Lesson video

This lesson is called Transport Systems in plants: phloem and translocation 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 sugars are transported around a plant through vessels called phloem in a process called translocation.

Now, we're gonna come across a number of keywords in our lesson today, and they're listed 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 how transporting sugars and nutrients around the plant occurs and then we're gonna have a look at the structure and adaptations of phloem before looking specifically at the process of translocation.

So are you ready to go? I certainly am.

Let's get started.

Now, plants need a whole host of nutrients in order to survive just like we do.

So they need glucose.

Glucose is required for cellular respiration to transfer energy for life processes.

So that's the most important nutrient.

Now glucose can be stored as starch and it can also be turned into other larger carbohydrates such as sucrose and cellulose.

Now, if a plant combines glucose with nitrogen, it can make amino acids.

And joining lots of amino acids together means that plants can make proteins.

So plants will also require a source of nitrogen.

In addition to that, plants will also need a range of other mineral ions.

For instance, magnesium, which combines with amino acids to make chlorophyll and phosphates, which are required to make DNA.

So you can see how plants require a wide range of nutrients in order to survive and grow.

Now glucose is produced in cells that photosynthesize, for instance, the palisade cells in the top layer of the leaves.

These contain many, many chloroplasts and therefore photosynthesize at a really rapid rate, and therefore they can produce a lot of glucose.

And this glucose is really important because it is used in process called cellular respiration to transfer energy for other life processes.

And all living cells require energy for life processes.

All cells require glucose for cellular respiration.

So glucose is an essential component for every single cell in every living organism.

Now, not all cells can produce their own glucose because not all cells can photosynthesize.

Looking at the root cells in the picture on the screen, you can see that there's no chlorophyll within those cells because they aren't green and therefore they cannot photosynthesize.

So root cells, which are buried underground with no access to light, cannot photosynthesize, neither can the flowering parts of a plant which are busy making eggs and pollen and creating seeds upon fertilisation.

They require a lot of energy, they require a lot of glucose, but they cannot make their own glucose because they cannot photosynthesize.

And the parts of the plant, which are very deep underneath the photosynthesis layers like the middle of the stem, be that a thick trunk or a more narrow ordinary flowering stem, those cells in the middle can't photosynthesize either because they're too far away from the light source and therefore they cannot produce their own glucose for respiration either.

However, all cells respire so all cells require glucose.

So let's quickly check our understanding.

Which cells in a plant require energy from cellular respiration? Is it A, all living cells in a plant, B, only cells in the leaves or C, only cells that contain chloroplasts? I'll give you five seconds to decide.

Okay, so which cells require energy from respiration? Well, you should have chosen all living cells because all living cells respire.

Well done.

So we can see why glucose which is made in photosynthesis must be transported around a plant to the parts of the plant which cannot make their own glucose.

Those parts of the plant which cannot photosynthesize because those parts of the plant still require glucose in order to respire, but they cannot make it themselves.

So in order to allow glucose to be transported around the plant, it is converted into a double form of glucose.

So two are stuck together called sucrose.

And sucrose, which is essentially table sugar, is then transported around the plant.

Now whilst a plant is transporting sugar, it can also transport other organic substances such as amino acids through the plant as well also to where they are needed.

And all of this transportation goes on via vessels called phloem.

So sugars and amino acids and other organic substances are all transported around the plant via the phloem.

Now we can see phloem if we have a look at peeled banana.

Those strings that come off of the banana between the fruit and skin are phloem.

And this is why the banana is so sweet and nutritious because it is covered in really heavy layers of phloem which is transporting sugars and other nutrients into the flesh of the banana for storage.

And that is why bananas are sweet and nutritious.

So let's quickly check our understanding.

Which two statements when combined together explain why roots do not produce glucose? A, they do not contain chloroplasts; B, they do not contain mitochondria; C, so they cannot carry out cellular respiration; D, so they cannot photosynthesize.

Which two are you going to combine? I'll give you five seconds to decide.

Okay, so you should have put together, A, they do not contain chloroplasts with D, so they cannot photosynthesize.

And this is why roots do not produce glucose.

So let's summarise our learning so far.

What I would like you to do is to firstly describe what plants require the following nutrients for.

What do they need glucose, amino acids, magnesium, and phosphates for? Then I would like you to explain why some parts of the plant such as roots, the inner stems and flowers cannot photosynthesize.

Before finally considering that maple syrup is extracted from the phloem of the maple tree, so can you explain why it is so sweet? So pause the video and come back to me when you are ready.

Okay, let's see what you wrote.

So firstly, what are these nutrients for? Glucose is used in cellular respiration and also can be made into other substances such as cellulose and amino acids.

Amino acids are used to make proteins.

Magnesium is used to make chlorophyll and phosphates are used to make nucleotides in DNA.

Well done for getting those.

Then I asked you to explain why some parts of the plant can't photosynthesize.

So you should have written that some parts of the plant can't photosynthesize because they do not contain chloroplasts, so they don't have any chlorophyll.

And this is because they are either located underground or deep inside the plant so do not receive sunlight.

And then finally, I asked you to explain why maple syrup is so sweet and you should have said that this is because phloem transports sugar.

Well done on getting those answers.

Okay, let's move on to look at phloem and the structure and adaptations of this vessel.

Now phloem is found within the roots, the stems, and the leaves of a plant.

And you can see on those micrographs where phloem is found in each of those three parts of the plant.

Now phloem is a very specialised, highly adapted set of tubes that run throughout the plant joining the roots, to the stem, to the leaves.

And by being joined up together, they provide a constant connection through the plant and that allows sugars, amino acids, and other dissolved nutrients to be transported around the plant very, very quickly and efficiently in a process called translocation.

So you can see phloem in the micrograph image on the slide, and these are long tubes with no interruptions.

Now phloem is made of living cells and these living cells are stacked on top of each other and joined end to end.

Now, when they are joined end to end, they have holes punctuated into those end plates and this means that they are called sieve end plates.

So I hope you can see why they're called sieves because if you think about a sieve that you might use in the kitchen to drain your pasta, for instance, there's lots of little holes in the sieve and that is true of the ends of the cells where they're stacked end to end.

There's lots of holes between each cell which joins them up to each other, and this allows the solutes to pass through.

Now, substances can travel both up and down through the phloem, through these holes in the sieve end plates.

And because phloem is made of living cells, it therefore has cytoplasm.

But that cytoplasm is really pushed out the sides of the phloem cells in order to make the tube as wide as possible to maximise the amount of dissolved substances that can be transported through it.

So phloem has these very specific adaptations to allow it to do its job effectively.

So let's quickly check ourselves there.

Which of these adaptations are present in phloem? A, a lignified cell wall; B, no cytoplasm or C, sieve end plates? I'll give you five seconds to think about it.

Okay, you should have said that the sieve end plates are the only adaptation of this list that is present in phloem.

Well done if you spotted that.

So what I'd like you to do now is to firstly describe the adaptations of phloem that allow it to transport substances around the plant.

Then I'd like you to consider the problem of aphids.

So aphids, as you can see from the photograph, there are little green insects and they are real garden pests because they damage plants by removing sugar.

And if you haven't spotted them before they've caused significant damage, the first proper signs of damage are usually shrivelled up dead leaves.

So what I'd like you to do is to suggest into which part of the leaf do they insert their long mouthpiece, which is called a stylet, where are they gonna insert that into the leaf and can you explain why you have concluded that.

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

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

So firstly, I asked you to describe the adaptations of phloem.

So you should have said that phloem is made of living cells, which contains cytoplasm, but this is pushed to the sides, making the vessel as wide as possible to increase the amount of substances that it can transport.

You should have also said that it has perforated ends called sieve end plates, and these allow substances to pass through in both directions.

Then I asked you to suggest into which part of the leaf, aphids insert their long stylet and why you have concluded this.

So you can see in the diagram there the outline of the aphid and its very long stylet and the stylet has been pushed through the leaf into the phloem.

So aphids are inserting their stylet into the phloem vessel because the phloem contains a lot of sugar, and this is a great food source for the aphid.

Now the aphids remove the sugar from the phloem and this means that the leaf will not have enough glucose for cellular respiration.

It means that there's not enough energy for the life processes that need to take place within that leaf, and it means that the leaf will swirl up and die, which is why that's one of the first telltale signs.

If you haven't spotted the aphids already, that's how you can tell.

So did you get most of that written down? Well done if you did.

Okay, let's move on to the last part of our lesson looking at this process of translocation.

So translocation is the process of transporting sugars, amino acids, and other dissolved nutrients around the plant as required.

Now substances start at what is called the source.

This is where they are made or stored.

They then move through the phloem and they are delivered to the sink.

So we've got this process of moving nutrients around the plant through the phloem from the source to the sink.

So there's some really key words that you'll need to remember when you are describing this process of translocation.

Now, sources can be quite varied actually.

They can either be places where the substance required has been made, such as in the palisade cells of the leaf where glucose is made through photosynthesis, or it can be places where the substance has been stored.

For instance, potatoes.

Now potatoes are tubers.

They are stores of sugar.

But sugar is stored as starch within potatoes, which is why potatoes are so starchy and glucose is stored as starch within potatoes until such time as it is needed by the plant later on in its lifecycle.

So potatoes collect a lot of sugar in the form of starch in the potato tubers and then use it over winter to sustain itself until it can grow new shoots and start photosynthesizing again.

So sources are either places where the substance is made or the substance is stored.

Now sinks can be literally any part of the plant which requires the substance that is being transported.

So we've already identified a few of these parts of the plant, such as developing roots or the inside of the stem or flowers when they are creating eggs and pollen or when they are creating seeds following fertilisation.

Now all of these parts of the plant cannot photosynthesize and therefore will always need a constant supply of glucose so that they have that store of energy ready for use in cellular respiration.

So how does this process of translocation work then? Well, sugars and other dissolved nutrients in the source are transported into the phloem by the process of active transport, and this occurs across the membranes of the living phloem cells.

So the things that are being transported, the nutrients that are being transported are actively transported into the phloem.

This means that water can then osmos into the phloem behind it, and that increases the pressure within the phloem and pushes the sugars and the other dissolved nutrients along the phloem tube.

And then when they get to wherever they are required, they can then be actively transported out of the phloem to the sink cells.

So the sugars and other nutrients are actively transported into the phloem, water osmosis is in behind, pressure increases and pushes all of those substances to where they need to get to, and then the nutrients are actively transported out of the phloem to the sink.

So what I'd like you to do is to put these statements in the order of the process of translocation.

So A says dissolved substances are loaded into the phloem from the source by active transport.

B is substances are unloaded from the phloem at the sink by active transport.

C is substances are moved through the phloem due to the high pressure, and D is water moves by osmosis into the phloem.

So in which order do they go? I'll give you five seconds to think about it.

Okay, let's check our work.

So you should have started at A, then put D, then C, before finishing with B.

Well done if you got that correct.

So what I'd like you to do now is to firstly draw a flowchart to show the process of translocation, and then I'd like you to complete the table to show examples of sources and sinks before finally explaining how active transport and osmosis are used in the process of translocation.

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

Okay, let's check your work.

So firstly, I asked you to draw a flowchart to show the process of translocation.

So you should have started that sugars from the source are transported into phloem by active transport across the membranes of the living phloem cells.

Then added that water moves into the phloem cells by osmosis, then added that the pressure increases and pushes the sugars along the phloem tube before finishing that sugars are transported out of the phloem by active transport at a sink.

Well done if you got all of that detail in.

And even if you've only got some of that detail, make sure that you've got your process in the correct order.

Then I asked you to complete the table to show examples of sources and sinks.

So for sources you should have said any photosynthesizing cells such as the palisade cells in the leaves and any stores of starch such as potatoes.

Then for sinks, you could have included developing roots and stems, flowers, and the deep inner parts of plants like the inner parts of stems which cannot photosynthesize themselves.

So well done if you've got that, and especially if you've added a bit more detail to that as well.

That's an excellent job.

Then I asked you to explain the rules of active transport and osmosis in translocation, and you might have written that active transport is used to move sugars and other nutrients from the source cell into the phloem, and then water enters the phloem by osmosis.

This is because there is suddenly a very high solute concentration in the phloem, and so water is osmos down the concentration gradient of water from where there's a lot of water in the xylem into where there's a much, much less water in the phloem.

And then active transport is used to unload the substances from the phloem at the sink cell.

So active transport loads the phloem and unloads the phloem at the source and at the sink, then water moves by osmosis into the phloem after the solutes have entered.

And again, well done if you've got any of that detail there.

Okay, we've come to the end of our lesson today.

So what have we learned? Well, we've seen that plants transport sugars, amino acids, and other nutrients around the plant via phloem vessels in a process called translocation.

And that phloem, in order to be able to fulfil this function, are highly adapted living cells, which have sieve end plates and allow substances to move around the plant both up and down.

Now, substances start at the source, for instance, in photosynthesizing cells and then move into the phloem by a process of active transport followed quickly by water behind, which is moving by osmosis, and then moving through the phloem to be unloaded at the sink by active transport again.

And sink cells are any cells which require the nutrients such as roots and flowers and the insides of stems. So thank you for joining me today.

I hope you've enjoyed our lesson, and that maybe next time you eat a banana, you'll notice those phloem vessels within it.

So thank you for joining me and hope to see you again soon.

Bye!.