Endomembrane system | Structure of a cell | Biology | Khan Academy

Summary

This video offers an insightful overview of the endomembrane system in eukaryotic cells, highlighting its integral components such as the cell membrane, nuclear envelope, and endoplasmic reticulum. It explains the phospholipid bilayer structure and delves into the synthesis and maturation of proteins within the system. The script also touches on the Golgi apparatus's role in protein modification and the significance of vesicles in protein transport. Furthermore, it mentions other organelles like vacuoles and lysosomes, emphasizing the complexity and beauty of cellular processes.

Takeaways

• 🌐 The endomembrane system in eukaryotic cells comprises all the membranes that interact within the cell, including the cell membrane and various organelles.
• 🧬 Phospholipid bilayers are the fundamental structure of the membranes, with hydrophilic heads facing outward and hydrophobic tails facing inward.
• 🔬 The cell membrane, outer nuclear membrane, and endoplasmic reticulum are continuous and share the same phospholipid bilayer structure.
• 🌟 The endoplasmic reticulum (ER) can represent more than 50% of the cell's phospholipid membrane and is involved in protein and lipid synthesis.
• 🛠️ The lumen of the ER is the site of protein and lipid synthesis, with the smooth ER playing a role in further processing these molecules.
• 🚀 The transitional ER is where proteins are packaged into vesicles for transport to other parts of the cell, such as the Golgi apparatus.
• 🔄 The Golgi apparatus is responsible for maturing proteins, which can include adding saccharides to form glycoproteins and preparing them for various cellular roles.
• 🚪 Vesicles are small compartments with membranes that transport proteins and other molecules throughout the cell.
• 🌱 In plant cells, vacuoles serve as large, membrane-bound organelles for storage and structural support.
• 🗑️ Lysosomes in animal cells are membrane-bound structures that recycle or break down cellular materials in an acidic environment.
• 🔧 The endomembrane system highlights the complexity and beauty of cellular processes, involving a coordinated network of membranes and organelles.

Q & A

What is the endomembrane system in eukaryotic cells?

The endomembrane system in eukaryotic cells is a network of membranes that interact with each other within the cell, including the cell membrane, nuclear membrane, and membranes of various organelles.

What are the main components of the bilayer of phospholipids?

The bilayer of phospholipids consists of hydrophilic heads pointing outwards and hydrophobic tails pointing inwards, forming the basic structure of cell membranes.

How is the endoplasmic reticulum (ER) related to the nuclear envelope?

The endoplasmic reticulum is continuous with the outer membrane of the nuclear envelope, forming a part of the endomembrane system.

What percentage of the cell's phospholipid membrane is associated with the endoplasmic reticulum?

The endoplasmic reticulum can represent up to or even more than 50% of the phospholipid membrane associated with the cell.

What happens in the lumen of the endoplasmic reticulum?

The lumen of the endoplasmic reticulum is the site where proteins and lipids are synthesized.

What is the transitional endoplasmic reticulum and its function?

The transitional endoplasmic reticulum is the area where newly synthesized proteins are transported out of the ER in vesicles, moving towards the Golgi apparatus for further processing.

What is the role of the Golgi apparatus in the endomembrane system?

The Golgi apparatus is responsible for the maturation of proteins, including the addition of saccharides to form glycoproteins and tagging for various cellular functions.

How do proteins move from the ER to the Golgi apparatus?

Proteins are transported from the ER to the Golgi apparatus in vesicles, which then merge with the Golgi membrane, releasing their contents for further processing.

What are vesicles and their purpose in the endomembrane system?

Vesicles are small compartments with a membrane around them, used for transporting proteins and other molecules within the cell.

What is the function of vacuoles in plant cells?

In plant cells, vacuoles are used for storage and provide structural support, sometimes becoming quite large and contributing to the plant's shape.

What is a lysosome and its role in the endomembrane system?

A lysosome is a membrane-bound organelle in animal cells where materials are broken down and recycled, functioning as a site for degradation and digestion of cellular components.

Outlines

00:00

🌐 Overview of the Endomembrane System

This paragraph provides an in-depth exploration of the endomembrane system in eukaryotic cells, highlighting its composition and function. The endomembrane system encompasses all interacting membranes within a cell, starting with the cell membrane, which is composed of a phospholipid bilayer. The narrator clarifies the structure of this bilayer, with hydrophilic heads facing outwards and hydrophobic tails facing inwards, and explains that this structure is consistent across various parts of the system, including the nuclear membrane and the endoplasmic reticulum. The paragraph delves into the role of the endoplasmic reticulum in protein and lipid synthesis, the transitional ER's role in protein transport via vesicles, and the maturation of proteins within the Golgi apparatus. The summary underscores the complexity and interconnectedness of the endomembrane system in cellular processes.

05:02

🛍️ Additional Components of the Endomembrane System

The second paragraph expands on the endomembrane system by discussing additional components such as vacuoles and lysosomes. Vacuoles are described as membrane-bound organelles that serve various functions, including storage and providing structural support, with a particular emphasis on their significance in plant cells. Lysosomes, on the other hand, are highlighted as membrane-bound structures in animal cells where materials are recycled or broken down. The paragraph illustrates the process of a vesicle merging with a lysosome to release its contents for digestion and recycling. This section of the script reinforces the theme of the endomembrane system's role in cellular maintenance and waste management, emphasizing the beauty and complexity of cellular structures.

Keywords

💡Endomembrane System

The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins. It includes the cell membrane, nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles. This system is crucial for maintaining the cell's internal organization and facilitating various cellular processes.

💡Phospholipid Bilayer

A phospholipid bilayer is a double layer of phospholipids that makes up the cell membrane and other membranes within the endomembrane system. Each phospholipid molecule has a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails. This structure forms the fundamental barrier that separates the cell's interior from its external environment and helps in the formation of various organelles.

💡Cell Membrane

The cell membrane, also known as the plasma membrane, is the outermost layer of the cell, composed of a phospholipid bilayer. It regulates the entry and exit of substances, maintaining the cell's homeostasis. In the endomembrane system, it interacts with other membranes to facilitate the movement and processing of molecules.

💡Nuclear Envelope

The nuclear envelope is a double membrane structure that surrounds the nucleus in eukaryotic cells. It consists of an outer and an inner membrane, both of which are phospholipid bilayers. The outer membrane is continuous with the endoplasmic reticulum, connecting the nucleus to the rest of the endomembrane system and allowing the transport of molecules between the nucleus and the cytoplasm.

💡Endoplasmic Reticulum (ER)

The endoplasmic reticulum (ER) is an extensive network of membranes in the cell, divided into rough ER (with ribosomes) and smooth ER (without ribosomes). It plays a key role in synthesizing proteins and lipids. The rough ER is involved in protein synthesis and folding, while the smooth ER is involved in lipid synthesis and detoxification processes. The ER's membrane is continuous with the nuclear envelope, making it a vital part of the endomembrane system.

💡Transitional ER

The transitional ER is the region of the endoplasmic reticulum where vesicles form and bud off to transport proteins and lipids to the Golgi apparatus. This region is crucial for packaging and shipping synthesized molecules to their next destination within the endomembrane system.

💡Golgi Apparatus

The Golgi apparatus, also known as the Golgi complex, is an organelle that further modifies, sorts, and packages proteins and lipids received from the endoplasmic reticulum. It consists of flattened membrane sacs and plays a key role in processing molecules for secretion or delivery to other parts of the cell. The Golgi apparatus adds carbohydrates to proteins to form glycoproteins and tags them for their final destinations.

💡Vesicles

Vesicles are small membrane-bound compartments that transport molecules between different parts of the endomembrane system. They bud off from the transitional ER and carry proteins and lipids to the Golgi apparatus or other destinations. Vesicles play a critical role in maintaining cellular organization and facilitating the movement of substances within the cell.

💡Lysosome

Lysosomes are membrane-bound organelles containing enzymes that break down waste materials and cellular debris. They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. Lysosomes are involved in recycling the cell's organic material, making them an essential part of the endomembrane system's maintenance and repair functions.

💡Vacuole

Vacuoles are large membrane-bound sacs found in plant cells and some animal cells. In plant cells, vacuoles store nutrients, waste products, and help maintain turgor pressure, which gives structural support to the plant. In animal cells, vacuoles are smaller and involved in storage and transport. Vacuoles are an integral part of the endomembrane system, contributing to the cell's storage and structural stability.

Highlights

The endomembrane system in eukaryotic cells consists of all the interacting membranes within the cell.

The cell membrane and other membranes are composed of bilayers of phospholipids with hydrophilic heads and hydrophobic tails.

The endoplasmic reticulum and the Golgi apparatus are integral parts of the endomembrane system.

The endoplasmic reticulum can represent over 50% of the cell's phospholipid membrane.

Proteins and lipids are synthesized in the lumen of the endoplasmic reticulum.

The transitional endoplasmic reticulum is where proteins are transported in vesicles.

Vesicles are small compartments with a membrane that transport proteins and other molecules.

Proteins mature in the Golgi apparatus where they may become glycoproteins through the addition of saccharides.

The Golgi apparatus can tag proteins for various cellular destinations, including the cellular membrane or outside the cell.

Proteins can be embedded into the cellular membrane, excreted from the cell, or used within the cell.

Vacuoles are membrane-bound organelles used for storage and structure, especially in plant cells.

Lysosomes in animal cells are membrane-bound structures for recycling and breaking down cellular material.

The endomembrane system showcases the complexity and beauty of cellular structures.

Membrane fusion is a key process in the transport and function of proteins within the endomembrane system.

The endomembrane system plays a crucial role in the synthesis, maturation, and transport of cellular components.

Different parts of the membrane may have distinct phospholipid bilayers but share the common structure of a bilayer.

Transcripts

- [Voiceover] What I wanna do in this video is give

an overview of the endomembrane system in eukaryotic cells.

Endomembrane system.

And at a very high level, the endomembrane system is all

of the membranes that interact with each other

inside of a cell.

So what membranes are we talking about?

Well, you can start off by talking

about the cell membrane itself.

And all of these membranes,

these have bilayers of phospholipids.

Sometimes my brain malfunctions

and I call them bilipid layers,

but these are bilayers of phospholipids.

So if I were to zoom in right over here,

if I were to zoom in right over there, that line,

it really is a bilayer of phospholipids,

so it would look like this.

So you have your...

hydrophilic heads pointing ouwards,

and your hydrophobic tails pointing inwards.

So hydrophilic heads pointing outwards,

hydrophobic tails pointing inwards,

and it keeps going.

If we think of it from left to right,

you have a layer of two,

or you have a bilayer, I should say, of phospholipids.

That's going to be true of the cellular membrane,

that's going to be true

of the outer nuclear membrane right over here.

We drew this one on the video on the endoplasmic reticulum.

And so over here you see these two membranes.

You might say, "OK, is this a bilayer?"

No, this is actually two bilayers.

So this membrane right over here has a phospholipid bilayer,

and this membrane over here also has a phospholipid...

a phosopholipid bilayer.

Let me do this in another color.

This one that I'm starting to trace in magenta,

that's the outer membrane of the nuclear envelope,

and it's continuous with the membrane

of the endoplasmic reticulum,

which I'm starting to highlight right over here.

And then the one that I'm highlighting

in this purple color,

this is the inner membrane of the nuclear envelope,

and all of this is part of the endomembrane system.

I've already started talking

about the endoplasmic reticulum,

and we go into some depth on that on the video

on the endoplasmic reticulum and the Golgi apparatus,

but it's also part of the endomembrane system.

And the endoplasmic reticulum in particular can represent

up to, or even more than, 50%...

of the phospholipid membrane associated with the cell.

And we've talked about what goes on

in the lumen of the endoplasmic reticulum.

So this area right over here...

we've talked about what happens there.

Proteins can get synthesized.

Actually, other molecules, like lipids,

can get synthesized there.

And then they can go to the smooth ER,

and then the place where they exit from the smooth ER,

and we saw that in the video on the endoplasmic reticulum,

how they can kind of butt out.

We call this area, it's often called the transitional ER.

So this area right over here,

we would call the transitional endoplasmic reticulum.

Transitional...

transitional ER is this place

where these proteins are being butted off,

and they're butting off in vesicles.

So this is the transitional ER.

And all vesicles are are little small compartments

that have a membrane around it,

that things like a protein can be transported in.

And I don't wanna beat a dead horse here,

but all of these lines that I'm drawing,

even though I drew it as a single line,

these are phospholipid bilayers.

So the membrane might be different,

the phospholipid bilayers might be different

when we go from one piece of the membrane to another,

but they all have that same general notion

of having this bilayer of phospholipids.

But just as a review, these proteins,

they can emerge from the transitional ER,

they can make their way to the Golgi apparatus.

And we've already talked about how

in the Golgi apparatus these proteins can be matured.

And when I say being matured,

there's a bunch of enzymes in here,

there's a bunch of Golgi enzymes in here,

they can do all sorts of things to the proteins:

tag them, they can actually add...

add saccharides to them,

so that they become glycoproteins.

They can tag them so they can be used

in the cellular membrane,

or be used outside of the cellular membrane,

or to be used other places in the cell.

So for example, this protein right over here butted off

as a vesicle, it makes its way to the Golgi apparatus,

the membrane can then merge,

and then dump the protein into the Golgi apparatus.

From there it can be matured.

It might turn into a glycoprotein,

who knows what happens to it?

And then it could butt off again,

and then this protein that's now butted off,

it could go to be embedded into the cellular membrane.

The protein could be excreted from the cell,

or it could go to other parts of the cell.

Everything I've just talked about,

those aren't the only parts of the endomembrane system.

You have things like vacuoles,

which are membrane-bound organelles in a cell.

In plant cells, a vacuole can be used for storage,

it could be used for structure.

Vacuoles can get quite large,

and they can actually give, kind of,

the structure of the actual plant.

In animal cells, you might have something called a lysosome.

And a lysosome is a membrane-bound structure

where, essentially, things go to, for the most part,

be recycled, or to be torn apart.

So maybe something got packaged from someplace.

Let me do this in another color.

And I drew that vesicle a little bit too big.

But maybe this stuff, it needs to be destroyed,

so this membrane, it can then merge with that membrane

and dump its contents in here,

and this has a low pH, and it can actually, kind of,

break apart this stuff, and it can digest this stuff,

and recycle it into its, I guess you could say,

more constituent material.

So all of this is part of the endomembrane system.

So once again, I don't think there's an appreciation

for how complex

and, on a lot of levels, beautiful cells can be.