My physiology textbook mentions that potassium has the greatest concentration ( 155 mEq/L ) in the Intracellular fluid and that I thought would make it the most osmotically active but the answer given is phosphate and I can't understand how this is the case. Is it because a compound of phosphorus would contribute more osmoles than potassium?

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    $\begingroup$ I don't have an answer, though I have a suspicion it may have something to do with the charge of their ionic forms. Potassium ions have +1 charge. Phosphate ions (the most common form of phosphorus in cells) have a -3 charge. $\endgroup$
    – acvill
    Mar 11, 2022 at 16:24
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    $\begingroup$ Can you provide a short quote from your actual textbook? $\endgroup$
    – Bryan Krause
    Mar 12, 2022 at 15:07
  • $\begingroup$ @BryanKrause actually this is a review question given at the end of the chapter. In the chapter they have given the respective concentrations of potassium, phosphate and other ions only and explained about osmosis and other transport mechanisms in a cell. $\endgroup$
    – Sayak Roy
    Mar 12, 2022 at 16:58
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    $\begingroup$ @acvill I've thought of a solution but I am not sure.Osmosis is defined as the movement of water from the region of its higher concentration to lower concentration through a semipermeable membrane which is impermeable to the solute. Although potassium is more concentrated in ICF(155 mEq/L) it can pass through the membrane whereas phosphate which is less concentrated (90 mEq/L) cannot pass the membrane and hence is more osmotically active. Can this be the reason ? $\endgroup$
    – Sayak Roy
    Mar 12, 2022 at 17:15
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    $\begingroup$ I'm guessing if you go to the textbook section that the question pertains to you'll find what specifically the author is wanting you to take away. $\endgroup$
    – Bryan Krause
    Mar 12, 2022 at 17:26


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