I'm wondering what is an evolutionary adaptation to the evolution of symporters and antiporters instead of just uniporters.

Antiporters might help preserve electrical neutrality by pumping in/out an ion of the same charge as is the case in H+/K+ ATPase, and perhaps transporting 2 ions at the same time is more efficient than just one.

But the same can't be said for Na+/K+ ATPase, which actually contribute to a net negative charge, so why not instead use a uniporter here?

Additionally, could the efficiency hypothesis also be applied to symporters or are here other reasons at play?

Hence in brief, what justifies the existence of symporters and antiporters as compared to uniporters?

  • 1
    $\begingroup$ The Na/K ATPase hardly contributes to any charge. The (lack of) importance of the 2/3 ratio is often misleading to students. There are many questions here about this. $\endgroup$
    – Bryan Krause
    Jun 12, 2021 at 14:38

1 Answer 1


Uniporters function as passive transporters and can not achieve concentrative accumulation. Symporters and antiporters are examples of active transport (they couple the movement of a subtrate to an additional process that yields energy). Since some solutes are present at very low concentrations on one side of the membrane (e.g. in the blood) systems had to evolve which can achieve concentrative uptake (as opposed to equilibrative uptake as mediated by passive transporters). Therefore, it makes sense that not only uniporters exist, but also symporters and antiporters (and primary active transporters as well).

  • $\begingroup$ For me the more interesting question is, why do uniporters have to exist when you have pores and channels? (Uniporters differ from the latter two in that the function according to the alternate access mechanism and are not open to both sides of the membrane) $\endgroup$
    – RtoR
    Apr 29, 2022 at 21:05
  • $\begingroup$ Uniport: a membrane transport process in which a single kind of sub- strate is transported across the mem- brane, either by the mechanism of mediated diffusion where the substrate equilibrates at the two membrane sides (e.g., monosac- charides in mammalian erythrocytes; TC 2.A.1.1.12), or by active transport where an external source of energy is used to achieve movement against a chemical potential or an electrochemical po- tential gradient (e.g., plasma membrane H+-transporting ATPase TC 3.A.3.3.1 or EC Such transport systems may be designated as uniporters. $\endgroup$
    – Treex
    May 1, 2022 at 6:41
  • $\begingroup$ The above quote comes from Oxford Dictionary of Biochemistry and Molecular Biology and according to this definition active uniporters seem to exist. $\endgroup$
    – Treex
    May 1, 2022 at 6:43
  • $\begingroup$ @RtoR Most channels are specific to certain very small molecules and ions like sodium. If there were channels that were large enough to accommodate molecules like glucose, many other molecules would be able to pass through as well and the membrane would lose its selective permeability, and would be unable to maintain concentration gradients. $\endgroup$
    – Eonema
    Jan 25, 2023 at 19:23

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