What is the purpose of co-transport?
My current understanding of co-transport is that, first, a substance is actively transported across a membrane, establishing a concentration gradient across said membrane. This same substance then diffuses down the established concentration gradient, effectively travelling back to where it began, but this time travelling via a transmembrane protein that transports another substance also, regardless of its concentration gradient. Thus, this second substance is transported to wherever it is necessary.Would it not then make more sense for the latter substance to simply be actively transported to its destination, as opposed to involving another substance which has no net movement at the end of the process? What is the advantage of co-transport?