Lesson video

This lesson is called, "The Importance of Exchange Surfaces and Transport Systems in Plants", and it's 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 exchange surfaces and transport systems work together in multicellular plants to help to transport substances around the plant, enabling them to stay alive.

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 and 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 at the supply and demand problem that plants have before we have a look at two solutions, firstly, transport systems, and secondly, exchange surfaces.

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

Let's get started.

Now, all the plants that we see around us are multicellular.

That means that they are made of many, many cells.

And you can see if we zoom into any of those plants, we'll see just a fraction of the cells that they are made up of.

Now, every cell in every plant's body requires nutrients.

Glucose, amino acids, water and mineral ions are some examples.

Also, every cell in that plant's body makes waste.

Carbon dioxide is probably most notable, but nitrogenous compounds are also very commonly produced.

And what this means is that every cell has stuff it needs and every cell has stuff it needs to get rid of and so every cell has a supply and demand problem.

For example, if we look at a leaf, the palisade cells in the leaf are where the vast majority of photosynthesis occurs, and therefore these cells within the leaf, and you can see them indicated there in the micrograph, require water and carbon dioxide in order to produce glucose through photosynthesis.

However, they cannot absorb carbon dioxide directly from the atmosphere because there's a waxy cuticle stopping carbon dioxide from getting straight to them from the air.

And there's also a whole load of cells beneath them as well, also preventing carbon dioxide from reaching them directly.

Also, they cannot absorb water directly from the ground because they are far, far away from the ground.

If you just think about where leaves are on a plant, they're not lying on the ground or buried within it.

They're up a loft on the plant within the atmosphere.

So how are they gonna get hold of water? How are they gonna get hold of carbon dioxide? Let's take another example.

Let's consider a flower on a plant.

So flowers make pollen, they make ova, petals, other reproductive parts, and ultimately they make seeds.

And the cells within the flower are responsible for carrying out all of these functions.

Now, in order to do that, they need many different nutrients to build very complicated new structures, to make seeds which have a food storage within them.

All these things require nutrients, glucose, amino acids, water, mineral ions.

But how are they gonna get them? Because they cannot photosynthesize.

These cells within flowers don't have chloroplasts because they're just getting in the way of everything else that they need.

So how are they gonna make their own glucose? How are they gonna make amino acids? Because they can't make them at the moment, they don't make their own glucose.

They also have the same problem that leaves have.

They're far above the ground, so they don't have ready access to the water stored in the soil.

So how are they gonna get water to their cells in order to keep them alive as well? Let's take another example.

Roots.

So roots have direct access to the water in the ground.

Water is not a problem for them to access at all.

However, they are below the surface of the ground, therefore they cannot photosynthesize.

It's dark.

And so they don't make chloroplasts and they cannot photosynthesize, and therefore they cannot make their own glucose even though they need it.

So they need to make their own food.

They just can't because they can't photosynthesize, they don't have any chloroplasts and they've also got an abundance of water, they don't have any carbon dioxide.

So roots have a different problem, but a problem nevertheless.

And you can see therefore across the whole plant, every cell within the plant has a supply and a demand problem.

They either need nutrients that they can't get or they get nutrients that they don't necessarily need them or can't use them in the way that they need to.

So plants need to develop ways of circumventing these issues because every single cell within their body has these issues in one way, shape, or form.

So let's quickly check our understanding.

Root cells cannot make their own glucose, amino acids or water? Five seconds to think about it.

Okay, you should have said that root cells cannot make their own glucose.

Well done! What I'd like you to do is to summarise this issue of supply and demand by choosing two of the following types of plant cells and outlining their supply and demand issues.

So choose from either the palisade cells and the leaf, the cells and the flowers or root hair cells in the root.

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

Okay, let's see what you might have written.

So you might have said that palisade cells in the leaf require water for photosynthesis but cannot absorb it directly from the soil.

They also need carbon dioxide, but they can't absorb that directly from the air because of the waxy cuticle.

Or for cells in the flower, you might have said that they require many nutrients, but they can't photosynthesize and they are not near the ground to directly absorb water.

Or for the roots, you might have said the root hair cells can absorb water from the ground, but they cannot use it to make glucose because they can't photosynthesize.

And therefore in all of these scenarios, there is a problem.

Plants can either get nutrients but not use them or need nutrients but can't get them.

So how are plants going to resolve these issues? Well, let's have a look at some transport system solutions.

So let's consider roots again.

Roots have a glucose supply problem.

They cannot photosynthesize, so they cannot make their own glucose.

Now, if glucose were to simply diffuse from where it is made within in the plant, in the leaves through to the roots, it would have to diffuse through hundreds of different cells.

And particularly in tall plants, the glucose would probably never get to the roots because of all of the other cells that it would have to move through in order to get there.

And so the glucose would never reach the roots and the roots would not have glucose in order to stay alive.

So what about leaves? Well, the palisade cells within the leaves require water for photosynthesis, but they are far, far away from the ground.

And if it were left to diffusion to move water from the roots, up through the plant to the leaves, well water would have to osmos through the plant and it would have to move through an awful lot of cells in order to get to the leaves.

And in most plants, it would simply take too long for the water to osmos its way through the plant and it would therefore never get there.

So plants overcome this issue of transporting substances around their structure by having transport systems made of vascular tissues.

So in humans, we have arteries, veins, and capillaries, and these provide the essential role of moving nutrients and waste products around the body.

Well, plants have an equivalent to that.

They have xylem and phloem vessels, and these are part of vascular tissue, which are essentially veins in the plant which move substances around it.

So xylem works because water is lost from the leaf through a process of transpiration, and this causes water to be pulled up through the plant and in at the roots and moved through the plant via the xylem vessels.

So xylem is moving water through the plant through these great big pipes.

Phloem, on the other hand is moving sugar and amino acids and other nutrients around the plant.

Now these nutrients are loaded by active transport into the phloem vessel.

Then water moves in behind by the process of osmosis and the pressure builds up.

The solute moves through the plant by the process of translocation and out at the sink where those nutrients are needed so where the sugar is needed, where the amino acids are needed, and where the other dissolved nutrients are required also.

So this is the role of phloem and if we put them together with transpiration in xylem and translocation via phloem, we can see that nutrients are transported very quickly around the plant.

And this means that because the distance has been reduced over which the substances need to diffuse because they're being piped to where they need to get to pretty much, they get there much faster and they actually get there as well.

So which of these are part of the plant's vascular system? A, the xylem; B, the palisade; or C, the phloem? I'll give you five seconds to think about it.

Okay, so you should have chosen that the parts of the plant's vascular system include the xylem and the phloem.

Well done if you chose both of those.

And which process uses phloem cells? Is it transpiration, translocation, or transportation? Again, five seconds.

So phloem uses the process of translocation.

Well done if you got that correct.

So what I'd like you to do now is to describe how the vascular system of a plant enables nutrients to be transported quickly to where they are needed.

So make sure you talk about both the xylem and the phloem in your answer.

And then I would like you to explain how diffusion, osmosis, and active transport are used to transport substances through the plant, via the vascular system.

So consider both translocation and transpiration and those processes and how diffusion, osmosis, and active transport fit into those.

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

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

So firstly, I asked you to describe how the vascular system of a plant enables nutrients to be transported quickly to where they're needed.

So you should have talked about the xylem, saying that this transports water and mineral ions through the plant and water is lost by the process of transpiration at the leaf and this pulls water and mineral ions up through the plant from the root via the xylem.

Then you should also have included about phloem and said how phloem transports sugars, amino acids and other dissolved nutrients throughout the plant.

And these nutrients are actively transported into the phloem and water is then moved by osmosis afterwards.

And these substances then are translocated via the phloem to wherever they are needed within the plant.

So just review your answer, make sure you've got all of those keywords and well done.

Then I asked you to explain how the processes of diffusion, osmosis, and active transport are used to transport substances around the plant's vascular system.

So you should have included that water diffuses out of the leaves through the process of transpiration so that's diffusion.

That sugars and amino acids are actively transported into and out of the phloem.

So that's active transport.

That water also moves by osmosis into and out of the phloem.

So that's osmosis.

And you might also have included that water moves by osmosis into the root hair cells from the soil and that mineral ions are actively transported into the root hair cells also from the soil.

So you might have added those two extra pieces of information.

Well done if you did! Okay, let's have a look at another solution and this time, looking at exchange surfaces.

Now humans have highly specialised exchange surfaces such as the alveoli in the lungs and the villi in the digestive system.

And over which these surfaces are able to exchange great quantities very rapidly of specific nutrients such as carbon dioxide and oxygen within the lungs, and a whole host of nutrients from our food into our bloodstream across the villi.

Now, plants also have specialised exchange surfaces and one of the ones we need to consider is the root hair cell.

So root hair cell is an example of an exchange surface and you can see that root hair cells are made up of a well of the bulk of the cell plus this protrusion, which we call the hair.

So this hair protrudes from the surface, sticks out from the surface and greatly increases the surface area of the whole cell.

And this is really important because what that does is greatly increase the rate of absorption of water by osmosis and mineral ions by active transport.

So this sticky out bit, this hair is protrusion greatly increases the surface area, which greatly increases the ability of the root to absorb really essential nutrients including water and mineral ions.

But by how much does it increase the surface area? Because it's obviously quite a large protrusion, but can we quantify that in some way? Now if we think about the surface area of a cube, we know that we can calculate that using the surface area of one surface.

So the width times the height times by the number of faces of a cube, which is six.

So let's say we've got a 15 micrometre wide cube, pretending to be our root hair cell.

Then the surface area of this without the hair would be 15 times 15 times six faces.

And that would be a total surface area of 1,350 micrometres squared.

So the surface area of a standard root hair cell without the protrusion is approximately 1,350 micrometres squared.

Now if we add the hair to that calculation, we'll see what happens to the surface area.

So the surface area of the green part of the cell, the main part of the cell is 1,350 micrometres squared.

Now that protrusion is perhaps about 1500 micrometres long and about five micrometres high.

Now we can ignore the ends of the hair in terms of surface area calculations because we've already incorporated them in the surface area calculation of the main green cube itself.

So we're just gonna ignore the end measurements.

And instead what we're gonna do is calculate the area of the four long sides of the hair, which is yellow in the diagram.

So we can see that the long hair is 1500 micrometres long and each face is five micrometres high.

So we've got five micrometres times 1500 micrometres and there are four of these faces so we're gonna times that by four and together that makes 30,000 micrometres squared.

So, we can see how the root hair cell, the hair part of that, the surface area has increased from 1,350 micrometres squared to a total of 31,350 micrometres squared simply because of the hair.

That means it's an increase of 23 times larger.

That hair, that sticky out bit is increasing the surface area by more than 23 times.

But actually if we stop and think about it slightly more, the impact of the hair is even greater still because when you look at a root, all those root hair cells are stacked on top of each other and next to each other and they're surrounded.

You don't just have a single root hair cells sticking out all sides into the water field soil.

You have one cell next to many, many other cells.

And so it's actually only the surface where the hair is sticking out, that surface which is available to the soil.

So that means that the exchange surface goes from being one surface of the cube, about 225 micrometres squared to 30,225 micrometres squared when you add the root hair cell.

So actually the surface area of the root hair cell increases by more than 134 times because of the presence of the hair part of the cell.

So you can really see how absolutely vital the hair part is of the root hair cell in terms of significantly increasing the surface area of the hair.

Now the hair parts of the root hair cells massively increase the surface area of the exchange surface.

And if you look at roots of a plant, they often look a bit fuzzy because the root hair cells are really, really small and they kind of.

All we can discern from them is this kind of fuzziness.

You can see that in the picture there on the screen.

But by greatly increasing the surface area and therefore the exchange surface area of the roots, it means that plants are able to maximise the amount of water and mineral irons they can absorb and do so at a high a rate as possible.

And this means that the whole plant can be supplied with water from the roots via the vascular system.

So an exchange surface is good and a vascular system is important, but the two together is absolutely fundamental to the way a plant can survive.

So the surface area of the cell does not affect absorption of nutrients.

Is that true or false? So you should have said that's false, but can you explain why? So you should have said that this is because a greater surface area means more rapid absorption of nutrients over the exchange surface.

Well done if you've got those right.

So what I'd like you to do now is to summarise those ideas that I've just been discussing by firstly describing how the root hair cell is an exchange surface and then explaining how the hair of the root hair cell affects the cell surface area and why this is important for plants.

Now you don't have to include calculations in this, but you may find some measurements useful to help you illustrate your point.

So include them if you want to.

Then finally, I'd like you to explain how the survival of a plant depends on the exchange surface of the root hair cells and the vascular system together.

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

Okay, let's check our work.

So firstly, I asked you to describe how the root hair cell is an exchange surface.

So you should have said that the cell membrane of the root hair cell is an exchange surface because substances are exchanged across it and that is what makes it an exchange surface.

Now you could have expanded that by saying that in roots, water and mineral ions are water being absorbed into the plants.

That's what's being exchanged at the roots.

Then I asked you to explain how the hair of the root hair cell affects the cell surface area and why this is important for plants.

So you should have said that the hair of the root hair cell extends into the soil and water and mineral ions are absorbed into the cell across the membrane.

Now without the hair, the surface area of the cell would be relatively small and only one part of the surface would be externally facing and able to absorb water and mineral ions.

So the ability for absorption would really be quite low, but the presence of the hair greatly increases the surface area and therefore greatly increases the rate of absorption.

And this is important for plants because water and mineral ions are only being absorbed at the roots but are required throughout the entire plant.

And so it's really important that water and mineral ions are effectively absorbed into the plant so that they can be distributed where acquired.

Then I asked you to explain how the exchange surfaces and the vascular work together.

So you might have written that the root hair cells provide a very large exchange surface to maximise absorption of water and mineral ions into the roots.

And from here, water and mineral ions are then transported from the roots to all parts of the plant by the xylem in the vascular system.

And what this means is that the distance over which water and nutrients have to diffuse to reach cells is significantly reduced because water is piped to the doorway, essentially the entrance of the cell, and therefore there's only a very short distance for that water and those mineral ions to osmos or diffuse into the cell from that point.

And therefore you can see how exchange surfaces and the vascular system are so important to work together to enable plants to transport substances that they need from where they are made or absorbed to where they are required.

Okay, so that's the end of our lesson.

So in our lesson today, we have seen that cells in all parts of a multicellular plant need water and nutrients such as mineral ions and glucose in order to stay alive.

And the xylem and phloem are part of the vascular system of the plant and are essential for transporting substances around the plant.

And what this does is minimise the distance that those substances have to diffuse over in order to reach the cells that require them.

So this means that supply can be maintained quickly and at a sufficient quantity in order to keep the plant cells alive.

Now, this is greatly enhanced by exchange surfaces such as root hair cells and root hair cells have a highly adapted membrane, the root hair, the protrusion, which maximises the surface area of the cell over which diffusion, osmosis and active transport can take place.

Therefore, as much water and mineral ions can be absorbed into the plant and then transferred into the vascular system to be transported around the plant to wherever they are required.

So I hope you found that a very interesting lesson.

It's certainly lots of things to think about anyway.

And thank you very much for working hard today and for joining me, and I hope to see you again soon.

Bye!.