Endoplasmic reticulum and golgi apparatus | Cells | MCAT | Khan Academy

Summary

This educational video script delves into the endoplasmic reticulum, a vital cell organelle connected to the nucleus. It distinguishes between the rough and smooth ER, detailing their roles in protein synthesis and lipid production. The script explains the secretory pathway, highlighting the Golgi apparatus's role in protein modification and sorting. It also touches on post-translational modifications and the movement of proteins to their destinations, such as lysosomes or the cell membrane, emphasizing the interconnectedness of cellular organelles in the secretory process.

Takeaways

• 📍 The endoplasmic reticulum (ER) is an organelle connected to the nucleus and has two parts: rough and smooth.
• 🛢️ The smooth ER synthesizes lipids, including those for the cell membrane and secreted hormones like steroid hormones.
• 🍬 The smooth ER also metabolizes carbohydrates and aids in detoxifying drugs and toxins.
• 🏭 The rough ER, studded with ribosomes, is the site of protein synthesis and contains proteins that will be secreted or become part of cellular structures.
• 🔍 Proteins synthesized in the cytoplasm typically remain in the cytoplasm or are destined for other organelles like the nucleus or mitochondria.
• 🔗 Proteins made in the rough ER follow the secretory pathway, which includes post-translational modifications like disulfide bridge formation.
• 📦 The secretory pathway describes the journey of proteins from synthesis to their final cellular destination, either secretion or integration into membranes.
• 🔑 Proteins intended for the secretory pathway have a signal sequence that directs them to the rough ER for synthesis.
• 📚 The Golgi apparatus modifies, sorts, and directs proteins to their correct destinations, including secretion or use in other organelles like lysosomes.
• 🚀 Vesicles transport proteins from the ER to the Golgi apparatus, where they undergo further modifications before being sent to their final destinations.
• 🔄 The ER, Golgi apparatus, lysosomes, and cell membrane are all part of the cell's secretory pathway, facilitating the transport and integration of proteins.

Q & A

What is the endoplasmic reticulum and how is it related to the nucleus?

The endoplasmic reticulum (ER) is an organelle in the cell that is connected to the nucleus. It has a lumen that is continuous with the space within the nuclear envelope, which is a double membrane structure.

How many parts does the endoplasmic reticulum have and what are they?

The endoplasmic reticulum has two parts: the rough endoplasmic reticulum, which is studded with ribosomes, and the smooth endoplasmic reticulum, which lacks ribosomes.

What is the primary function of the smooth endoplasmic reticulum?

The smooth endoplasmic reticulum synthesizes lipids, including those that become part of the cell membrane or are secreted from the cell, such as steroid hormones. It also metabolizes carbohydrates and aids in the detoxification of drugs and toxins.

What role does the rough endoplasmic reticulum play in protein synthesis?

The rough endoplasmic reticulum is the site of protein synthesis. It contains ribosomes and is responsible for synthesizing proteins that will either be secreted, become part of the cell membrane, or remain within the cell's organelles like the ER, Golgi apparatus, or lysosomes.

How does the rough endoplasmic reticulum differ from ribosomes in the cytoplasm?

Proteins synthesized in the cytoplasm by free ribosomes typically remain in the cytoplasm or are destined for organelles like the nucleus or mitochondria. In contrast, proteins synthesized in the rough endoplasmic reticulum often follow the secretory pathway and are destined for secretion or incorporation into the cell membrane.

What is a signal sequence and how does it relate to protein synthesis in the rough endoplasmic reticulum?

A signal sequence is a specific sequence of amino acids in a protein that indicates it should be synthesized in the rough endoplasmic reticulum. This sequence is detected during translation, causing the ribosome to attach to the ER and complete protein synthesis there.

Can you explain the secretory pathway in the context of protein synthesis?

The secretory pathway describes the route a protein takes from its synthesis in the rough endoplasmic reticulum to its final destination, which could be secretion out of the cell, incorporation into the cell membrane, or residence in organelles like the Golgi apparatus or lysosomes.

What is the role of the Golgi apparatus in the secretory pathway?

The Golgi apparatus modifies, sorts, and directs proteins to their correct destinations. It receives proteins from the endoplasmic reticulum, modifies them, and then packages them into vesicles for transport to their final destinations, such as lysosomes, the cell membrane, or secretion out of the cell.

How does a protein move from the rough endoplasmic reticulum to the Golgi apparatus?

A protein synthesized in the rough endoplasmic reticulum is packaged into a vesicle that buds off from the ER. This vesicle then fuses with the Golgi apparatus, delivering the protein to the cis face of the Golgi, where it begins its journey through the Golgi's stacks.

What happens to a protein within the Golgi apparatus?

Once inside the Golgi apparatus, a protein undergoes further modifications in the cis, medial, and trans stacks. After modification, it is packaged into a vesicle that buds off from the trans face of the Golgi, destined for its final location such as the lysosome, cell membrane, or extracellular space.

Why are the endoplasmic reticulum, Golgi apparatus, lysosomes, and cell membrane grouped together?

These organelles are grouped together because they are all part of the cell's secretory pathway, which handles the synthesis, modification, and transport of proteins and lipids to their final destinations within or outside the cell.

Outlines

00:00

🔬 The Endoplasmic Reticulum and Its Functions

The endoplasmic reticulum (ER) is a cellular organelle closely associated with the nucleus, characterized by its double membrane structure. It is divided into two parts: the rough ER, studded with ribosomes, and the smooth ER, which lacks ribosomes. The smooth ER is involved in lipid synthesis, including those for the cell membrane and secreted substances like steroid hormones. It also metabolizes carbohydrates and aids in detoxifying drugs and toxins. The rough ER, with its ribosomes, is the site of protein synthesis. Proteins synthesized here may be secreted, become part of the cell membrane, or remain within the ER, Golgi apparatus, or lysosomes. A key difference between proteins synthesized in the cytoplasm and those in the rough ER is their destination; cytoplasm-synthesized proteins are for internal use, while rough ER proteins follow the secretory pathway. Proteins destined for the secretory pathway have a signal sequence that directs them to the rough ER for translation completion. Post-translational modifications, such as disulfide bridge formation, also occur in the rough ER.

05:04

📦 The Golgi Apparatus and the Secretory Pathway

The Golgi apparatus is an organelle that processes and sorts proteins synthesized in the rough endoplasmic reticulum. It is composed of stacked sacs known as cisternae. Proteins from the ER are transported to the Golgi in vesicles, where they undergo further modifications. The Golgi apparatus is divided into the cis, medial, and trans stacks, with proteins moving from the cis to the trans stack, undergoing changes at each stage. The final destination of these proteins can vary: they may be sent to lysosomes, secreted outside the cell, or integrated into the cell membrane. Vesicles bud off from the trans stack, carrying proteins to their destinations. If a protein is destined for secretion or to become part of the cell membrane, the vesicle fuses with the cell membrane, releasing its contents. This process illustrates the secretory pathway, highlighting the interconnected roles of the ER, Golgi apparatus, lysosomes, and cell membrane in protein transport and modification within the cell.

Keywords

💡Endoplasmic Reticulum

The endoplasmic reticulum (ER) is an organelle found in eukaryotic cells that plays a crucial role in protein synthesis, lipid metabolism, and detoxification. It is connected to the nuclear envelope and consists of two types: rough and smooth ER. In the script, the ER is described as having a continuous space with the nuclear envelope, highlighting its close association with the cell's nucleus.

💡Nuclear Envelope

The nuclear envelope is a double membrane that surrounds the nucleus of a eukaryotic cell, separating the contents of the nucleus from the cytoplasm. It contains nuclear pores that regulate the passage of molecules in and out of the nucleus. The script mentions the nuclear envelope to illustrate the connection with the endoplasmic reticulum.

💡Rough Endoplasmic Reticulum

The rough endoplasmic reticulum is characterized by the presence of ribosomes on its surface, which are responsible for protein synthesis. It is involved in the folding and modification of proteins that are destined for secretion or inclusion in the cell membrane. The script explains that proteins synthesized in the rough ER are either secreted or become part of the cell membrane or organelles like the Golgi apparatus.

💡Smooth Endoplasmic Reticulum

The smooth endoplasmic reticulum lacks ribosomes and is involved in the synthesis of lipids, including those that are components of the cell membrane or are secreted from the cell, such as steroid hormones. It also plays a role in carbohydrate metabolism and detoxification of drugs and toxins, as mentioned in the script.

💡Ribosomes

Ribosomes are cellular structures that facilitate protein synthesis by translating mRNA into polypeptide chains. They are found in the cytoplasm and attached to the rough ER, where they are involved in the synthesis of proteins destined for the secretory pathway or cell membrane. The script describes ribosomes as 'purple dots' on the rough ER.

💡Secretory Pathway

The secretory pathway refers to the route proteins take from their site of synthesis in the rough ER to their final destination, which could be secretion out of the cell, incorporation into the cell membrane, or localization within organelles like the Golgi apparatus or lysosomes. The script details this pathway, emphasizing the role of the ER and Golgi apparatus in protein processing and transport.

💡Golgi Apparatus

The Golgi apparatus is an organelle that modifies, sorts, and packages proteins and lipids for transport to their final destinations, such as the cell membrane or secretion outside the cell. It is described in the script as a series of 'sacks' that work in a sequential manner, with the cis, medial, and trans stacks each performing specific modifications on proteins.

💡Post-translational Modifications

Post-translational modifications are chemical changes made to proteins after their initial synthesis. These modifications can include the addition of sugar groups, phosphate groups, or the formation of disulfide bridges, which can alter protein function, stability, or localization. The script mentions the formation of disulfide bridges as an example of such modifications occurring in the rough ER.

💡Signal Sequence

A signal sequence is a specific amino acid sequence found in proteins that are destined for secretion or localization to specific organelles. This sequence is recognized during protein synthesis and directs the nascent polypeptide chain to the rough ER for further processing. The script explains that proteins with a signal sequence are targeted to the ER for translation, distinguishing them from those synthesized in the cytoplasm.

💡Vesicles

Vesicles are small, membrane-bound sacs that transport molecules within the cell. They are crucial for the secretory pathway, as they carry proteins from the ER to the Golgi apparatus and then to their final destinations, such as the cell membrane or lysosomes. The script describes the process of vesicle budding from the ER and fusion with the Golgi apparatus.

💡Lysosomes

Lysosomes are organelles containing digestive enzymes that break down waste materials and cellular debris. They play a role in the degradation of proteins and lipids, and can also fuse with vesicles containing proteins from the Golgi apparatus for further processing. The script groups lysosomes with the ER and Golgi apparatus as part of the cell's secretory pathway.

Highlights

The endoplasmic reticulum is an organelle that extends from the nucleus and has a continuous space with the nuclear envelope.

The endoplasmic reticulum is divided into the rough endoplasmic reticulum with ribosomes and the smooth endoplasmic reticulum without ribosomes.

The smooth endoplasmic reticulum synthesizes lipids, including those for cell membranes and secreted steroid hormones.

It also metabolizes carbohydrates and aids in detoxifying drugs and toxins.

The rough endoplasmic reticulum is the site of protein synthesis, with ribosomes attached.

Proteins synthesized in the cytoplasm may end up in various cellular locations, including the nucleus, mitochondria, peroxisomes, or remain in the cytoplasm.

Proteins synthesized in the rough endoplasmic reticulum are destined for secretion, the cell membrane, or to remain within the ER, Golgi apparatus, or lysosomes.

Post-translational modifications of proteins, such as disulfide bridge formation, occur in the rough endoplasmic reticulum.

The secretory pathway describes the journey of a protein from synthesis to its cellular destination or secretion.

Proteins destined for the secretory pathway have a signal sequence that directs them to the rough endoplasmic reticulum for translation.

The Golgi apparatus modifies, sorts, and sends proteins to their final destinations and synthesizes molecules for secretion.

Proteins synthesized in the rough endoplasmic reticulum are packaged into vesicles that bud off and merge with the Golgi apparatus.

The Golgi apparatus has a cis stack, medial stack, and trans stack, each involved in different stages of protein modification and sorting.

Proteins can be directed to the lysosome, secreted from the cell, or integrated into the cell membrane via vesicles from the trans stack.

The endoplasmic reticulum, Golgi apparatus, lysosomes, and cell membrane are all part of the cell's secretory pathway.

Vesicles carrying proteins to be secreted or integrated into the cell membrane merge with the cell membrane, releasing or embedding their cargo.

The cell membrane reforms after vesicles merge with it, completing the process of protein secretion or membrane integration.

Transcripts

- [Voiceover] The endoplasmic reticulum

is an organelle in the cell that butts off of the nucleus.

So, let's say that this is the nucleus of a cell.

It has a nuclear envelope,

which is a double membrane structure.

And this is the endoplasmic reticulum.

Notice that the space in the nuclear envelope

is continuous with the lumen, or space,

of the endoplasmic reticulum.

So, what does the endoplasmic reticulum do?

Well, the endoplasmic reticulum

actually has two parts to it.

It has one part that's known

as the rough endoplasmic reticulum.

And that has a bunch of ribosomes,

those are the purple dots that you see.

And then, the endoplasmic reticulum has a smooth part.

That part does not have any ribosomes.

Let's start with the smooth endoplasmic reticulum first.

So, what does it do?

The smooth endoplasmic reticulum synthesizes lipids...

Including those that will end up

being part of the cell membrane.

And those that are secreted from the cell.

For example, steroid hormones.

The smooth endoplasmic reticulum

also metabolizes carbohydrates.

And it also aids in the detoxification

of drugs and other toxins.

Let's go to the rough endoplasmic reticulum.

So, the rough endoplasmic reticulum, as we mentioned,

has ribosomes, which means it's the site

of protein synthesis.

But, we know that there are also ribosomes

that are in the cytoplasm, so,

what's the difference between those proteins

that are translated in the cytoplasm,

and those that are translated

in the rough endoplasmic reticulum?

So, let's split protein synthesis into two.

We have those...

That are made in the cytoplasm and those

that are synthesized in the rough endoplasmic reticulum.

So, proteins that are synthesized in the cytoplasm

will end up in a number of places.

They might end up in the nucleus.

They might end up in mitochondria,

which, I'm just gonna abbreviate like that.

They might end up in peroxisomes.

Or, they might just stay in the cytoplasm.

Maybe that's where they belong.

In contrast, proteins that are synthesized

in the rough endoplasmic reticulum

will either be secreted into

the extracellular environment...

Or, they will end up becoming into raw proteins.

In the cell membrane.

Or, they might remain in the endoplasmic reticulum,

Golgi apparatus, or lysosomes.

And you'll see in a few moments

why I'm grouping these three organelles together.

And I just wanna mention as a side point

that another thing that happens

in the rough endoplasmic reticulum

is post-translational modifications of proteins.

For example, the formation of disulfide bridges in proteins,

that happens in the rough endoplasmic reticulum.

Okay, let's go back to the protein synthesis

that happens in the rough endoplasmic reticulum.

So, proteins that are secreted from the cell,

or that become part of the cell membrane,

follow what we call the secretory pathway.

The secretory pathway describes the pathway a protein takes

from when it's synthesized until it leaves the cell

or becomes part of a cell membrane.

But, you might be thinking, how does a protein,

quote, unquote, know that it's supposed to be

following the secretory pathway,

and therefore, that it should be synthesized

in the rough endoplasmic reticulum

as opposed to the cytoplasm?

So, the answer to that question is

that old proteins begin to be translated in the cytoplasm.

But, those that need to follow the secretory pathway

have what's called a signal sequence.

That signal sequence is detected early on in translation

and will cause the polypeptide that's being synthesized

to be pushed in to the rough endoplasmic reticulum

where translation is completed.

In order for us to understand the secretory pathway,

we need to talk about another organelle.

That organelle is the Golgi apparatus.

The Golgi apparatus is an organelle that's found

near the endoplasmic reticulum.

And it's basically a group of sacks

that are stacked together.

What happens in the Golgi apparatus?

So, the Golgi apparatus, number one,

modifies proteins that are made

in the rough endoplasmic reticulum.

Number two, the Golgi apparatus sorts

and sends proteins to their proper destinations.

And number three, the Golgi apparatus

synthesizes certain molecules that

need to be secreted from the cell.

So, let's take a look at a protein

that was synthesized in the rough endoplasmic reticulum,

let's say that this part had a couple of ribosomes

and there was a protein made.

Let's say this is the protein.

So, what will happen to it?

So, this protein has to end up

either at the lysosome or outside of the cell,

or as a protein that's part of a cell membrane.

So, it'll butt off in a vesicle.

Here's a vesicle butting off the endoplasmic reticulum.

Of course, the protein is inside of it.

And that vesicle will merge with the Golgi apparatus,

and the protein will end up inside the Golgi apparatus.

And this part of the Golgi apparatus

is known as the cis stack.

The cis stack is the part that's closest

to the endoplasmic reticulum.

Now, this protein that's in the Golgi apparatus

will undergo modifications.

It'll get transferred to the middle part

of the Golgi apparatus.

The middle part is known as the medial stack.

In the medial stack, it'll also be modified

in different ways, and then, it'll eventually land up

in this part of the Golgi apparatus.

This part is known as the trans stack.

The trans stack is the part that's furthest away

from the endoplasmic reticulum.

And from the trans stack, a vesicle will kind of butt off

and that vesicle will be holding the protein in it.

And from here, this protein can take a couple

of different paths.

One thing that might happen to it

is maybe it's destined to land up in the lysosome.

So, let's say this is a lysosome.

So, in this case, the vesicle will move

towards the lysosome, merge with it,

and land up in the lysosome.

I'm gonna digress for just a moment.

If you recall earlier, I grouped together

the endoplasmic reticulum with the Golgi apparatus,

with the lysosomes, and I'm gonna add

one more organelle to this group, the cell membrane.

And the reason that I grouped all these organelles together

is that they're all part of the secretory pathway.

Take a look at the protein we just spoke about.

It was made in the endoplasmic reticulum

where a vesicle butted off, then,

the protein landed up in the Golgi apparatus,

and then, another vesicle popped off

and the protein ended up in the lysosome.

So, all of these organelles have ways

of transferring proteins between them.

Let's say the protein we mentioned

was not supposed to go to the lysosome.

Maybe it was supposed to go to be secreted from the cell,

or maybe, it's supposed to become a protein,

that's part of the cell membrane, so,

let's butt off another two vesicles.

Let's say that this vesicle has in it

proteins that need to be secreted from the cell.

And let's say this vesicle has, on its membrane,

a protein that needs to end up

on the membrane of this cell.

Let's see what happens to these two vesicles.

So, here is the vesicle with the proteins

that need to be secreted.

And here's the vesicle with the protein

that needs to end up in the cell membrane.

So, what'll happen is the vesicle will move

toward the cell membrane, merge with it,

and release the protein.

The same will happen with the other vesicle,

it'll move towards the cell membrane,

merge with it, and that protein will end up

embedded in the membrane of the cell.

And, of course, at some point, these indentations

will flatten out and the cell membrane

will go back to its original shape.