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I am very confused about how exactly substances are transported across cells. For example, if a cell is placed in a hypertonic solution, the cell loses water. If the cell is placed in a hypotonic solution, it gains water. This happens due to osmosis.

My confusion is, however, why don't any of the solutes within the cell move in or out of the cell instead of the water, to maintain an equal concentration inside and outside the cell? Why is it always the water in these cases that moves in and out of cells? Is it because the cell allows free movement of water, but it controls the movement of the solutes according to its needs using carrier or channel proteins? How exactly does the cell control what substance will move in and out during diffusion?

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  • $\begingroup$ What does your source/textbook say about transport of materials across membranes? That's usually where these discussions begin, before getting into osmosis. $\endgroup$
    – Bryan Krause
    Apr 5, 2023 at 14:38
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    $\begingroup$ Really, basic osmosis is Chemistry, and in my experience always refers to semi-permeable membranes which, by definition allow water but not solutes across. As the most cursory research would make clear — research that we expect from posters to this site — what prevents polar solutes passing across a cell membrane is its non-polar lipid nature, making specific transporters necessary. $\endgroup$
    – David
    Apr 6, 2023 at 8:31
  • $\begingroup$ Before membranes and membrane transport, there's something called osmosis and diffusion to become familiar with. Because one of the roles of membrane transport of cells is to maintain osmotic pressure :) $\endgroup$
    – bonCodigo
    Apr 27, 2023 at 6:27

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A few short inputs:

  1. The membrane is permeable for some molecules but not for others. Small and uncharged molecules can pass easily (such as H2O). Others like Sodium (Na+), Chloride (Cl-) etc. have a harder time doing that, due to electric charge (the membrane is hydrophobic, hence, energetically it doesn't really like to mix with something that is that hydrophilic). However, larger molecules can pass through the membrane, if they are very hydrophobic (think about different steroid hormones).
  2. Furthermore, many ions, such as sodium, are actively transported in a particular direction. Water is usually not.
  3. That is also, why when doing an osmosis experiment in a physics book you have a "semipermeable" membrane which only allows water to move across, but not ions. A cells membrane is basically "semipermeable".

I would suggest to read the chapters about transporters, ion channels etc. in a molecular biology book, such as "Molecular Biology of the Cell" from Alberts.

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  • $\begingroup$ First you say 1) uncharged molecules can pass easily (such as H2O) but thereafter 2) the membrane is hydrophobic, hence, energetically it doesn't really like to mix with something that is that hydrophilic. These statements bite each other. Water is hydrophylic, since it likes itself :) Hence, it is lipophobic $\endgroup$
    – AliceD
    Apr 6, 2023 at 23:58
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    $\begingroup$ Well, not necessarily. Water also doesn't mix very easily with membranes (water is still polar, membranes are largely not, at least on the inside). Otherwise they wouldn't be able to exist as stable entities. Think about oil (the membrane) and water. However, water, because it is fairly small and uncharged, can at least pass it regularly. That's why bacteria have things like aquaporins, where lots of water can get through the outer membrane (but not the inner membrane). $\endgroup$
    – Felix H.
    Apr 7, 2023 at 6:26
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    $\begingroup$ That's interesting, I'll read on it a bit :) $\endgroup$
    – AliceD
    Apr 7, 2023 at 6:52
  • $\begingroup$ Felix, Cell or Berg & Stryer still would be quite up the ladder in my view. Start with AP chemistry and bio would be of some value. $\endgroup$
    – bonCodigo
    Apr 27, 2023 at 6:30
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    $\begingroup$ By that comment do you agree that it is difficult for water to diffuse into the membrane and easier to pass through aquaporins? I am quite confused of what you finally have to say. $\endgroup$
    – Patrick
    May 19, 2023 at 17:36
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I would first like to define some terms first based on the permeability.
Impermeable: A membrane that does not allow any substance to pass through it is called an impermeable membrane. e.g. Cuticle layer
Permeable: A membrane that allows the free passage of substances through it is called a permeable membrane. e.g. Primary cell wall
Semi permeable: Semi permeable membranes allow the movement of most substances if they are small enough by osmosis or diffusion. It does not require any energy or in other words uses passive transport. e.g. Egg membrane
Selectively permeable: As the name suggests these membranes are more selective and the substance that can pass through are limited. Substances that cannot pass through by passive transport is transported by active transport through integral proteins embedded in the cell membrane.

Though the cell membrane acts semi-permeable to the small oxygen and carbon dioxide molecules it is best to call the cell membranes as selectively permeable membrane because of the degree of the control they have over on what to allow and not to. Try reading this page for the difference between semi permeable and selectively permeable.

You were right in saying that the cell membrane allows the free movement of water but controls the movement of other solute particles according to its needs. If the molecules are small enough and they are non-polar, they can easily diffuse inside. If they are polar , like water and glucose, it is difficult to push their way through the non-polar tails of the phospholipid bilayer. The channel proteins will aid in the movement of such substances through passive transport. The channel protein that allows the rapid flow of water through is called an aquaporin. If the substances need to move against the concentration gradient, from a lower concentration to a higher concentration, they use energy and pass through the proteins. This is active transport e.g. Sodium potassium pump.

And for the last question, diffusion is the net movement of substance from a region of higher concentration to a region of lower concentration. Note that this happens along the concentration gradient and does not need any energy. The cell cannot exactly "control" that movement. If the particle is small enough and the polarity is right, it can pass along the gradient. Movement of $CO_2$ out of the cell and movement of $O_2$ inside the cell is by diffusion.

Correct me if I went wrong somewhere.

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"Water can pass through biological membranes via two pathways: simple diffusion through the lipid bilayer, or water-selective facilitated diffusion through aquaporins (AQPs)" Ibata et al., 2012

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