Asklemmy

43746 readers

1462 users here now

A loosely moderated place to ask open-ended questions

Search asklemmy 🔍

If your post meets the following criteria, it's welcome here!

Open-ended question
Not offensive: at this point, we do not have the bandwidth to moderate overtly political discussions. Assume best intent and be excellent to each other.
Not regarding using or support for Lemmy: context, see the list of support communities and tools for finding communities below
Not ad nauseam inducing: please make sure it is a question that would be new to most members
An actual topic of discussion

Looking for support?

Looking for a community?

Lemmyverse: community search
sub.rehab: maps old subreddits to fediverse options, marks official as such
[email protected]: a community for finding communities

~Icon~ ~by~ ~@Double_[email protected]~

founded 5 years ago

MODERATORS

[email protected]

Where does the energy come from to lift water by osmosis? (feddit.uk)

submitted 4 days ago* (last edited 4 days ago) by [email protected] to c/[email protected]

14 comments fedilink hide all child comments

For example in a tree, the water is lifted from the high concentration in the soil to the low concentration higher up in the tree. But at the end of that process the water has been elevated, which should take energy (=mgh), but it seems like it kind of gets lifted for free without spending any energy?

Similarly, dipping a paper towel into a bowl of water, the water "climbs" the towel (by capillary action?) and absorbs upwards, meaning the water was lifted upwards (so gained potential energy) seemingly for free?

you are viewing a single comment's thread
view the rest of the comments

[–] [email protected] 37 points 4 days ago (4 children)

What you are describing is not osmosis, it is capillary action. Capillary action is caused by the forces between the water molecules and the molecules of the tube overcoming the force of gravity. You can read more here: https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/States_of_Matter/Properties_of_Liquids/Capillary_Action

Briefly, the water molecules are attracted to the molecules of the tube by adhesive force. The liquid molecules are also attracted to each other by cohesive force. The interplay of these forces causes capillary action.

However, it seems that tree sap moves by more than just capillary action. If you scroll down part way in this book they talk about it a bit: https://pressbooks.online.ucf.edu/phy2053bc/chapter/cohesion-and-adhesion-in-liquids-surface-tension-and-capillary-action/

[–] [email protected] 4 points 4 days ago (3 children)

So is it ultimately down to electromagnetic attraction on the microscopic scale?

[–] [email protected] 4 points 4 days ago (2 children)

Generally, yes, but in the case of trees there is also negative pressure (vacuum) exerted from transpiration when water leaves the top of the tree and "pulls" other water up behind it.

[–] [email protected] 1 points 4 days ago (1 children)

So the energy comes from sunlight and ambient heat, which provides the energy to evaporate that water, overcoming its adhesion and cohesion.

[–] [email protected] 2 points 4 days ago

I think it's more like sunlight energy + adhesion energy - cohesion energy, because the capillary action is also helping to lift the water but generally yes the energy is coming from a combination of the chemical forces in the water and the pressure gradient from the sunlight/heat.