Lesson video

This lesson is called "Transport systems in plants: Xylem and transpiration," and is from the unit transport and exchange surfaces in plants.

Hi there, my name's Mrs. McCready, and I'm here to guide you through today's lesson.

So thank you very much for joining me today.

In our lesson today, we're going to explain how water and mineral ions are transported through the xylem by a process called transpiration.

Now, we're gonna come across a number of keywords in our lesson today, and they're shown up here on the screen for you now.

You may wish to pause the video to make a note of them, but I will introduce them to you as we come across them.

Now, in our lesson today, we're going to first of all look how water flows through a plant, before we look at the structure and adaptations of xylem, and then consider the process of transpiration specifically.

So, are you ready to go? I certainly am.

Let's get started.

Now, water and mineral ions including nitrates, are essential for plants to grow and survive.

Water is an essential requisite of cells in order to fill them up and make them turgid, and that provides strength to the plant and helps it fulfil its shape.

Water is also a required reactant in photosynthesis, and mineral ions are required for proteins, for chlorophyll, and for DNA besides many other things.

So, there are many reasons why water and mineral ions are required by plants.

Now, water and mineral ions enter a plant through its roots using the root hair cells to help maximise the amount of absorbance that happens.

Then water moves its way up through the plant via vessels called xylem, which are present in the stem and the leaves.

And when it gets to the leaves, it can be continually lost via little pores in the leaves called stomata.

So, water is entering the plant at its roots, moving through the plant via the xylem, and being lost from the leaves via stomata.

Now, in this way, water and mineral ions are moving up through the plant, and this is driven by a process starting in the leaves.

So although water is entering at the roots, it's actually being driven by water being lost into the atmosphere via the leaves.

And when water is lost into the atmosphere from the leaves, that pulls water with its dissolved minerals up through the xylem in the stem and into the roots by osmosis from the soil.

So although water is entering at the roots, it's really being pulled up the plant by the process of water loss from the leaves.

Now, the vessels through which mineral ions dissolved in water move are called xylem, and we can see them very, very easily if we look at celery.

So if we break the stem and kind of pull it away from each other, you can see the tough fibres that stick out and they are the xylem.

You can see them in the image there.

Now, xylem, they're the stringy bits essentially of celery.

These are the parts of the plant which are transporting water with their dissolved mineral ions.

So if we took some celery and put it into a beaker with coloured water, what do you think the celery stems might look like after an hour or so? What do you reckon? Well, if you've said that the celery stems would become coloured, then you'd be absolutely right.

Because the coloured water would be drawn in from the bottom of the celery, and up through the xylem, and it'd be really easy to see all those xylem vessels within the stem of the celery.

So let's quickly check our understanding.

Water evaporates from a plant through the.

A, xylem, B, root hair cells, or C, stomata? I'll give you five seconds to decide.

Okay, so you should have said that water evaporates from the stomata.

Well done if you remembered that correctly.

So what I'd like you to do is to firstly describe the roles of the following parts of the plant in moving water and mineral ions around the plant.

The roles of the root hair cells, the xylem, and the stomata.

And then I would like you to suggest what might happen if the rate of water lost from the leaves is greater than the rate of water being absorbed by the roots? What do you think might happen? So pause the video and come back to me when you're ready.

Okay, let's see what you've written.

So firstly, I asked you to describe the roles of different parts of the plant in moving water and mineral ions around.

So for the root hair cells, you should have said that these absorb water into the plant by osmosis.

For xylem, you should have said that water moves up through the plant, through the xylem vessels.

And for the stomata, you should have said that water is lost continuously through open stomata in the leaves, and what this does is pull water up through the plant.

Then I asked you to suggest what might happen if the rate of water lost from the leaves is greater than the rate of water being absorbed by the roots.

So you might have written that if the water is lost from the leaves faster than it can be absorbed by the roots, then water may move by osmosis out of the cells of the plant instead.

Now, what this will do is cause the cells to become flaccid and the plants will wilt, because the plants cells are losing water.

So, check over your work.

Did you get the right idea of that? Well done if you did.

Okay, let's move on to have a look at the structure and adaptations of xylem.

So xylem, as we have seen, is found in stems, but we can also find xylem in the roots and the leaves of the plant as well.

And you can see where xylem is found within the root, within the stem, and within the leaves, of the microscope images on the screen there.

So xylem is found throughout the plant.

Now, xylem is found throughout the plant because it forms specialised tubes which provide a constant flow of water connected from the roots through the stem to the leaves.

This is why we find xylem throughout the plant, because if it were missing in any part of the plant, then there wouldn't be this constant connection and flow of water through the plant.

So water can be transported really quickly through the plant because where it is entering and where it is leaving are joined up with this constant pipe work, a bit like a garden hose.

Now, xylem is made of empty dead cells, which are joined end-to-end.

Now they have a number of adaptations which allow them to transport water effectively.

And the very fact that they are dead and joined end-to-end is one of them.

But in addition to that, the walls of the cells aligned with a polymer called lignin, and this is what makes them really tough, which helps to keep them round and open.

So when you are peeling the xylem out of celery, for instance, the fact that it is so tough and fibrous is because of the lignin in the walls of the cells.

Now those walls, because of the lignin, are impermeable to water.

Water cannot pass out of the xylem via the walls, and that means that water can actually get to where it's supposed to be getting to rather than being lost on the way.

Now, there are no end plates between each empty dead cell.

So where they are stacked end-to-end, joined one on top of the other, there's nothing separating those cells from one another.

And that means there is essentially an unobstructed tube formed through the plant from the roots all the way up through the stem to the leaves.

And because the cells are dead, there is no cytoplasm, so there's nothing getting in the way and narrowing the tube, it's just that outer strong lignified shell.

So put all of that together and you've got a very effective hose pipe running through the plant from the root via the stem to the leaves.

Now, xylem carries water to all of the parts of the plant because of those features, specifically because the wall is impermeable to water and heavily lignified, because there are no end plates separating the cells that stack on top of each other from one another.

So there's nothing to interrupt the flow between those dead cells, and therefore water is not blocked and it is not lost on the way.

And this is why xylem is so effective at moving water through the plant.

So let's quickly check our understanding.

Which two features of xylem vessels allow water to move uninterrupted from the roots to the leaves? Is it A, they have an impermeable lignified cell wall, B, they are only present in the stem and leaves, C, they have no end walls, or D, they transport water and dissolved mineral ions? I'll give you five seconds to think about it.

Okay, so which two features have you chosen? Well, you should have selected A and C, but B is incorrect because xylem is also found in the roots as well as the stem and leaves.

And whilst D is correct, it doesn't explain why water can move uninterrupted.

So well done if you got all of those correct.

So what I'd like you to do now, is firstly on the diagram of the xylem, I'd like you to label the key features of xylem that enable it to transport water up through the plant.

And then I'd like you to think about the fact that xylem is not found in the growing tips of shoots where new leaves and buds and stems are growing.

And I'd like you to use that information to suggest why animals often prefer to eat shoots rather than older leaves and parts of the stem.

So have a think about that, pause the video, and come back to me when you're ready.

Okay, let's see what you've done then.

So on this diagram, I firstly asked you to label the key features of xylem.

So you should have added that there are no end wall plates and no cytoplasm, and these cause no obstructions in the vessel.

You should also have labelled the lignified impermeable cell wall, and identify that this causes the vessels to stay open and does not let water out.

Then I asked you to suggest why animals often prefer to eat shoots rather than older leaves and parts of the stem.

And you might have written that this is because xylem is very fibrous because of the lignin in the walls, and also because it has no cytoplasm or other cell parts.

So the xylem tissue is very hard, it's very hard to chew, and it's very difficult to digest.

Whereas new shoots which lack xylem, are much less fibrous because of the lack of xylem, and that means that they are easier to eat and they're easier to digest.

It also means that because they've got other things there as well, like cytoplasm in the cells, they are more nutritionally beneficial, they are usually tastier, and that is why animals often prefer to eat the younger parts of a plant.

Did you get those ideas written down? Well done if you even got a few of those.

Good job.

Right.

Let's move on to the last part of our lesson now, which is looking at this process of transpiration.

Now, we've seen how water can leave the plant from the leaves.

Now, leaves have little holes, pores in their surface called stomata, and you can see them identified there in the electron micrograph.

Now, these stomata are opened and closed using special cells called guard cells.

So the guard cells are the cells which make the hole.

The stomata is the hole itself, and the guard cells are those sort of elongated sausage-like cells, which can open and close the pore by swelling and thinning themselves.

So the guard cells are really important in forming the pore and also controlling the size of the pore, the stomata.

Now, water is lost from leaves, and this process of water loss is called transpiration.

Now if we look at a cross-section through a leaf, we can see that most water is lost through the open stomata.

And the stomata is the hole at the bottom of the leaf between the two guard cells.

Now, what happens is that water evaporates out of the cells in the central part of the leaf to form water vapour, which collects within the air spaces in that middle section of the leaf behind the stomata.

And as it's evaporated out into water vapour within those air spaces, it can then diffuse out of the leaf via the stomata.

So water is evaporating out of the leaf into the air spaces and then diffusing out of the air spaces via the stomata into the atmosphere.

So that's one way, and it's the main way that water is lost from a leaf via these opens stomata.

Some water also evaporates from the surface of leaves.

However, most leaves have a waxy cuticle over the top of the surface which greatly reduces evaporation and therefore reduces water loss.

And this is why water loss from a leaf is mainly through the open holes, stomata, rather than from the general surface of the leaf.

So water is leaving the leaf via this process of transpiration.

So the loss of water from the leaf is called transpiration, and losing water from the leaf draws water up through the plant.

And this constant stream of water flowing up through the plant is called the transpiration stream.

Now it's really important that you understand the difference between the process of transpiration and the transpiration stream.

So transpiration is the loss of water from the leaf, whereas the transpiration stream is the flow of water through the xylem and the transpiration stream, being this pull of water, this flow of water up through the plant, draws water in from the root essentially to replace the lost water that is leaving the plant via the stomata at the leaves in this process of transpiration.

So transpiration is really important in creating this pull up through the plant.

And transpiration is really critical to ensuring that water gets right to the top of even the tallest plants.

And even a relatively short plant needs transpiration to help it get water all the way up to the top.

And if you have a look at these tall palm trees, you can see that the top of the plant is a very long way away, and these aren't at all the tallest trees in the world.

So transpiration is responsible for moving water vertically at what can be many tens of metres high.

It's really quite impressive.

So, whose description of transpiration is correct? Now Sam says, "Transpiration is when water is lost from the leaves." Alex says, "Transpiration is the movement of water through the plant caused by osmosis in the roots." And Jacob says, "Transpiration is when the plant breathes out water and oxygen." But who is correct? I'll give you five seconds to think about it.

Okay, so you should have said that Sam is correct and Alex is wrong because osmosis doesn't cause transpiration.

And Jacob is wrong because transpiration is when water leaves or diffuses from the leaf.

It's nothing to do with oxygen.

So what I'd like you to do in our final task today is to firstly expand Laura's description of transpiration.

So she says, "Transpiration is about water loss." But can you add more detail to that partial description for a more complete definition of transpiration? Then I'd like you to consider this situation.

There is a plant disease called Verticillium wilt, and it's caused by a fungal infection.

So the fungus infects the plant through the roots and then it grows up through the xylem.

And what I'd like you to do is to suggest what effect this disease might have on the plant.

And there's a bit of a clue in the title of the disease.

So pause the video and come back to me when you're ready.

Okay, let's see how you got on.

So firstly, I asked you to expand Laura's description of transpiration.

So you should have said that transpiration is the loss of water from the leaves by evaporation and diffusion through opens stomata.

What this does is create a constant stream of water flowing up through the plant, and this is called the transpiration stream.

So you may have added this second sentence in to add more detail, but do make sure you've distinguished between transpiration, the loss of water from leaves, and the transpiration stream, the constant flow of water through the xylem.

You may also have added that water is replaced by osmosis from the soil into the root hair cells, for an extremely thorough description of transpiration and its associated processes.

So well done if you've added all of that detail.

You may wish to add in extra bits if you haven't added it all.

And do make sure you've got the definitions of transpiration and transpiration stream completely separate in your mind.

Then I asked you to consider this disease called Verticillium wilt, and what effect this might have on the plant.

So you might have suggested that Verticillium wilt blocks the entry into the roots and also blocks the passage of water up through the xylem.

And what this means is it will prevent water from being pulled up the plant by the transpiration stream, causing the plant to wilt and eventually die.

So Verticillium wilt can be really quite a troubling disease for a gardener.

Well done if you suggested those problems. Okay, we've come to the end of our lesson now.

So what we've seen in our lesson is that water and mineral ions are transported from the roots up the stem to the leaves via the xylem vessels.

And xylem is made of dead empty cells with no cytoplasm or end plates, and with tough cell walls made of lignin, which are impervious to water.

And these adaptations make the xylem vessels open and unblocked, and allow water to move without hindrance up through the plant.

Now, water is continually lost from the leaves of the plant by evaporation and diffusion through open stomata in a process called transpiration.

And the process of transpiration pulls water up through the plant from the roots via the xylem, creating the transpiration stream.

So, well done for today's lesson.

Thank you very much for joining me today, and I hope to see you again soon.

Bye.