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My textbook says this:

Osmosis is the diffusion of water.

Wikipedia says this:

The diffusion model of osmosis is rendered untenable by the fact that osmosis can drive water across a membrane toward a higher concentration of water.

It's my understanding that diffusion is passive transport, and it moves material from a highly concentrated area of that material to an area where it is in lower concentration. By this definition, it looks like Wikipedia is correct. But if for example, a cell is put in saltwater, water from within the cell would go out. Is this diffusion now? Which definition is correct?

I would just go with the Wikipedia definition because it actively describes its own definition as a refinement of the textbook definition, but the paragraph above in the article is a bit confusing to me.

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    $\begingroup$ Have you tried reading the sources at the end of the article? $\endgroup$ – John Jun 22 '20 at 3:44
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    $\begingroup$ This question is off-topic for Biology. It seems like you understand the process of osmosis, and are just curious if it's also a kind of diffusion. The answer to your question is either going to be based in physics or in the semantics of various definitions. $\endgroup$ – MikeyC Jun 23 '20 at 14:43
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    $\begingroup$ you will likely have better luck in the chemistry stack as this is only tangentially connected to biology. $\endgroup$ – John Jun 24 '20 at 12:59
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Yes, In most cases, osmosis is the diffusion of water. To my understanding, what I think Wikipedia is trying to do is to assert the fact that osmosis is also be the movement of water from a region of lower water potential to a region of higher water potential. These kinds of contradictions, as much as scientists try to avoid them, still come to play because science is very complicated. Just when we start accepting a theory, it gets discarded by another theory.

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    $\begingroup$ Welcome to Biology.SE! First, while this looks like it might be a good answer it is to a question that is off-topic for this site (because it isn't primarily a biology question) and it is better to leave such questions unanswered. Second, for future posts please note that answers are much more likely to receive a favorable response if you include supporting references (primary literature is best). Without that support, your answer is indistinguishable from opinion. ——— Please take the tour and consult the help pages for additional advice on How to Answer effectively on this site. Thank you! 😊 $\endgroup$ – tyersome Jun 23 '20 at 18:02
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Osmosis is about solutions (solvent + solute).

Osmosis is a movement of the solvent through certain types of membranes. These membranes let the solvent move across them, but not the solute. The process tends to balance the solute concentration on both sides of the membranes.

If we say that the solvent is water and the solute is salt (NaCl). Water will move from the side of the membrane where the salt concentration is low toward the side where the salt concentration is high. At the end of the process both concentrations will be equal.

In biology it is a very important process at a cellular level (cell membranes are semi-permeable membranes) but also plays a key role in food digestion or kidney reabsorption, for instance. Cells (bacteria) that are put in solutions with high concentration of salt die of loosing the water they contain.

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    $\begingroup$ First, while this looks like it might be a good answer it is to a question that is off-topic for this site (because it isn't primarily a biology question) and it is better to leave such questions unanswered. Second, for future posts please note that answers are much more likely to receive a favorable response if you include supporting references (primary literature is best). Without that support, your answer is indistinguishable from opinion. ——— Please take the tour and consult the help pages for additional advice on How to Answer effectively on this site. Thank you! 😊 $\endgroup$ – tyersome Jun 23 '20 at 18:06

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