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Does active transport go only against concentration gradient? Is there any scenario that active transport moves molecules from high to low?

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  • $\begingroup$ I'd parse this into two sub-questions for yourself. 1) What would the benefit be of using active transport to move something down a concentration gradient? 2) If you artificially or pathologically or experimentally changed concentrations such that something that would ordinarily be moved up a concentration gradient is now moved down, would that interfere with an active transport system already in place? $\endgroup$
    – Bryan Krause
    Apr 24, 2023 at 14:01
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    $\begingroup$ These questions are the exactly why I'd ask this question. Some of the textbooks and my teacher said the active transport is not always against concentration gradient. I don't think that's right too. An example introduced by my teacher was the active transport was done by small intestine cells from low to high after eating dinner. $\endgroup$
    – William G
    Apr 24, 2023 at 14:36
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    $\begingroup$ If you include the quotes from the textbooks in your question it would significantly help. $\endgroup$
    – E Tam
    Apr 24, 2023 at 17:15

2 Answers 2

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If the molecule to be transported has a ionic charge, then the change in free energy is not only due to the concentration gradient, but also on the membrane potential (Vm) and molecule charge (z).

If the electrical free energy necessary to move the molecule surpass the free energy released by the chemical gradient, then an active transport is necessary.

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Does active transport go only against concentration gradient? Is there any scenario that active transport moves molecules from high to low?

  • What's active transport?
  • Have you considered the ion transport mechanism of gastric pump?

There are two types of Active transporters. 1) Primary Active Transport is generally energy driven by ATP Hydrolysis. 2) The other category of active transporters do not rely on ATP directly.

One:

The primary term refers to transport mechanisms that are directly coupled to the consumption of metabolic energy ATP. Thus "pumps" encompasses all transporters capable of thermodynamically uphill transport (so-called active transport). Let's consider a couple popular active transporters (ion pumps) of the cell membrane that are directly fueled by ATP.

Within ion pumps itself there are those active transporters that pumps ion against a concentration gradient e.g. H+,K+-ATPase (gastric proton pump) where high concentration of H+ is actively transported across the appical membrane of parietal cells towards the low concentration of H+ of gastric lumen. Gastric pump maintains a 1H+:1K+ ion stoichiometry, yielding overall electroneutral ion pumping mechanism and cycel. This is a type of active transport mechanism that is specific to a particular physiological task such as digestion.

Then a more popular ion pump of the same family, Na+,K+-ATPase which actively transports ions (3Na+:2K+) across an electrochemical gradient (a combined concentration gradient and an electrical gradients) in basolateral membrane by pumping Na+ from cell interior (cytosol) to cell exterior in exchange for K+ from cell exterior to interior where cytol has high [Na+] concentration and extracellular is high in [K+]. The electrochemical gradient is necessary for the pump's ion pumping activity and infact gradient is maintained by the underlying concentration and asymmetric selective substrate ions (Na+, K+) stoichiometry (3:2). Na+K+-ATPase's active transport is at work in all cells of the body (not just human).

Primary active transporters, such as Ion pumps do infact pumps ion across high to low concentration (concentration gradient). Have you considered H+K+-ATPase?

Two:

The second type of active transporters depend on the electrochemical gradients of other ions as an energy source. They are usually called ion exchangers. These active transporters are able to carry more than one ion up its electrochemical gradient and at the same time taking another ion (most often Na+) down its gradient. e.g. the Na+/Ca2+ exchanger that shares Ca2+ with the PMCA Ca2+ pump.The exchanger keeps intracellular Ca2+ concentrations low. The Na+/H+ exchanger regulate intracellular pH, in this case by acting directly on the concentration of H+.

Such that It is not quite just to simply to characterize that 'all' active transport mechanisms are 'only' based on concentration gradients alone. Further, just because a molecule (substrate) to be actively transported by a transporter, is an ion, it does not engage in a concentration gradient. There are transporters such as P-Type 4 ATPases which are evolved to actively transport phospholipids as substrates that are not cations/ions.

I am not going to contaminate the answer by introducing passive transporters such as ion channels which are crucial to the co-existence and function of the ion regulation of the cells. e.g. ion channels produce electrical signals by diffusing down substrate ions across electrochemical gradients (again a combination of ion concentration and signaling. That means ionic currents depend on the electrochemical gradient across the cell membrane (of some type). In the case of nerve cells, such an electrochemical non-equilibrium is crucial. Diffusion will continue until this gradient has been eliminated. Imagine, with ongoing electrical signaling and ion diffusion, if there's no reserves to constantly replace these ions, these necessary ion concentrations would reach a zero, so is the nerve signaling.

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