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Secondary active transport uses electrochemical gradients as a source of energy for the uphill transport of substrates (coupled to downhill transport of the ion).

However except for in a few cases (e.g. Electron transport chains) the electrochemical gradient is formed by ATPase pumps which use energy from ATP to pump the ions.

So why not just use ATP in the first place (and not have to set up the gradient). (I do understand that the gradients are useful for signalling, action potentials etc. but that is a seperate issue.) i.e. What is the advantage of secondary transport.

My (complete) speculation: Is secondary transport perhaps used because it allows a sort of inherent regulation. Stop the gradient forming pump and you immediately stop all the secondary transport using that particular gradient? If that is the case are secondary transport proteins in groups with similar functions/pathways utilising the same gradient?

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  • $\begingroup$ I'm not sure if I understand the question properly. Essentially, it seems like you are asking why the protein network is one way compared to another simpler design. The answer to that is likely to be speculative; something to do with evolutionary ancestry of the proteins and possible functional redundancy built into the network. These types of questions at the organism level are often closed as duplicates of "Why do some bad traits evolve, and good ones don't?" $\endgroup$ – James Apr 11 at 9:58
  • $\begingroup$ @James, I was actually presuming that there is an evolutionary reason for secondary transport, i.e. it has some sort of advantage and thus is so pervasive. My question is whether there are any hypothesises (preferably with evidence) as to what that advantage may be? I've tried to make the question more explicit. $\endgroup$ – Mirte Apr 11 at 10:04
  • $\begingroup$ I have not read the post but the title question (Why have secondary transport?) is grammatically wrong and does not mean anything. You should edit that. $\endgroup$ – Remi.b Apr 11 at 16:49
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I think an easy analogy for why this approach evolves is to think of how humans use electricity (or energy more broadly).

Although it is feasible to have localized electricity generators (for example, a solar panel on a simple calculator), it's typically more efficient to have centralized electricity production. With that organization, changes to the electricity generating machinery can proceed without any changes needed at the end user. If the power plant improves efficiency by 10%, that improves the efficiency of the entire system without a change needed in every lightbulb, refrigerator, and phone charger.

Once an ion gradient is established, different independent systems can use it to perform otherwise energetically unfavorable tasks. Of course this is not the only way evolution could proceed: as linked in a comment there are several general explanations for why evolution doesn't produce every possible combination. There are in fact a lot of proteins that use ATP directly.

The sodium-potassium ATPase that powers one of the major secondary transport systems is particularly interesting because it moves 5 ions at a time, making it very efficient. It would waste ATP if it only exchanged 2 ions at a time. By producing that gradient, other proteins can then evolve to use this gradient at an appropriate energy level rather than needing to work on a "1 ATP = 1 cycle" basis.

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  • $\begingroup$ also keep in mind a common active ion being transported out that drives it is sodium and it is easy to see why the transport of sodium out of the cell would evolve by itself in some environments. once it exists it can be exploited for other purposes. the other common ion is H+ which can be used on it own to regulate PH. $\endgroup$ – John Apr 12 at 11:25

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