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‘Rise above yourself and grasp the world’ Archimedes – attribution

Posts Tagged ‘transpiration

transporting water in trees – the finale, perhaps

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‘If I do not succeed today, I will attack [the problem] again on the morrow’

Mary Fairfax Somerville (1780-1872), mathematician, physicist, autodidact, genius

 

trees are so interesting… some more than others

So far my journey into this subject has proved fascinating but inconclusive, but a Veritasium video has helped me with the final solution, though it’ll still take a while to get my head around it.

There’s more than one problem involved here, as I’ve mentioned. There’s the transport problem and the ‘knowledge’ problem; how does the tree ‘know’ when it needs to bring up water, and how much to bring up?

Let’s look at transpiration again. Think of it like our perspiration. When we exercise, or even just when we’re in the sun, we sweat. The sweat then evaporates, cooling our bodies, and if we need to, we produce more sweat. It’s not conscious, we don’t have to know how much more sweat, or water, to produce, it’s just a ‘process’, no doubt a very complex one, like the process in trees and plants. However, I’ll come back to transpiration later.

Water can move up the xylem tube to the leaves of a tall tree at a maximum rate of a third of an inch per second, according to Peter Wohlleben (obviously translated into American). That’s around 2.5cms every 3 seconds, or 50cms per minute. Or 30 metres per hour. That’s rather impressive. When I told a friend about this, she said you can hear the water gushing up the trunk if you put your ear to it. Maybe that’s true.

The Veritasium video starts with another fascinating question – how can trees get so tall? And of course it’s worth noting that different species of trees have their own ‘natural’ height limits, levels of ‘bushiness’ and so forth, which is obviously affected by their particular environments as well as their genes. The tallest are around 100 metres. And one major limiting factor is that they need to transport water from roots to topmost leaves. You can’t suck water up a straw for more than ten metres, because you’ll have sucked all the air out, creating a vacuum, a pressure difference of one atmosphere. To suck the water ten times that high would create a difference of 10 atmospheres or more. So even if trees could suck somehow, the task seems impossible…

Back to transpiration – when water evaporates from the leaves, this ‘pulls up the water molecules behind it’, according to the video, though it doesn’t give an account of this ‘pulling’ mechanism, which in any case couldn’t account for the 100-metre movement, again because of the pressure limitations. Interestingly, many websites, including Wikipedia, describe transpiration as the whole process of water transportation in plants, rather than the process of evaporation and replacement in the region of the leaves, so it can be confusing. In any case the ‘pulling’ up of water to replace molecules lost in evaporation is explained by the cohesion-tension theory, as referred to in a previous post. It’s about hydrogen bonding and the adhesive and cohesive properties of water. Yet it seems miraculous that this process can explain such a vast movement against gravity. The xylem inside trees – those dead, hardened, hollowed-out cells – provide an uninterrupted column for water to pass through (the apoplastic pathway), but the distance would seem to cause pressure problems. The video discounts osmotic action, and here I have to take a quick primer on osmosis, because I don’t get it:

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 in.

I don’t know why that seems counter-intuitive to me. Is it because I don’t think (sufficiently) scientifically?

Right, after a glance at a couple of videos, I think I get it. I remember the mantra that osmosis is the passage through a semi-permeable membrane (and when does a fully permeable membrane stop being a membrane?) from high to low concentration. But of course it’s the concentration of water molecules that passes from higher to lower, not the concentration of the solute. Duh. And its continued movement up the tree would have something to do with the polarity of water, its bonding properties. But anyway, osmosis isn’t the answer, as mentioned. And neither is capillary action, as explained before.

So now, to the actual explanation, which, at this moment, I certainly don’t get, but I’m going to try. It has to do with gases, liquids, vacuums, pressure and the properties of water. The video provides its final solution, so to speak, in less than two minutes of air-time, but for unscientific me, after a few viewings, it raises more questions than answers. So I’m going to analyse it bit by bit, and this may turn out to be the longest single post I’ve ever written.

So first it’s pointed out, by Hank in the video, that the lowest you can go, pressure-wise, is a vacuum. But that’s only for gases. So a perfect vacuum equals zero pressure. You need something to exert pressure – if there’s nothing there, no pressure:

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 -15 atmospheres in an average tree.

This negative pressure or tension idea doesn’t come easily to me, and it’s the key to the explanation. It’s certainly accepted science, though there are questions about how much negative pressure water can withstand, as this scientific paper explores, before cavitation. The negative pressure of -15 atmospheres is approximately -1.5 Mpa (megapascals). Experiments described in the scientific paper show that, depending on circumstances, liquid water can sustain far greater negative pressures than -1.5 MPa.

I might be wrong, but it seems to me that negative pressure is like pressure from within (hence tension) rather than from without. I’m going to have to accept this as true, and try and make sense of the rest of the explanation:

Think about the air-water interface at the pore [of the cell wall – is he talking about the whole xylem tube as a cell?]. There’s 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 2.5 nanometers in diameter. At this scale, water’s high surface tension ensures the air-water boundary can withstand huge pressures without caving [cavitation].

So there’s an air-water boundary (the meniscus) at the pore, which presumably means the top of the column of water. But why does he call it a pore, which we usually think of as a hole, e.g. in the skin. This term isn’t explained at all, it’s just suddenly introduced. Does he mean the xylem column is 2.5 nm wide? No, the average xylem diameter is 25 to 75 micrometers, and 1 micrometer is 1,000 nanometers. In botanical terms, we think of the stomata on the underside of leaves. Is this what is meant? It seems so, from what comes next:

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.

I’m still not quite sure how this might be so, and perhaps for that reason the rest of the explanation drifts away from me, though I’m sure it’s trustworthy, and it certainly helps explain why transpiration is indeed an essential part of the entire water movement explanation. But I’ll continue the explanation together with any questions I can come up with. Derek Muller, our Veritasium creator, now asks himself, shouldn’t the water boil at this high negative pressure?

Changing phase from liquid to gas (boiling) requires activation energy. What is this? It’s often defined as the minimum energy required to set off a chemical reaction:

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 xylem tubes contain no air bubbles. 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.

Slow down, two more terms are introduced here, a nucleation site and a metastable state. A nucleation site is basically a site where a phase change can begin, and start to spread, as in crystallisation from a solution. In order for water to boil, it has to start to boil somewhere – which is a molecular change. At this site, aka the nucleator, the opportunity for another, freer molecular arrangement (a gas) becomes available, and this will communicate itself to surrounding molecules. Okay, not the best explanation, but it helps me. For a better explanation you should go to the Khan Academy – and so should I. A metastable state, and I quote, is an excited state of an atom or other system with a longer lifetime than the other excited states. However, it has a shorter lifetime than the stable ground state. … A large number of excited atoms are accumulated in the metastable state (Optics 101).

This explains why Muller says the water high in the xylem ‘really should be boiling’, or that it should be in a gaseous state, for that would be its ground state under normal circumstances.

So that’s as far as I can go. It’s been an odyssey for me as well as it was for Muller, and I’m definitely not as sure on it as I could/should be, but I’ve made a lot of headway, and it really is amazing to think of what not only trees but the plants on my balcony ‘garden’ are doing all the time – sucking water through their bodies and into the atmosphere…

 

Written by stewart henderson

February 22, 2018 at 9:42 am

How do plants transport water? Part 1: xylem, transpiration and a mysterious water potential difference

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roots, xylem, upward flow, transpiration – but how does it work? Find out in the next thrilling episode, maybe.
Stolen from Nature Education, with apologies

This post could fit well in the ‘How Stuff Works’ series, always a useful resource, but I doubt if they’ve done a piece on today’s subject. Maybe I’ll check later.

I’ve been reading a book called The hidden life of trees, by Peter Wohlleben, a Chrissy present from a good friend. One of its shortest chapters is titled ‘The mysteries of moving water’. The reason for its brevity is essentially that there’s as yet no solution to the mystery of how water gets from the soil to the leaves of a tree, or any plant for that matter. At least, according to Wohlleben.

This strikes me as amazing, if true. After all, it’s a simple, everyday scenario for any home gardener. You notice on a hot summer day that the leaves of your capsicum plant are wilting. You apply a two-litre dose of H2O to the base, et voilà, within an hour or two (I don’t know, I’ve never timed it), those leaves have become as turgid as much of my writing. And it just may cross your mind that it’s pretty miraculous how plants can do that. But if it’s true that we don’t know how plants manage such an everyday miracle, surely working it out is Nobel Prizeworthy for any ambitious team of botanico-chemists out there, or whatever.

Of course it’s much more likely that botanists have been trying to solve this mystery for decades – isn’t it? But before I look into it, here’s what Wohlleben says in his book:

…water transport is a relatively simple phenomenon to research – simpler at any rate than investigating whether trees feel pain or how they communicate with one another – and because it appears so uninteresting and obvious, university professors have been offering simplistic explanations for decades… Here are the accepted answers: capillary action and transpiration.

Upon reading this I tried to recall what I knew of these terms. With capillary action I drew a blank, though I feel sure I knew about it once. Transpiration, though, was clear enough: it was like perspiration, the evaporation of water from the leaves, rather than the skin (or is perspiration the secretion of water through the pores rather than the evaporation? Later). So transpiration is only about the movement of water from the surface of a leaf to the atmosphere by means of solar energy; it surely has nothing to do with movement through the stem or trunk, though the loss of water from the leaves is presumably a signal to the plant to draw up more water from the earth, but how can we talk of signals when a plant has no brain or command centre to receive them? And how can water be ‘drawn up’ when it has no muscle power or other obvious energy source?

As to capillary action, Wohlleben explains:

Capillary action is what makes the surface of your coffee stand a few fractions of an inch higher than the edge of your cup. Without this force, the surface of the liquid would be completely flat. The narrower the vessel, the higher the liquid can rise against gravity. And the vessels that transport water in deciduous trees are very narrow indeed: they measure barely 0.02 inches across. Conifers restrict the diameter of their vessels even more, to 0.0008 inches. Narrow vessels, however, are not enough to explain how water reaches the crown of trees that are more than 300 feet tall. In even the narrowest of vessels, there is only enough force to account for a rise of 3 feet at most.

Needless to say, plenty of research has been done on the subject of water transport in plants, but I have to agree with Wohlleben that there’s a lot that’s missing. The key to the process is a material called xylem, a structure made from hollow, dead, reinforced cells. Here’s how a BBC science site tries to explain it:

Transpiration explains how water moves up the plant against gravity in tubes made of dead xylem cells without the use of a pump.

Water on the surface of spongy and palisade cells (inside the leaf) evaporates and then diffuses out of the leaf. This is called transpiration. More water is drawn out of the xylem cells inside the leaf to replace what’s lost.

As the xylem cells make a continuous tube from the leaf, down the stem to the roots, this acts like a drinking straw, producing a flow of water and dissolved minerals from roots to leaves.

Water doesn’t flow upwards, however. It has to be pumped up, or sucked, as we do when we apply our lips and energy to a straw. The BBC also describes the whole process as transpiration, which just seems wrong to me. Obviously much transpires here, but it isn’t just transpiration. What?

What obviously needs explaining is where the energy comes from to draw the water up against gravity, and how the plant ‘knows’ that water needs replenishing.

A more comprehensive, and richly referenced, attempt at an explanation is provided by Nature, the well-known science magazine, on one of its educational websites. There we’re told that ‘plants retain less than 5% of the water absorbed by roots for cell expansion and plant growth’. This is fascinating, as is the reason for the lack of retention – photosynthesis. Water is lost to the atmosphere from the leaves’ stomata, which are like our pores. These stomata are used to absorb CO2 for the photosynthesis of sugars, but their openness to CO2 increases the transpiration rate, so there’s a tricky balance between the two – water loss versus CO2 and sugar gain.

The xylem mentioned above doesn’t reach down all the way to the base of the root system. First the water must pass through several cell layers that act as a filtration system. But how does it do this? What is the force being applied and where does it come from? The Nature article gives this complex explanation:

The relative ease with which water moves through a part of the plant is expressed quantitatively using the following equation:

Flow = Δψ / R,

which is analogous to electron flow in an electrical circuit described by Ohm’s law equation:

i = V / R,

where R is the resistance, i is the current or flow of electrons, and V is the voltage. In the plant system, V is equivalent to the water potential difference driving flow (Δψ) and i is equivalent to the flow of water through/across a plant segment. Using these plant equivalents, the Ohm’s law analogy can be used to quantify the hydraulic conductance (i.e., the inverse of hydraulic R) of individual segments (i.e., roots, stems, leaves) or the whole plant (from soil to atmosphere).

Got that? I may be wrong, but isn’t this just an analogy? Don’t analogies tend to break down with a little bit of analytic pressure? The idea of hydraulic conductance is clearly drawn from electrical conductance, but electrical conductance relies on a power source, doesn’t it? What is the plant’s power source? Yes, I can see that certain parts of the plant have a greater resistance to the water’s mostly upward movement than others, and that this resistance is measurable by examining the time it takes for water to pass through the different parts with their particular structure and chemistry, but it says nothing about the energy source. In Ohm’s law, V, voltage is the amount of power, which comes from a source of that power, such as a battery. In the above analogy, Δψ is described as the water potential difference that drives flow. I’m possibly being dumb, but how does that happen? What’s meant by ‘water potential difference’?

The Nature article, I must say, is very good at telling us about the materials and obstacles negotiated by water molecules on their journey. First they pass through the root’s epidermis, then the cortex and the endodermis and then on to the xylem. They travel by an apoplastic pathway (more of that next time), or else a cell-to-cell pathway (C-C), and the role of ‘water-specific protein channels embedded in cell membranes (i.e., aquaporins)’ is mentioned, but this role is apparently still much of a mystery. Anyway, the xylem continues into the petiole, to which the leaves are attached, and then into the mid-rib, the main central vein of the leaf. From there the water passes into the smaller branching veins of a dicot leaf, which also contain tracheids – elongated xylem cells for the transport of water and mineral salts. It’s from this network of veins that transpiration takes place.

So I’m learning a lot, but the ‘water potential gradient’ and how it pulls or pushes water upwards, that’s still very much a mystery to me. But there’s more to come.

References

https://www.nature.com/scitable/knowledge/library/water-uptake-and-transport-in-vascular-plants-103016037

http://www.bbc.co.uk/schools/gcsebitesize/science/add_gateway_pre_2011/greenworld/planttransportrev1.shtml

Peter Wohlleben, The hidden life of trees, Collins 2017

 

Ok, the usual update on Trump’s downfall. Some are saying that the Mueller enquiry is winding up (and I’m not talking about GOP hardheads), but I’m hoping not, because I reckon the financial stuff alone will take years to wade through properly. In the meantime though, I’m hoping that more really dramatic developments occur to light a fire under Trump’s capacious backside, sooner rather than later. The latest news is that the Mueller team are looking at the cover-up re Trump Jr’s meeting with Russian agents. So maybe the cover-ups and the endless obstructing will lead to some justice action soon, while the ‘follow the money’ aspect will continue for some time, and hopefully do the really lasting and permanent damage to the Trump horrorshow.

 

Written by stewart henderson

February 1, 2018 at 11:13 pm