Phloem & translocation | Life processes | Biology | Khan Academy

Summary

This educational video script delves into the fascinating process of translocation in plants, illustrating how they transport nutrients from one part to another. It explains the vital role of phloem, a vascular tissue, in moving food and the active process of loading sugar molecules into it. The script clarifies how osmosis causes water to flow into the phloem, creating pressure that drives the bulk flow of sap towards areas of lower sugar concentration. It also highlights the importance of phloem cells being alive, thanks to companion cells, to facilitate this energy-dependent process. The summary provides a clear and concise explanation of how plants ingeniously manage their internal food distribution.

Takeaways

• 🌱 Plants and trees can create their own food from sunlight and efficiently distribute it through their bodies.
• 🚀 Excess food is stored in organs like carrots or potatoes for future use during times of scarcity, such as winter.
• 🔄 The process of moving food around within a plant is called translocation.
• 🧬 Plants don't have a brain but can still sense which parts need food and transport it accordingly.
• 🌿 Vascular tissues, specifically xylem and phloem, are crucial for transporting substances within the plant.
• 💧 Xylem transports water and minerals, while phloem is responsible for moving food.
• 📦 Phloem contains a thick sugary sap made of dissolved sugar molecules in water, which serves as the plant's food.
• 🔄 Translocation occurs within the phloem and involves the active loading of sugars into the phloem tubes, requiring energy.
• 🌀 Osmosis causes water to flow from xylem into phloem, creating pressure that drives the bulk flow of sap.
• 📉 The direction of sap movement is determined by the pressure differences caused by varying sugar concentrations.
• 💉 Companion cells support the living cells of the phloem, enabling the active processes required for translocation.

Q & A

What is the process by which plants move food around their bodies called?

The process by which plants move food around their bodies is called translocation.

How do plants store excess food during times of plenty?

Plants store excess food in their storage organs, such as roots, tubers, or stems, for use during times when photosynthesis is not possible, like winter.

What are the two types of vascular tissues in plants and what do they transport?

The two types of vascular tissues in plants are xylem, which transports water and minerals from the soil to different parts of the plant, and phloem, which primarily transports food.

What is the role of the xylem in the context of translocation in phloem?

While xylem primarily transports water and minerals, it plays a role in translocation by influencing the direction and pressure of the movement of food in the phloem through osmosis.

How does the phloem know in which direction to transport food?

The phloem transports food from areas of high sugar concentration to areas of low sugar concentration, which is determined by the plant's needs and the pressure differences created by water movement.

What is the term used for the process of moving sugar from a lower to a higher concentration in the phloem?

The process of moving sugar from a lower to a higher concentration in the phloem is called loading, and it is an active process that requires energy.

What happens when water from the xylem moves into the phloem due to osmosis?

When water moves from the xylem into the phloem due to osmosis, it increases the pressure in the phloem, causing the sugary sap to move towards areas of lower pressure and sugar concentration.

What is the term used to describe the movement of the entire sugar solution in the phloem?

The term used to describe the movement of the entire sugar solution in the phloem is bulk flow, which is driven by pressure differences.

Why do phloem cells need to stay alive for the translocation process to work?

Phloem cells need to stay alive because the loading of sugar into the phloem is an active process that requires energy, which can only be generated by living cells.

What is the role of companion cells in the phloem?

Companion cells are partner cells to the living cells in the phloem. They provide the necessary components to keep these cells alive and assist in the loading and unloading of sugars into and out of the phloem.

How does the unloading process of sugars from the phloem work?

The unloading process involves the diffusion of sugars out of the phloem into cells with lower sugar concentration, which can then be taken up by the cells for their needs.

Why are phloem cells different from xylem cells in terms of their structure and function?

Phloem cells are different from xylem cells because they are alive and require a bit of cytoplasm and intact cell walls to function in the translocation process. In contrast, xylem cells are dead and can afford to lose their cytoplasm and cell walls since their function relies on physical forces rather than cellular activity.

Outlines

00:00

🌿 Understanding Plant Translocation

The first paragraph introduces the concept of translocation in plants, which is the process of moving food around within the plant. It explains that plants have the ability to store excess food in their storage organs, such as carrots or potatoes, and retrieve it when needed, like during winter when sunlight is limited. The paragraph raises questions about how plants, lacking a brain, can sense which parts need food and what mechanisms they use for food transport. It also briefly introduces vascular tissues, xylem, and phloem, and suggests revisiting previous videos for a refresher on these topics. The focus is on phloem, which is responsible for transporting food, and the paragraph ends with a setup for a detailed explanation of how translocation occurs within the phloem.

05:02

🚀 Active Loading and Bulk Flow in Phloem

The second paragraph delves into the mechanics of how plants move food through their phloem. It describes the process of 'loading' sugar molecules into the phloem, which is an active process requiring energy due to the concentration gradient. The high sugar concentration in the phloem draws water from the xylem through osmosis, leading to an increase in pressure. This pressure causes the sugary sap to move from high to low pressure areas, a process known as 'bulk flow.' The direction of sap movement is determined by the need for sugar in different parts of the plant, which could be either upwards or downwards. The paragraph also explains the role of companion cells in maintaining the living state of phloem cells, which is crucial for the active loading process.

10:03

🔄 The Cycle of Translocation in Plants

The final paragraph summarizes the entire process of translocation in plants. It revisits the steps of loading sugars into the phloem, the subsequent increase in water due to osmosis, the movement of sap due to pressure differences, and the unloading of sugars into cells where they are needed. The paragraph clarifies that the phloem cells must remain alive to perform the active loading of sugars, which requires energy. It also explains that the xylem cells can be dead since their function is based on physical forces. The summary concludes by emphasizing the importance of the living state of phloem cells for the translocation process to work effectively.

Keywords

💡Translocation

Translocation is the process by which plants move nutrients and sugars throughout their bodies. In the context of the video, it is a vital mechanism that allows plants to distribute food to areas that require it, such as growing regions or storage organs. The term is central to the video's theme as it describes how plants manage their internal resources without a central nervous system, using the example of how excess food is moved down to storage organs like carrots or potatoes and then back up to areas needing growth.

💡Storage Organs

Storage organs in plants are parts such as roots, stems, or tubers that store nutrients for later use. The video script mentions carrots or potatoes as examples of storage organs, highlighting their role in retaining excess food produced by the plant through photosynthesis. This concept is integral to understanding how plants survive periods of low light or photosynthetic activity, such as during winter.

💡Vascular Tissues

Vascular tissues are the conduits within plants that transport water, nutrients, and food. The script distinguishes between two types of vascular tissues: xylem, which carries water and minerals, and phloem, which is responsible for transporting food. Understanding vascular tissues is crucial to grasping the script's explanation of how plants move nutrients and sugars, as it forms the structural basis for translocation.

💡Xylem

Xylem is a type of vascular tissue that transports water and dissolved minerals from the roots to the rest of the plant. The video script explains that xylem works in conjunction with phloem to facilitate the movement of substances within the plant. The concept of xylem is important for understanding the broader context of plant nutrition and the contrast between the functions of xylem and phloem.

💡Phloem

Phloem is the vascular tissue responsible for transporting food, primarily in the form of sugars, throughout the plant. The script focuses on the role of phloem in the process of translocation, detailing how sugars are moved from areas of surplus to areas of need. Phloem's function is central to the video's narrative, as it is the tissue where the active transport of food occurs.

💡Sugar Molecules

Sugar molecules, particularly in the form of sucrose, are the primary form of food that plants produce through photosynthesis. The script describes how these sugar molecules are dissolved in water to form a sugary sap, which is then transported via the phloem. The concept of sugar molecules is fundamental to the video's theme, as they represent the 'food' that is being translocated within the plant.

💡Cytoplasm

Cytoplasm is the jelly-like substance within cells, containing the cellular machinery necessary for life. The video script mentions that phloem cells retain some cytoplasm, which is essential for their function in transporting food. Understanding the role of cytoplasm helps to clarify why phloem cells must remain alive, in contrast to xylem cells, which are dead.

💡Companion Cells

Companion cells are specialized cells associated with the living cells of the phloem. They provide the necessary organelles and substances to keep the phloem cells alive. The script explains that companion cells are crucial for the active loading of sugars into the phloem, as this process requires energy that only living cells can produce. The concept of companion cells is key to understanding the video's explanation of how phloem cells can function without typical cellular components.

💡Osmosis

Osmosis is the movement of solvent molecules, such as water, across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. In the script, osmosis is described as the process that causes water to flow from the xylem into the phloem when sugar is loaded, creating a pressure that drives the bulk flow of the sugary sap. Osmosis is a critical concept in the video, as it explains the physical mechanism behind the movement of water and the creation of pressure within the phloem.

💡Bulk Flow

Bulk flow refers to the movement of a large volume of fluid or particles through a system, driven by pressure differences. In the video script, bulk flow is used to describe how the sugary sap moves through the phloem from regions of high pressure to regions of low pressure. This concept is central to the video's explanation of how the pressure created by osmosis leads to the directional movement of food within the plant.

💡Active Process

An active process is one that requires energy to move substances against their concentration gradient. In the context of the video, loading sugars into the phloem is described as an active process because it requires energy to move the sugars from an area of lower concentration to an area of higher concentration. This concept is important for understanding how plants can control the direction and rate of sugar transport, despite not having a brain or nervous system.

Highlights

Plants and trees can create their own food from sunlight and efficiently move it to storage organs like carrots or potatoes.

During winter, when sunlight is scarce, plants can translocate stored food to areas that require growth.

Translocation is the process by which plants move nutrients to where they are needed.

Plants consist of vascular tissues, xylem and phloem, for transporting water, minerals, and food respectively.

Phloem is crucial for the translocation of food within the plant.

Phloem contains a thick sugary sap, which is the plant's food source.

Phloem cells have gaps between them, allowing the sap to move throughout the tissue.

Translocation involves loading sugar into the phloem, which is an active process requiring energy.

The direction of food transport in phloem is determined by the concentration gradient and pressure differences.

Water from xylem moves into phloem by osmosis, creating pressure that drives the bulk flow of sap.

The movement of sap is like squeezing a water balloon, flowing from high to low pressure areas.

Unloading is the final step where sugars are removed from the phloem and taken up by the plant's cells.

Phloem cells must remain alive to perform the active loading of sugars, unlike xylem cells which are dead.

Companion cells support the living phloem cells, providing the necessary components for their survival.

The living state of phloem cells is essential for the active transport of sugars, which requires energy.

Translocation in plants is a complex process involving active loading, osmosis, pressure-driven bulk flow, and unloading.

Transcripts

- [Instructor] Plants and trees are not just great

at creating their own food from sunlight,

but they're also excellent in moving that food around.

When they have excess of food,

they move it to their storage organs.

For example, if this was a carrot or a potato plant,

then they would move that excess food down.

So they move the excess food down and store it for future.

And now let's say maybe during the winter season

there isn't much sunlight so they cannot photosynthesize.

So they're not able to get a lot of food.

Now, they can move this food up to wherever they want.

So they can move it back from the storage organs

to the place where they need, maybe the growing areas.

And this process of moving the food around

wherever they want, we give a name to it,

we call it translocation.

Translocation.

This is basically plants and trees moving the food around

to the places they need.

But the big question is, how do they do that?

I mean, for one, how do they even understand

which part of the plant body needs food

because they don't have a brain?

And secondly, what mechanism do they use

to transport that food up or down according to their needs?

Well, let's find out.

Now before we continue, let's back up a little bit.

We've already seen in previous videos

that plants and trees consists of pipe-like structures

to transport stuff around,

that starts from the roots all the way to the leaves,

and we call these structures vascular tissues.

And there are two kinds,

xylem that transports water and minerals

from the soil to the different parts of the body,

and phloem, which mostly transports food.

Now if these terms look new to you

or you need a refresher on this,

then you can go back and watch our previous videos

on intro to vascular tissues

and videos on xylem and transportation.

But if you feel comfortable, then let's continue.

Now in this video, since we wanna talk about translocation,

meaning transportation of food,

we are going to be concentrating on phloem.

So translocation happens inside the phloem.

Food gets transferred in them, but how?

Well, let's see.

Let me draw xylem and phloem.

So let's say this is the xylem tissue which has water in it,

this is going to be important for us as well,

and here is our phloem.

Phloem mostly contains sugar molecules dissolved in water

forming a very thick sugary sap, which is food for them.

And if you're wondering what these things are

over here at the sides, that is some leftover cytoplasm.

The cells don't lose all of their cytoplasm.

And similarly, this is the cell walls.

They have developed gaps in between.

So they have not lost their cell walls like in xylem,

some gap is developed so that the food can, this whole sap

can move throughout the phloem.

And if you're wondering why it is structured like this,

we'll talk a little bit about that towards the end.

Anyways, imagine a couple of cells next to phloem.

Let's assume these are far apart, okay, not so close

and let's assume this cell

has a lot of sugar molecules in it.

And imagine this cell doesn't have much sugar molecules,

it needs a lot.

Let's say it's one of the growing regions of this plant.

So it needs a lot of sugars.

So how do we transport the sugar from here to here

is the question?

Well, you can kind of guess the process.

We take some of the sugars, put it into the phloem,

then we make sure it transports

through the phloem till here,

and then we remove the sugar and put it into this cell.

That makes sense, right?

But a couple of questions could be how does the phloem know

in which direction to transport this food?

Because that cell which is in need of sugars

can be anywhere.

It can be below it or it can be above this as well.

So how does it know whether it has to go down or up?

And secondly, how do you even move this thick sugary sap

through the phloem?

So let's put this back over here

and let's see how this works.

So the first step as you predicted

is to put the sugar into the phloem tube.

I'm just gonna call that loading.

And this process is an active process.

What does that mean? Active.

Well, think about it.

If the concentration over here of sugars

is less than over here,

then sugars will automatically diffuse

from higher to lower concentration.

But pretty soon the concentration will equalize.

How do you move even more sugars there?

Well, that requires energy.

The cells have to spend some energy

to move the sugar from lower to higher concentration.

So, by using energy,

the sugars need to be transported into the phloem.

And because energy is utilized for this process,

we say it's an active process.

Now see what happens.

Because of a lot of sugar,

the concentration of sugar is much higher in this region

compared to any other regions.

And remember, nature always likes

to balance the concentration out.

So one way to reduce the concentration

is to put the sugar back.

But remember that's not possible, we're not doing that.

So what else can nature do?

Well, there's xylem right next with a lot of water.

So you know what's gonna happen?

Water will start flowing from xylem into the phloem,

to this part of the phloem because of osmosis.

So let me write that as step two.

Remember osmosis it's the process in which solvent flows,

like water starts flowing from a low concentration region

to a higher concentration region.

Basically water is flowing

to try and dilute this concentration.

Okay, what does that do?

Well, because a lot of water is flowing in,

this region of the phloem starts puffing up over there.

This increases the pressure over here.

And what I mean by that is you can imagine these walls

are pressing on this solution,

squeezing that solution a lot.

Now, what do you think will happen

if I squeeze this solution a lot?

Well, it'll automatically start moving from here

to a region where there is low pressure.

It's kind of like squeezing a water balloon.

And where do you think is the pressure lower?

The pressure is lower

where there is less sugar concentration, right?

Because if there's less sugar concentration,

there will be less water over here

and that's exactly where you need

to send your sugar molecules.

So if on the top there is a cell

which has very low sugar concentration,

automatically that part will have low pressure,

automatically this side will move up.

If it turns out that somewhere in the bottom

there is low concentration,

automatically the pressure over there will be very low

and the sap will move down.

And this is how the pressure decides

in what direction the sap will move.

So in our example, the sap will move down from here to here.

Let's call that as our step three,

and we can call this the bulk flow,

the bulk flow driven by pressure.

And the reason we are calling it bulk flow

is because this whole thing is a solution, remember.

Even though I've put dots over here,

this is one single solution.

And so the whole solution moves down, not just this part.

All right?

So let me show you what that will look like.

So as the solution moves, this pressure is relieved.

Let me get that back to normal.

And so due to the high pressure over here,

this whole solution will start moving like this.

The whole thing will move from high pressure to low pressure

until it reaches over here.

And then finally the sugar reaches the low pressure region.

Because there is less concentration of sugar,

it will move out of phloem

and it can now be taken up by this cell.

And so this we'll call it as step four, unloading happens.

The sugars get unloaded from the phloem tube.

So let's unload that sugar from the phloem

into the required cell.

And finally, remember that region

which had a lot of concentration of sugar had a lot of water

due to the osmosis.

Well now, that concentration has lowered

so, that water will move back to xylem.

So in step five, again, osmosis happens

and the water moves back

and that's how translocation happens inside plants.

Now before we summarize and wind up this video,

one question we need to address is why phloem structure

is a little different than that of xylem.

Why does it have cytoplasm, little bit of cytoplasm left?

Why are the end walls not completely gone like in the xylem?

Well, that's because these cells are alive.

Phloem has life cells.

In contrast, xylem has dead cells.

And so xylem cells can afford to lose all of their stuff

because they are dead, right?

But phloem cells need a little bit of cytoplasm

and their cell walls to stay alive.

Now that could raise even more questions.

First of all, you may be wondering, how can cells stay alive

without a nucleus or without a mitochondria?

How can these cells be alive?

Well, in fact, you know how they are staying alive?

They have a partner cells which I have not shown over here,

but each cell is connected to a partner.

Let me show you what would that look like

if I were to draw those.

So these are the partner cells

and they have all the stuff needed

to keep these cells alive.

In fact, they are life partners, okay?

And that's why these cells are called companion cells.

All right?

And so in reality, if you're wondering,

when you want to load the sugars,

you have to first load it into the companion cell

and then it goes into the phloem.

Similarly, while unloading,

it first goes into the companion cell

and then it goes out of the phloem.

But that's a small detail.

We don't have to worry too much about that,

so let me get rid of those.

So it's those companion cells that keep them alive.

But another question you might be wondering

is why do we have to go through all that trouble

to keep these cells alive?

Why is it necessary that these cells need to be alive?

Well, the answer is in this process.

You see, we just saw that in order to load the sugar

into the phloem, that requires energy.

It's an active process

and the cell can only generate energy if it is alive.

So if the cells were dead, like in xylem,

they wouldn't be able to generate energy,

they wouldn't be able to load sugar,

they wouldn't be able to accept that sugar molecules.

It would just diffuse back

and then the phloem transport wouldn't work.

And xylem cells don't need to be alive

because the mechanism of xylem

is only based on physical forces like suction

or the pressure from the bottom, from the roots, right?

You don't need the cells.

The cells don't need to use any energy

so they can afford to be dead, but phloem needs to be alive.

All right, that's pretty much it.

So, let's quickly summarize.

What did we learn in this video?

We saw that plants and trees can move the food up or down

based on their requirement and we called this translocation,

and this happens inside the phloem tissues.

And how do these sugars go from one place to another?

Well, in the first step, we have loading

where the sugars get loaded actively into the phloem tubes

and this requires energy,

that's why it's called as an active process

and that's why these cells need to stay alive.

And in doing so,

because the concentration over here has increased,

water starts flowing from xylem into the phloem

to try and decrease that concentration due to osmosis.

And because of that, a lot of water comes in

and the cell now kind of all there puffs up.

I like to imagine that way because now you can see

that there's a lot of pressure over there.

And then because there is high pressure region over here,

it automatically wants to move

towards the low pressure region.

And the low pressure region is the region

where there is less concentration of sugars.

And as a result, the whole sap starts moving

towards the low concentration region.

And so this is that step three, which is a bulk flow.

It starts moving, the whole sap moves

from higher to lower concentration,

and now that pressure is relieved.

And then in step four, the sugars automatically diffuse out

because outside there is less concentration

and that can now be taken up by the cell,

which we will call it as unloading.

And then finally,

the excess water drains back into the xylem

because the concentration has decreased.

So it goes back

and that's how translocation works in plants and trees.