporter.

If I was to say that there was a disadvantage to primary active transporters it would be that they use ATP which is a very general energy source. By creating electrochemical gradients using a variety of different molecules in secondary active transporters, you can create a whole serious of specialized pumps that will only operate when the necessary molecules are present to couple with the gradient you established. It just allows for more control and diversity to establish specialized secondary pumps that can ultimately function whether ATP is present or not.

[nj_bartel]

Thanks, that makes sense   Would another possible benefit be that, as there are certain gradients that are pretty constantly maintained (e.g. membrane potential through the sodium potassium ATPase), you can use those already present gradients to carry out your transport rather than expending additional ATP?

[renge ishyo]

Yeah, you can expand on this in any number of ways. The general idea is that the secondary pumps can "store" electrochemical gradients until they are needed, and one secondary pump can operate without disturbing another secondary pump by the selectivity of the molecules that are needed to operate the secondary pump. So yeah, some pumps can operate continuously while others only once in awhile based on need. If everything relied on ATP only, it would be much harder to select one process over the other because both processes would be triggered by the same molecule. Establishing secondary pumps lets the cell diversify electrochemical processes. The downside to this scheme however, is that you actually do lose some extra energy by establishing secondary pumps (you lose some extra energy when you spend ATP on the primary pump to establish a secondary pump to begin with). I guess it is worth the loss to gain the diversity of functions (it just means we have to eat more ).