How Trees Bend the Laws of Physics

Key Takeaways

[Applause] sometimes the simplest questions have the most amazing answers like how can trees be so tall it's a question that doesn't even seem like it needs an answer trees just are tall some of them are over a 100 m why should there be a height limit I'll tell you why trees need to transport water from their Roots up into their topmost branches in order to survive and that is no trivial task there is a limit to the height that water can be sucked up a tube it's 10 m if you suck on a long vertical straw the water will go no higher than 10 m at this point there will be a perfect vacuum at the top of the straw and the water will start to boil spontaneously for a tree to raise water 100 m it would have to create a pressure difference of 10 atmospheres how would trees do that when I posed this conundrum a lot of people said the answer is transpiration and that's when water evaporates from the leaf pulling up the water molecules behind it now that's clearly a mechanism a tree can use to create create suction but it doesn't help us overcome this 10 m limit the lowest the pressure can go is a pure vacuum which I imagine is not happening inside of tree leaves right right Hank so you might suspect that a tree does not contain continuous straw-like tubes the tree effectively has valves in it so you don't have a column of water this big tube which you're saying needs to be filled with water is actually made up of cells although these are good speculations they don't turn out to be correct scientists who stud trees find that the xylm tubes that transport water do contain a continuous water column so how else could the tree transport water from the roots to the leaves they don't suck they don't use a vacuum okay so how do they do it squeezing like a cow like you're squeezing a cow all the way up there's little tree muscles in there yeah besides being a giant waste of energy all of the cells that make up the xylm tubes are dead what about osmotic pressure If there is more solute in The Roots than in the surrounding soil water would be pushed up the tree but some trees live in mangroves where the water is so salty that osmotic pressure actually acts in the other direction so the tree needs additional pressure to suck water into the tree then it must be capillary action the thinner the tube the higher the water can climb but the tubes in a tree are too wide at 20 to 200 micrometers in diameter water should rise less than a meter so how do trees do it well one of the assumptions we made is wrong the lowest the pressure can go is a pure vacuum P vacuum your vacu in a gas this is true when you eliminate all of the gas molecules the pressure is zero and you have a perfect vacuum but in a liquid you can go lower than zero pressure and actually get Negative pressures in a solid we would think of this as tension this means that the molecules are pulling on each other and their surroundings as the water evaporates from the pores of the cell wall they create immense negative pressures of minus5 atmospheres in an average tree think about the the air water interface at the pore there is one atmosphere of pressure pushing in and - 15 atmospheres of suction on the other side so why doesn't the meniscus break because the pores are tiny only 2 to 5 nanm in diameter at this scale water's high surface tension ensures the airwater boundary can withstand huge pressures without caving As you move down the tree the pressure increases up to atmospheric at the roots so you can have a large pressure difference between the top and the bottom of the tree because the pressure at the top is so negative but hang on if the pressure at the top is5 atmospheres shouldn't the water be boiling yes yes it should but changing phase from liquid to gas requires activation energy and that can come in the form of a nucleation site like a tiny air bubble that's why it's so important that the xylm tubes contain no air bubbles and they can do this because unlike a straw they've been water filled from the start this way water remains in the metastable liquid state when it really should be boiling it's just like super cooled water remains liquid even though it should be ice so you could say that the water in a tree is super sucked because it remains liquid at such negative pressures and why are trees moving all this water up the tree I want you to make a guess say it out loud for photosynthesis actually no less than 1% of the water is used in photosynthetic reactions any other ideas okay what about growth well 5% of the water is used to make new cells so what happens to the other 95% of the water it just evaporates for each molecule of carbon dioxide the tree takes in it loses hundreds of molecules of water wa can you believe how amazing this is trees create huge negative pressures of tens of atmospheres by evaporating water through nanoscale pores sucking water up 100 Metter in a state where it should be boiling but can't because the perfect xylm tubes contain no air bubbles just so that most of it can evaporate in the process of absorbing a couple molecules of carbon dioxide I will never look at trees the same way again eat anything how does that make any sense don't eat no they don't eat anything I'd like to say a huge thank you to Hank Henry and Professor poov for making on camera hypotheses this is an essential part of the scientific process even if your hypothesis turns out to be wrong as Einstein said a person who's never made a mistake has never tried anything new i' always wondered what it would be like to be on this side video now I'd be surprised if you weren't already subscribed to these guys but if you're not go click on these annotations and check out their channels and you may just learn something I'd also like to thank professor John Sperry from the University of Utah he walked me through all of this in an hourong Skype conversation so I'm going to put a link to his website in the description we're looking at pressures here below at atmospheric is that right that's right mm below atmospheric this is liquid pressure not gas pressure so it's a common misconception that oh you can't have you know negative pressures because there's no molecules left you know the definition of pure vacuum is zero molecules that's for gas okay so just to just to be clear I think this was one of my big big problems in understanding this this video would have been impossible without cgp gray when I told him about this idea in London and I felt like like mind just blown with this whole thing he said it was going to be really hard to explain and you know when he says it's hard to explain you know things are going to be tough so thank you for all your input to this script and thank you for watching making this video has been a real Odyssey for me so thank you for joining me on that journey I really appreciate all of your comments and if you haven't subscribed to the channel already uh you can click The annotation or click the link above and join me on my next scientific Adventure I made a video promising to make a video about the answer to this I I proposed the problem like right a couple months ago and I was like subscribe to the channel I'll give you the answer next week drive it at the right frequency oh yes success is frightening