What is the purpose of co-transport?

Question

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?

Answer 1

Co transport is basically secondary active transport. The reason this type of transport is used is due to the fact that sometimes when a molecule (glucose, ion etc) needs to move across the plasma membrane, the reaction is thermodynamically unfavorable (endergonic). In order for the reaction to proceed, thermodynamic coupling is used which essentially means that the unfavorable (endergonic) process is being driven by an exergonic (favorable) process. This allows for the reaction to occur. The combined Gibbs free energy change of this coupling should be negative. Delta G must be less than 0/negative in order for the reaction to proceed favorably.

Answer 2

The cell does this to save energy. Take for example glucose piggy backing off of the sodium transport. The cell wants glucose but would require excessive energy to obtain it. However, by having the glucose transported with sodium together, the cell uses less energy to get glucose into the cell. This is true for any other secondary substance that uses co-transport to transverse the membrane.