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

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?

But where would the energy for the transport come from! Thermodynamics tells us that the universe tends to increase the amount of disorder (Entropy). If we have active transport of a solute, then by definition, this is going against a concentration gradient. So we could end up with a substance being entirely on one side of a plasma membrane, which wouldn't be disordered at all - clearly thermodynamics wouldn't let this happen by itself since we would be reducing entropy.
This is why energy is needed for active transport. We need to put in the leg-work by using some energy to locally reduce entropy, and ultimately move something to one side of the plasma membrane. Think of the concentration gradient like the pent up energy of a spring. We can release this energy when we want, in order to make it thermodynamically feasible to move a solute against its concentration gradient.

What is the evolutionary advantage of co-transport?

We've established that we need an energy source to drive active transport. But indeed, why would we use a concentration gradient? Couldn't we equivalently use a different energy source? Well yes, we could use ATP (for instance in Na$^+$/K$^+$-ATPase). But a concentration gradient might already be established over a plasma membrane, so its already there as an existing source of energy. Perhaps this is why co-transport evolved with this mechanism.

Answer 2

@Jam is correct — if a little long-winded — in pointing out that one of the benefits of co-transport (no need to involve evolution) is that the concentration gradient of one co-transported molecule can provide the thermodynamic energy for the transport of the other molecule against a concentration gradient. This is dealt with in a quantitative manner in Section 13.1.2 of Berg et al. and in a qualitative manner in Section 13.4.

However, there is another factor to consider. This is the maintenance of electrical neutrality. Typically it is charged ions that are transported (anions in the examples shown below, taken from Ch. 18 of Berg et al.) so that co-transport of a counter ion is needed to maintain electrical neutrality.

The diagram above also highlights another important point. You need to look at the biochemistry of the co-transporting systems, rather than just considering them in abstract. One of the sites of many co-transporters is the mitochondrial membrane, where transport is part of a co-ordinated process.

Consider for example ATP, ADP and phosphate. ATP is synthesized in the mitochondrion from ADP and phosphate, and then much of it must be transported into the cytoplasm. At the same time ADP and phosphate must enter the mitochondrion as substrates for the generation of more ATP. Active transport is clearly a non-starter here (it would use the ATP that is to be transported!) and co-transport ensures that the influx of ADP is balanced by the efflux of ATP. Phosphate influx (which is balanced by hydroxide efflux, to preserve neutrality) must be coupled to this process, although I am not aware of how this achieved (contributions welcome here).

Analogous considerations apply to NAD⁺ and NADH, but it is the electrons that are transported, rather than these compounds themselves, using surrogate oxidized or reduced compounds. Shuttle systems of this sort integrate the biochemical functions of the mitochondrion with those in the cytoplasm, and need to be studied individually to understand the choice of co-transporters. Section 18.5 of Berg et al. is a convenient starting point.