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Pages I've read about worms' respiratory systems says that the skin needs to be wet (covered in mucus) or oxygen won't diffuse across the skin. Why? If there is more oxygen outside the worm's skin than inside, what prevents it from diffusing across the skin, even if the skin is dried out?

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The quick answer: When the skin dries, the lipids in the cell membranes of the skin tissue undergo a phase transition which makes the membranes less permeable for oxygen.

Explanation: The lipids of the cell membrane can exist in different phase states. In the liquid disordered phase the lipids are relatively flexible and mobile, making this phase more oxygen permeable compared to the liquid ordered phase, in which the lipids are more rigidly packed.

The phase transition temperature of lipids increases upon dehydration (another reference), meaning that at the same ambient temperature, a dry lipid membrane is in the liquid ordered state and a wet lipid membrane is in the liquid disordered state.

Therefore, a dry cell membrane is less oxygen permeable than a wet one.

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Because you are talking about diffusion, thus there needs to be a continuous pathway of gas or liquid for the molecule to move through. Membranes stop behaving like liquid when you dehydrate them. Oxygen cannot diffuse through extracellular solids (not at a meaningful rate anyway), which is all that is left why the skin dries out, so there needs to be a liquid path for it to follow. The water around a cell membrane helps weaken the binding energy allowing materal to pass between the lipid molecules, without the water is becomes a functional solid.

Skin does not replace water with air when it dries out, it collapses in on itself losing volume so all you have is solids (or things close enough to solid to act as a barrier), those no path for the oxygen. It is the same reason CO2 cannot diffuse through a coke can, it can't diffuse through a solid.

Edit, there is some gas exchange but it is very minor especially compared to what happens normally, and occurs because there is an occasional gas path. Cell membranes can be diffused across, but they slow it down, layer upon layer of them packed tightly as you have in dehydrated skin produce much the same effect as a solid, reducing the diffusion rate the point it can barely be measured and is nowhere near what the organism needs.

Animals that do not need ot gas exchange across the skin add layers of waxes to the skin making it an even better barrier and thus making it even more water tight.

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  • $\begingroup$ Isn't the worm's skin still solid underneath the mucus? $\endgroup$ – Mark Eichenlaub Sep 12 '17 at 0:02
  • $\begingroup$ @MarkEichenlaub No, like most other biological tissue it is a gel, a liquid held together by a matrix of solids. That is why the mucus evolved, to prevent the skin from dehydrating. There are air tight skins in other organisms, (by creating waxy barriers), generally they experience minimal gas exchange. $\endgroup$ – John Sep 12 '17 at 0:21
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Respiration in aquatic organisms involves diffusion of dissolved gases present in the aquatic medium across their permeable cell membrane . In worms which lives in moist environment the skin functions as a respiratory surface and requires the surface to be moist to enable exchange of gases.

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  • $\begingroup$ Thank you, but I don't see this an answer to the question. "to permit exchange of gasses across moist skin" why does the skin have to be moist? "which is stable when moist." What does it mean to be "stable"? Why is moisture important to that? $\endgroup$ – Mark Eichenlaub Aug 31 '17 at 23:51
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    $\begingroup$ Hi Mala. When directly quoting a source, it's required that the words be in blockquotes with the source listed (just listing the source isn't enough), or the material is considered to be plagiarized. It's kind to answer the question, but this is not the correct way to do it. $\endgroup$ – anongoodnurse Sep 1 '17 at 0:47
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    $\begingroup$ Yes, but why does the surface have to be wet? $\endgroup$ – Mark Eichenlaub Sep 1 '17 at 12:56
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    $\begingroup$ The dissolved oxygen in water needs moist surface to get diffused in to the cutaneous blood vessels located in the skin of worms . - basically to ensure that oxygen can be dissolved and absorbed by the circulatory system to fulfill the requirements of the organisms to reach to the deeper parts of the body. Required amount of oxygen can not be delivered directly in its gaseous form. $\endgroup$ – Mala Sep 1 '17 at 14:37
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    $\begingroup$ Dear Mark Eichenlaub -Thanks .The rate of diffusion varies with the area of diffusion, Concentration gradient and the thickness of the surface area as per Fick's law . A molecule of water which is at higher concentration inside the body than outside - diffuses out and makes the diffusion surface moist. Gas exchanging surface in land animals are internal to prevent much loss of water from the body. $\endgroup$ – Mala Sep 4 '17 at 6:06

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