Xylem & transpiration | Life processes | Biology | Khan Academy

Summary

This educational video script explores the fascinating process of how trees transport water against gravity to their highest branches. It reveals that over 95% of absorbed water is lost through evaporation, a process crucial for creating suction that pulls water upwards. The script explains the role of xylem, made of dead cells, in conducting water and its importance in plant structure. It also touches on the significance of transpiration for mineral transport and cooling, and introduces root pressure as a nocturnal force ensuring the water column's stability.

Takeaways

• 🌳 Trees and plants use a combination of transpiration and root pressure to transport water from the ground to their highest branches.
• 💧 Over 95% of the water absorbed by trees is lost to the atmosphere through a process called transpiration, which is crucial for water transportation.
• 🔬 The xylem, a vascular tissue in plants, is responsible for transporting water and dissolved minerals from the roots to the leaves.
• 🌱 Xylem is made up of dead cells that have lost their organelles and cell walls, forming a continuous column for water to rise.
• 🌡️ Transpiration not only helps in water transportation but also cools the plant down, similar to how sweating cools the human body.
• 🌱 The xylem's thick walls are necessary to withstand the suction force created by transpiration, also providing structural support to the plant.
• 🌿 Root pressure, which originates from the roots, is a secondary force that helps maintain the column of water in plants during the night when transpiration is minimal.
• 🚰 The active transport of minerals into root cells creates a concentration gradient, leading to osmosis and the generation of root pressure.
• 🌱 The continuous cycle of water and mineral transportation is essential for plant growth, as it delivers vital nutrients from the soil to every cell.
• 🌞 During the day, transpiration dominates the water transportation process, while at night, root pressure becomes the primary force keeping the water column intact.

Q & A

How do trees transport water from the ground to their top against gravity?

Trees transport water through specialized pipe-like structures called xylem, which is made up of dead cells that form continuous hollow tubes. The process is facilitated by two forces: transpiration and root pressure. Transpiration involves water evaporating from the leaves, creating a suction force that pulls water up from the roots. Root pressure is the force generated by the roots absorbing water and minerals, which pushes water upward, especially at night when transpiration is less active.

What percentage of water absorbed by trees is lost to evaporation?

More than 95% of the water absorbed by trees is lost to evaporation, which is a crucial process for water transportation and other functions like cooling the plant.

What is the role of xylem in plants?

Xylem is a vascular tissue in plants that primarily transports water and dissolved minerals from the roots to the rest of the plant. It is composed of dead cells that form continuous tubes, allowing water to move through the plant.

How are the xylem vessels formed in plants?

Xylem vessels are formed from modified cells that lose their cellular contents, including the nucleus, mitochondria, and cytoplasm, as they mature. The cell walls then break down, leaving behind a continuous hollow tube that can be filled with water.

What is the difference between xylem and phloem?

Xylem and phloem are two different types of vascular tissues in plants. Xylem is responsible for transporting water and dissolved minerals and is composed of dead cells. In contrast, phloem is responsible for transporting food, such as sugars, and is made up of living cells.

Why are xylem cells dead, and what is the significance of this?

Xylem cells are dead because they have lost all their cellular contents during maturation, which allows them to form continuous, hollow tubes for water transport. The absence of living cells ensures that there is no metabolic activity to interfere with the flow of water, making the system more efficient for transportation.

How does the process of transpiration contribute to water transportation in plants?

Transpiration is the process by which water evaporates from the leaves through tiny pores called stomata. This evaporation creates a suction force that pulls water up from the roots through the xylem. It also helps in cooling the plant and transporting minerals from the soil to the plant cells.

What is root pressure and how does it help in water transportation at night?

Root pressure is the force generated by the roots as they actively absorb water and minerals, increasing the concentration of solutes inside the root cells. This causes water to move into the root cells through osmosis, creating a pressure that pushes water upward. Root pressure is especially important at night when transpiration is reduced, ensuring that the column of water in the xylem does not collapse.

What is the role of stomata in the water transportation process?

Stomata are tiny pores on the surface of leaves that play a dual role in the water transportation process. They facilitate gas exchange, which is essential for photosynthesis, and they also serve as the exit points for water vapor during transpiration, contributing to the suction force that pulls water up from the roots.

How does the evaporation of water help in cooling plants?

The evaporation of water from the plant's surface, particularly through transpiration, helps in cooling the plant. As water evaporates, it requires energy in the form of heat, which is taken from the plant's tissues, thereby reducing the plant's temperature and preventing it from overheating, similar to how sweating cools the human body.

Outlines

00:00

🌳 Understanding the Water Transport Mechanism in Trees

The paragraph introduces the marvel of how trees, despite their towering heights, manage to transport water against gravity. It poses the question of how trees, some reaching hundreds of meters, push water to their canopies. The instructor reveals that the process is primarily driven by evaporation, with over 95% of absorbed water being lost to the atmosphere, a fact that might seem counterintuitive. The paragraph sets the stage for a deeper exploration into the mechanisms of water transport in plants, hinting at the role of vascular tissues like xylem and phloem, which are introduced as the conduits for water and food transport, respectively.

05:01

💧 The Role of Xylem and the Power of Transpiration

This section delves into the structure and function of xylem, which is composed of dead cells that form a continuous管道 for water and mineral transport. The instructor explains that xylem's dead cells create a vacuum that facilitates the upward movement of water through a process known as transpiration. As water evaporates from the leaves through stomata, it creates a suction force that pulls water up from the roots. The paragraph also touches on the structural strength of xylem, necessary to withstand the pull of water, and how this contributes to the rigidity of plants. The discussion concludes with the multifaceted benefits of transpiration, including mineral transport and cooling through evaporative processes.

10:03

🌱 Root Pressure and Osmosis in Water Uptake

The final paragraph explores the role of root pressure in water transport, particularly during the night when transpiration is less active. It describes how root hairs absorb minerals from the soil, creating a concentration gradient that triggers osmosis. As water moves into the root cells to balance the concentration, it generates pressure that pushes water upward, complementing the suction force from transpiration. The paragraph concludes by summarizing the dual forces of transpiration and root pressure that ensure continuous water and mineral circulation within the plant, highlighting the intricate balance maintained by these natural processes.

Keywords

💡Transpiration

Transpiration is the process by which water is lost from the surface of plants to the atmosphere in the form of water vapor. In the video, it is explained as a driving force for water transport in plants, where the evaporation of water from the leaves creates a suction force that pulls water up from the roots. This process is crucial for the transport of water and dissolved minerals from the soil to the plant's cells and also helps in cooling the plant, similar to how sweating cools the human body.

💡Xylem

Xylem is a type of vascular tissue in plants that transports water and dissolved minerals from the roots to the rest of the plant. The video describes xylem as being composed of dead cells that form long, hollow tubes, which allow water to be pulled up from the roots to the leaves. Xylem's structure, with thick walls, provides strength to the plant and acts as a backbone, with wood being a product of xylem tissue.

💡Phloem

Phloem is the other type of vascular tissue in plants, responsible for transporting food, primarily in the form of sugars, from the leaves to other parts of the plant. While the video focuses mainly on xylem, it contrasts phloem by mentioning that it is composed of living cells, unlike xylem, and is involved in the transport of food rather than water.

💡Stomata

Stomata are tiny pores found on the surface of leaves that facilitate gas exchange, including the intake of carbon dioxide for photosynthesis and the release of oxygen and water vapor. In the context of the video, stomata play a key role in transpiration, as they are the exit points for water vapor to be released into the atmosphere, contributing to the suction force that pulls water up through the xylem.

💡Hydrogen Bonding

Hydrogen bonding is a type of chemical bond that involves a hydrogen atom being attracted to a highly electronegative atom, such as oxygen or nitrogen. In the video, hydrogen bonding is mentioned as the force that allows water molecules to attract and pull on each other, creating a chain reaction that helps water move up the xylem against gravity.

💡Root Pressure

Root pressure is the force generated by the roots of a plant that helps push water and minerals up into the xylem, especially during the night when transpiration is minimal. The video explains that root pressure is caused by the active transport of minerals into the root cells, which creates an osmotic imbalance leading to water moving into the roots by osmosis, thus maintaining the column of water in the xylem.

💡Osmosis

Osmosis is the movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. In the video, osmosis is described as the process that occurs in root hairs where water moves into the root cells to balance the concentration of minerals, which have been actively transported into the cells, thus creating root pressure.

💡Active Transport

Active transport is the process by which cells move substances across their membrane using energy, often against a concentration gradient. In the video, active transport is discussed in the context of root cells absorbing minerals from the soil, which leads to an imbalance in concentration and subsequently triggers osmosis, contributing to root pressure.

💡Evaporative Cooling

Evaporative cooling is a cooling effect that occurs when water evaporates from a surface, such as when sweat evaporates from the skin, removing heat and lowering the temperature. The video explains how transpiration in plants serves a similar purpose, helping to cool the plant down by evaporating water, which is then replaced by more water being pulled up through the xylem.

💡Vascular Tissues

Vascular tissues are specialized tissues in plants that form conduits for the transport of water, nutrients, and food. The video discusses two types of vascular tissues: xylem and phloem. These tissues are crucial for the growth and survival of plants, enabling them to transport necessary substances throughout their structure.

Highlights

Trees can grow hundreds of meters tall, raising the question of how they transport water against gravity.

Over 95% of water absorbed by trees evaporates, which was a surprising fact.

Plants have evolved vascular tissues, specifically xylem and phloem, for transporting water and food respectively.

Xylem is formed from modified cells that lose their contents to form a continuous pipe for water transport.

Xylem is composed of dead cells, contrasting with the living cells in phloem.

Water climbs up the xylem vessels due to evaporation at the leaves, creating a suction force.

The suction force from evaporation is compared to drinking through a straw, illustrating how water is pulled up.

Xylem's thick walls are necessary to withstand the suction force, also serving as structural support for plants.

Wood is derived from xylem, which plays a crucial role in the water conducting system of plants.

Transpiration, the evaporation from plant leaves, is essential for water and mineral transport and also for cooling.

Root pressure, driven by osmosis, ensures the water column does not collapse at night when transpiration is minimal.

Active transport of minerals by root cells leads to a concentration gradient, driving osmosis and root pressure.

Osmosis, the movement of water from lower to higher solute concentration, contributes to root pressure.

The dual forces of transpiration and root pressure work together to maintain the water column in plants.

Transpiration not only pulls water up but also helps in cooling plants through evaporative cooling.

The video provides a comprehensive overview of how plants transport water and minerals, highlighting the importance of xylem and transpiration.

Transcripts

- [Instructor] Look at all these giant trees

standing tall and mighty.

When I used to look at them,

I used to wonder about one question.

How do these trees take up water from the ground

and push it all the way to the top

against the force of gravity?

I mean think about it.

Some of the trees can grow hundreds of meters tall.

So, how do they get that water all the way to the top?

The answer is evaporation, that's right.

It turns out and this blew my mind away

when I learned about this,

it turns out more than 95% of the water that they absorb

just evaporates into the atmosphere.

But doesn't that sound crazy to you?

I mean how can evaporation make water go up

and what's the whole point?

I mean if they are evaporating most of that water,

why do they even need water?

What's going on?

Well, let's find out.

Now, before we dive into this, let's back up a little bit.

We have seen before that plants and trees can grow so tall

because they have evolved pipe like structures within them

which are called vascular tissues.

They help in transporting the food and water

from one part of the body to another.

And since we have to transport two things, food and water,

there are two separate kind of pipes.

One is called xylem which transports mostly water

and some dissolved minerals

and the other one is called phloem

which mostly transports food,

f for food, ph for phloem.

Now, if you seem to be pretty unfamiliar

with these fancy names then we've talked a lot about them

in a previous video called intro to vascular tissues.

Be great idea to go back and watch that as a refresher.

If you feel you're confident, let's go ahead then.

So, in this video, since you wanna talk about water

we're going to be focusing only on xylem

and see how it works.

Now, when I was studying about this

even before thinking about water,

my main question was, where do these pipes come from, right?

I mean how did they get there?

Turns out that they come from modified cells.

For example let's think about how xylem is formed.

Well, here's how we like to think about it.

So, before xylem was formed we had normal plant cells

but as they mature they start losing

all of this stuff from inside.

They start losing their nucleus, their mitochondria,

their cytoplasm, all of that is gone

and eventually they lose even these

horizontal cell walls as well

and all the vertical cell walls join together to form

a tall empty pipe, that's our xylem.

And since it's empty we can now fill it up with water.

And so this might make you wonder,

if these cells have lost all of their organelles

how are they staying alive.

The answer is they aren't.

They're all dead.

And so important to remember that xylem

is made of dead cells, they're no longer alive

and just to contrast it turns out that phloem

is made of living cells.

We'll get to that in another video as to why and how

but xylem is made of dead cells.

And so now to the big question,

how does the water climb up these xylem vessels?

Well, like I said before it's due to evaporation.

When the water eventually reaches

all the cells of the leaves,

most of them go out of the tiny pores which are present

in these leaves.

You might know the names of these pores,

they're called stomata.

They're useful in exchanging gases.

When the stomata opens up, the water gets evaporated

in to the atmosphere.

But how does this evaporation help in pushing the water up?

Well, to figure that out,

let's zoom into one of these leaves.

So, let's say that this is that same xylem tissue

that we just talked about made of dead cells

and let's say this is one of the leaves,

highly magnified view you imagine

and this is one of those pores the stomata

through which the water can go out.

Now, if we draw some of the water molecules,

imagine these are individual molecules of water.

I know they seem too big.

Molecules are not so big but imagine, okay.

And so these are the water molecules

that are moving out of the leaves.

It turns out that these molecules pretty strongly attract

other molecules as well

and as a result when these molecules move out

they automatically start pulling

on the molecules behind them.

You'll learn in chemistry that this force

is called the hydrogen bonding, okay

but don't worry about the names as of now.

So, because of this attraction

they pull on the molecules behind

and as a result these start moving away

and then they start pulling on the ones behind them

and they start moving on the ones behind them.

And so can you see that because of this the force,

this pulling force starts from the leaves

and goes all the way down to the roots

because this is a single column of water,

a giant very tall column of water

and that's how the force gets transmitted.

This is kind of like how she's drinking that juice

from the straw.

She's sucking on that straw and the juice comes up.

Similarly, you can imagine the xylem to be a giant straw.

The evaporation causes that suction

because of which the water gets pulled up.

And so because there's a lot of suction force

within the xylem,

the walls of the xylem tissue needs to be super thick,

thick enough to withstand that force.

Now, what do you mean by that?

For example, just look at this water bottle.

When I start sucking on it look at how easily

the walls collapse.

There is a similar suction over here but way stronger

and so the walls need to be strong enough to not collapse.

And so xylem has pretty strong walls because of which

it also acts like a backbone to the plants and the trees.

And if you're wondering, wood is made of xylem.

In fact, the word xylem comes from the Greek xylon

which means wood.

So, the wood that is used in the furnitures

and tables and everything

were once a part of this amazing water conducting system.

Pretty awesome, right?

But you might be thinking,

why do plants have to go through all this trouble

to suck that water and then just make it evaporate?

Why are they doing it in the first place?

Well, for many reasons.

First of all, they need some of that water.

They don't throw all of it away.

Some of that water is useful in photosynthesis.

But more importantly this running stream of water

helps in transporting the minerals, the important minerals

from the soil to all the cells of the plant.

So, this water acts like a train

where the minerals hop in at the soil

and then when they reach their destination at the leaves

they get absorbed, the water gets thrown away

and the process repeats.

So, it's because of this the the trees

get the important minerals because of which they can grow.

And if that's not enough evaporation is also useful

in cooling them down

just like how when you go out in a hot sun

you start sweating because when that sweat evaporates

your body gets cooled down.

In a similar manner, these trees and plants

are always in the hot sun during the daytime.

So, they need to cool off as well

and so instead of sweat they just evaporate these water

and that helps them cool down.

And so you can see evaporation is a big deal

for these plants and trees.

It's such a big deal that scientists thought,

let's give it a different name altogether.

So, the evaporation that happens in the trees

from their plants, from the stomata,

it's called transpiration.

Okay, transpiration.

And why is this transpiration useful?

Because it causes suction force.

So, it causes suction

because of which the water gets pulled up

and all the minerals get transported

and also it causes cooling,

this is called evaporative cooling.

Both of these are essential

and that's how xylem pretty much works.

But if you're really curious,

you might ask one final question.

What happens during the night time?

Because transpiration requires heat

which you can get during the daytime

but during night time there's hardly any transpiration.

So, what keeps that column of water up and running?

Why doesn't it just fall down?

That's because there is a second force

that is causing this water to go up.

This force originates from the roots itself,

it's called the root pressure

because it comes from the roots

and it's this force that makes sure during the night time

the column just doesn't fall down.

So, there are both forces acting.

There's a suction force due to transpiration

and there is this root pressure.

During the daytime, the transpiration dominates

but during the nighttime because there is no transpiration,

root pressure is the only force.

But how does that work?

Well, to quickly give you an overview,

here we have zoomed in to the tip

of one of the tiny root hairs.

And what you're seeing over here is the water

and these dots represent minerals.

So, we have water and dissolved minerals in the soil

and we also have water and dissolved minerals

in these root hairs.

And the concentration is pretty much even

because diffusion makes sure of that.

But now the cells of these root hairs

start messing with the concentration balance.

They start absorbing the minerals from their surrounding.

So, look at the minerals carefully.

They start absorbing it from the soil

and this cannot happen all by itself,

the cells need to spend energy to bring those minerals in.

And because of that it's called an active process

or an active transport.

Active means they're using energy to transport the minerals

inside their cells.

And now look at what has happened.

Because the soil lost the minerals,

this solution has become dilute

because there's a lot of water but there's less minerals

and the solution inside the root has become concentrated,

more concentrated than outside.

And nature does not like that, nature needs balance.

So, either the minerals have to flow back out

but the cells don't allow that,

they use energy to make sure that doesn't happen.

But if that can't happen, you know what nature does?

It makes sure that the water starts going in,

the water starts rushing in.

Think about it.

Because the outside region is more dilute,

the water moves out from that

and enters the more concentrated region.

It starts diluting this as well.

And in the process notice the water starts rushing in.

You may have heard of this process.

This process is called osmosis.

It's a special kind of diffusion in which

particles don't move from higher concentration

to lower concentration but instead the water starts moving

from lower concentration to higher concentration.

but the idea is the same

to balance the concentration.

And as a result of this osmosis,

now can you see that because water is rushing in

it starts pushing on the column of the water

that's already present over here

and that's what causes the pressure from the bottom.

This is called the root pressure.

Let me just write that down.

This pressure is called root pressure.

And in more general because it is happening due to osmosis

you can also call this as the osmotic pressure.

And so this root pressure forces the water to go up as well.

It's not as strong as their suction produced

due to transpiration but during the night time

this is the only available force.

So, this makes sure that the column doesn't fall down.

And that's pretty much it.

So, to quickly summarize, what did we learn in this video?

We saw that xylem tissues are made of dead cells

which help in transporting water up

and it happens due to two forces.

One is the transpiration in which the water evaporates

and causes suction

which literally sucks the water from the roots.

And we saw that transpiration also helps

in cooling the plants off.

And the second pressure is the root pressure.

This is where the roots are actively absorbing the minerals,

increasing the concentration and thereby causing osmosis.

And so this continuous upstream of water makes sure

that every single cell gets the required minerals

from the soil and helps recycle the water from the ground

into the atmosphere.