Plant cell walls | Structure of a cell | Biology | Khan Academy

Summary

The video explores plant cells, with a focus on the structure and function of the cell wall. Unlike rigid brick walls, plant cell walls are more like a mesh made of polysaccharides, such as cellulose and pectin. These walls provide plants with shape and support, while internal pressure also plays a role. The video discusses how plant cells can communicate through plasmodesmata, tunnels between adjacent cells. It also touches on the distinction between primary and secondary cell walls, with the latter giving wood its rigidity, even when dehydrated.

Takeaways

• 🌿 **Plant Cell Structure**: Plant cells have a cubic or rectangular prism shape, which is different from the roundish shape of other cells.
• 🧱 **Cell Wall Function**: The cell wall provides the rigidity that allows plant cells to maintain their shape and enables plants to stand upright.
• 🌱 **Cell Wall Composition**: The cell wall is not a solid barrier but a mesh-like structure composed of polysaccharides like cellulose, hemicellulose, and pectin.
• 🍃 **Internal Pressure**: The internal pressure from the vacuole and cellular fluid, combined with the cell wall, contributes to the structural support of the plant.
• 🌳 **Secondary Cell Walls**: In more mature plants, a secondary cell wall forms, adding more layers of cellulose and molecules for increased rigidity, as seen in wood.
• 🌱 **Cellular Membrane**: The cellular membrane, or plasma membrane, is the outer layer that encloses the cell's contents.
• 🍃 **Middle Lamella**: The space between cells, known as the middle lamella, allows for the exchange of molecules and fluid.
• 🔗 **Plasmodesmata**: Direct tunnels called plasmodesmata connect adjacent plant cells, facilitating the flow of cytosol and small molecules.
• 🧪 **Active Research**: There is ongoing research to understand the exact functions, necessities, and transport mechanisms of cell walls and plasmodesmata.
• 🥗 **Diet Connection**: Cellulose and pectin from plant cell walls are part of dietary fiber, which has health benefits for humans, such as slowing glucose absorption and aiding digestion.

Q & A

What is the primary function of the cell wall in plant cells?

The primary function of the cell wall in plant cells is to provide structural support and maintain the shape of the cell, allowing the plant to stand upright and grow.

Why are plant cells often depicted as having a cubic or rectangular prism shape?

Plant cells are often depicted as having a cubic or rectangular prism shape because of the rigidity provided by the cell wall, which helps them maintain this structure.

What is the difference between the cell wall and the cellular membrane in plant cells?

The cellular membrane, or plasma membrane, is a lipid bi-layer that lies just inside the cell wall and regulates the passage of substances in and out of the cell. The cell wall, on the other hand, is a rigid layer outside the cellular membrane that provides structural support.

What is a vacuole and how does it contribute to the structure of a plant cell?

A vacuole is a large organelle within the plant cell that stores water, nutrients, and waste products. It contributes to the structure of the cell by providing internal pressure that helps the cell maintain its shape and rigidity.

What are the main components of the cell wall and how do they contribute to its structure?

The main components of the cell wall are polysaccharides such as cellulose, hemicellulose, and pectin. These components are arranged in a mesh-like structure that provides both flexibility and rigidity to the cell wall.

How does the cell wall differ from a brick wall in terms of permeability?

Unlike a brick wall, which is impenetrable, the cell wall is more like a mesh or fabric that allows small molecules to flow through it, providing some permeability.

What is the middle lamella and how is it related to the cell wall?

The middle lamella is the space between cells, composed mainly of pectin, which helps to bind cells together. It is the layer that connects the cell walls of adjacent plant cells.

Why do plants wilt when they are not watered?

Plants wilt when they are not watered because the internal pressure that helps maintain their rigidity and upright structure is reduced due to the loss of water.

What is a secondary cell wall and how does it differ from a primary cell wall?

A secondary cell wall is a thicker layer that can be built on top of the primary cell wall in more mature plant cells. It provides additional rigidity and is what gives wood its structural strength.

What are plasmodesmata and what is their function?

Plasmodesmata are small channels or tunnels that connect the cytoplasm of adjacent plant cells, allowing the direct flow of nutrients, signaling molecules, and other small substances between cells.

How are plasmodesmata different from the spaces between cells?

Plasmodesmata are direct channels that penetrate both cell walls and the membranes of adjacent cells, whereas the spaces between cells, or middle lamellae, are the gaps that exist between cell walls without direct cellular connections.

Outlines

00:00

🌿 Structure of Plant Cells

This paragraph discusses the structure of plant cells, emphasizing the role of the cell wall in giving plant cells their cubic or rectangular shape. The cell wall is composed of various polysaccharides like cellulose, hemicellulose, and pectin, which provide rigidity but also allow for flexibility and growth. The paragraph also describes the internal pressure from the vacuole and cytoplasm that, combined with the cell wall, gives plants their structural support. The presence of organelles such as chloroplasts, mitochondria, and the endoplasmic reticulum is acknowledged, but the focus remains on the cell wall.

05:01

🌳 The Rigidity of Cell Walls and Plasmodesmata

The second paragraph delves into the concept of how plant cells maintain their rigidity, particularly when they are not receiving water, which can lead to wilting. It explains that more mature plants develop a secondary cell wall that adds to the rigidity of the cell, allowing structures like wood to remain firm even when dehydrated. The paragraph also introduces plasmodesmata, which are tunnels that facilitate the direct exchange of cytosol and small molecules between adjacent plant cells. This communication system is highlighted as an area of active research, emphasizing the complexity and ongoing study of plant cell biology.

Keywords

💡Plant Cell

A plant cell is the basic unit of life in plants and is similar to animal cells but with unique structures like a cell wall and vacuoles. In the video, the focus is on the plant cell's cell wall, which gives the cell its shape and contributes to the plant's overall structure, allowing it to grow upright.

💡Cell Wall

The cell wall is a rigid layer surrounding plant cells, providing structural support and protection. It is composed mainly of cellulose, hemicellulose, and pectin, which form a mesh-like structure rather than a solid barrier. The script describes how the cell wall contributes to the plant's ability to maintain its shape and stand upright.

💡Cellular Membrane

Also known as the plasma membrane, it is a selectively permeable barrier that controls the movement of substances in and out of the cell. The video script mentions that beneath the cell wall lies the cellular membrane, which is crucial for the cell's function and survival.

💡Cytoplasm

The cytoplasm is the jelly-like substance within the cell, containing organelles and other cellular components suspended in it. The script describes the cytoplasm as being contained by the cellular membrane and filled with various organelles, including a large vacuole.

💡Vacuole

A vacuole is a membrane-bound organelle that appears in plant and fungal cells and functions in storage, waste management, and maintaining turgor pressure. The video script uses the vacuole as an example of an organelle within the cytoplasm that contributes to the internal pressure of the cell.

💡Chloroplasts

Chloroplasts are organelles found in plant cells that conduct photosynthesis, the process by which plants convert sunlight into chemical energy. The script mentions chloroplasts as 'key for photosynthesis' and associates them with the plant's ability to produce energy.

💡Mitochondria

Mitochondria are known as the 'powerhouses of the cell' because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy. The video script includes mitochondria as one of the organelles present in plant cells.

💡Nuclear Membrane

The nuclear membrane, also called the nuclear envelope, is a double membrane that surrounds the cell's nucleus, separating its contents from the cytoplasm. The script refers to the nuclear membrane as containing DNA, highlighting its role in protecting and housing genetic material.

💡Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a network of membranes involved in the synthesis of lipids, proteins, and carbohydrates. The script differentiates between the rough ER, which has ribosomes on its surface, and the smooth ER, which lacks ribosomes.

💡Golgi Apparatus

The Golgi apparatus is an organelle responsible for modifying, sorting, and packaging proteins and lipids for secretion or use within the cell. The video script includes the Golgi apparatus in the list of organelles present in plant cells.

💡Plasmodesmata

Plasmodesmata are microscopic channels that penetrate the cell walls of plant cells, allowing for direct communication and transport of substances between cells. The script describes plasmodesmata as tunnels that facilitate the flow of cytosol and small molecules between adjacent plant cells.

Highlights

Plant cells have a cubic or rectangular prism shape due to their cell walls.

The cell wall is composed of polysaccharide fibers such as cellulose, hemicellulose, and pectin.

Cellulose is a polymer of glucose and is a key component of the cell wall.

The cell wall is not impenetrable; it allows for the flow of small molecules.

The cell wall, combined with internal pressure, gives plants their structural support.

Vacuoles play a role in maintaining the shape of plant cells by exerting internal pressure.

Chloroplasts and mitochondria are essential organelles within plant cells for photosynthesis and energy production.

The nuclear membrane contains DNA and is crucial for cell function.

The endoplasmic reticulum has two forms: rough, with ribosomes, and smooth, without ribosomes.

The Golgi apparatus is involved in modifying, sorting, and packaging proteins.

The cell wall's structure is like a mesh or fabric, not a solid brick wall.

The middle lamella is the space between cells, filled with the cell wall material.

Plants can wilt when not watered due to a loss of internal pressure.

Mature plants develop a secondary cell wall for increased rigidity.

Wood's rigidity comes from its thick layers of cellulose and other molecules in the secondary cell wall.

Plasmodesmata are direct tunnels between adjacent plant cells, allowing the flow of cytosol and small molecules.

The study of plant cell walls and their components is an area of active research.

Understanding plant cells can give us appreciation for the natural world and our food.

Transcripts

- We've talked a lot about cells in general,

but what I thought I would do in this video

is focus on plant cells, and in particular

focus on the cell walls of plant cells.

So this right over here, this is a

drawing of a plant cell.

And the thing that might jump out

at you immediately is instead of

drawing it as just kind of a roundish shape

like that, the way I've drawn a lot of

other cells, I've drawn this as kind of

a cubic structure, or a rectangular prism,

and that's because plant cells

can have a structure like that.

And so the next question is, well,

what gives them that shape?

What allows them to form that, kind of,

cubic rectangular prism shape?

And the answer is, it's the cell wall.

So this is the cell wall.

So let's make sure we can orient

ourselves properly in this picture.

So clearly, if I didn't have this cut-out,

all I would be seeing is the outside.

All I would be seeing is the cell wall.

But we've cut it out, and we can see

the different layers.

We have the cell wall on the outside.

Right below that, right below that

we have the cellular membrane,

or the plasma membrane.

So that's the cellular membrane.

Cellular membrane, right under that.

And then under that, the cellular membrane

is containing the cytoplasm.

And inside of the cytoplasm

we have all sorts of things.

This big thing that is taking up

a lot of the volume inside of this plant cell,

that's a vacuole, which we have described

in other videos.

Vacuole.

It's the combination of this

internal pressure, things like the vacuole

and, just frankly, the pressure

from all of the fluid inside the cell

pushing outwards, plus the cell wall

kind of holding it all in.

That's what gives plants their structure.

That's why a plant is able to grow,

and be upright.

A plant is able to grow and be upright.

So that's my drawing of a plant.

I actually have a plant in my room

that I'm looking at right now,

and it's able to grow and be upright.

And so you have the cell wall,

you have the cellular membrane,

you have the other organelles,

I have some chloroplasts here,

key for photosynthesis, our good friends

mitochondria.

We have our nuclear membrane,

or I should say this yellow thing

is the inner nuclear membrane.

It has the DNA inside.

Then you have the endoplasmic reticulum,

kind of containing that.

The rough ER, containing the ribosomes

or having the ribosomes on the membrane.

The smooth ER, not having the ribosomes.

Golgi apparatus, so that's a little bit

of a review.

But our focus here is on the cell wall.

So let's go back to that.

So if we zoom in on this, if we zoom in

on the cell wall right over here, we can

look at this diagram.

And over here, it might be a little bit

surprising to you.

Because when I've always imagined

a wall, a cell wall, I imagine something

like a brick wall.

Something that's impenetrable.

But this drawing shows us

something different.

And just to be clear what's going on here,

so this is our cellular membrane --

sorry I wrote cellular membrane,

so right over here I have my

lipid bi-layer.

And then right on top of that,

I have the cell wall.

But you see, it isn't just a thick,

like a brick wall, like something

that is impenetrable.

You see you have all these polysaccharide

fibers running across it.

So you have things like cellulose,

which we saw as a polymer of glucose,

arranged in a certain way.

Hemi-cellulose, which has different types

of monomers associated with it.

We have pectin, which is another

polysaccharaide.

And all of these things -- you've actually

probably eaten, if not today probably in the

last week, when we talk about fiber

in your diet, you're talking about things

like the cellulose and the pectin.

Things that your body can't digest.

When you eat a plant you're getting it,

because you're eating their cell walls.

And it does cool things, like

slows the absorption of glucose in your

intestines.

It absorbs water, so I guess you could say

things pass a little bit easier.

But the key thing here is this isn't

a wall.

This actually allows --

Or, it is a wall,

It's officially the cell wall.

But it's not a thick, impenetrable wall

like you might associate the wall

of the room that you're in.

You can see that it has space

for small molecules to flow.

And it's really more like a mesh,

or like a fabric.

And so the cellular membrane

actually has access to the fluid,

and to the molecules, that are

between the cells.

And so just to be clear what we're

looking at.

This layer right here, this cellular membrane,

that's the lipid bi-layer.

This right over here, this is the

cell wall.

I'll do that in a different color.

That is the cell wall.

And then right above the cell wall,

that's the space between the cells,

which we cell the middle lamella.

So the space between the cells

we call the middle lamella.

So this also is, right over here, is also

the middle lamella.

So all of that is interesting, but you might say,

"Okay, well, how hard is a cell?

I get that it's a mesh, but clearly the cells,

the plants are able to stand upright.

Is that because the cell wall provides

all of that rigidity?"

And the answer is, kind of.

The cell wall is like this mesh.

It helps the cells have their shape.

But if you stop watering a plant,

you're going to see it kind of

wilt over.

And that's because part of its ability

to stand up is from the internal pressure

of the cells.

But also part of its shape is the actual

cell wall.

Now, some of you might be saying,

"Well I've seen plants that are much,

much more rigid than this plant

that you've just drawn.

What about things like trees?

What about wood?

Wood seems very rigid.

In fact, so rigid that we can build

actual walls out of wood."

And the answer there is these

more mature plants, actually once the cell

has stopped growing and you have your

cell wall, more layers of cellulose and

other molecules can be built to form

what's called a secondary cell wall.

So this could be viewed as a primary

cell wall.

And then a thicker, secondary cell wall

could be built, which gives much,

much more rigidity.

And so when you look at wood,

what gives wood its structure,

even if you were to take out all of the

water, even if you were to dehydrate

the wood, it's still going to have

its rigidity, because the cellulose layers

and the other molecules that are

so thick that it's able to have its

rigid form.

Now that last thing I want to talk about.

We've already seen that the

cellular membrane has access to

the molecules floating around between

cells, but there's actually also

direct tunnels between adjacent plant cells.

And those direct tunnels I've drawn here

on this, outside of the cell walls

these little yellow circles, these are

plasmodesmata.

These are plasmodesmata.

To get a better understanding of what

they're like, imagine this is one cell.

So I'm writing here Cell 1.

And let's say this is Cell 2.

Cell 2 right over here.

And I have a cross-section.

You see the plasmodesmata are these

tunnels that form between not just the

membrane of the cell wall and the

plasmodesmata.

It forms between the two cells.

And so you can actually have a

flow of cytosol and small molecules

directly between these two cells.

And if you want to get a little bit more

involved in the structure, you have this

kind of smooth endoplasmic reticulum pipe

going through it.

But I want to make it very clear.

'Cause a lot of times when you

study biology it's all explained,

it seems all neat and clean and textbook.

But people are still studying exactly

why do we have these things?

What are they necessary?

What gets transported across these things,

and how are they able to transport,

and under what conditions are they?

So all of these areas, when you were to

kind of dig one layer deeper

than frankly I'm talking about,

you're getting into an area of

active research.

So anyway, hopefully this whole thing

gives you a little bit more appreciation

for the wood around you, the

plants, the house plants around you,

and even the salad that you might have

for lunch.