Understanding the Structure and Function of the Golgi Apparatus
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Introduction
In this deep dive into cellular biology, we explore one of the most important organelles in the cell: the Golgi apparatus. Known as the cell's packaging and shipping department, the Golgi apparatus plays a crucial role in modifying, sorting, and delivering proteins throughout the cell. By the end of this article, you'll have a comprehensive understanding of its structure and function—two concepts that are pivotal in cell biology.
Structure of the Golgi Apparatus
Overview
The Golgi apparatus, or Golgi body, is made up of a series of flattened, membrane-bound sacs called cisternae. Its unique structure is not only stunning but functional, allowing it to meet the demands of its role in the cell.
Key Features
- Cisternae: The individual membranes of the Golgi apparatus are called cisternae, which stack to form the overall structure.
- CIS and TRANS Faces: The Golgi apparatus has two distinct sides:
- CIS Golgi: This side faces the rough endoplasmic reticulum (ER) and is responsible for receiving proteins in vesicles that bud off from the ER.
- TRANS Golgi: The opposite side faces the cell membrane and is where proteins are dispatched to their respective destinations.
- Lumen: The interior space of the Golgi apparatus where various modifications of proteins occur.
Relation to Other Organelles
To fully understand the Golgi apparatus, it’s essential to relate it to other organelles:
- Rough Endoplasmic Reticulum: Proteins synthesized and modified in the rough ER will be packaged into vesicles that travel to the CIS Golgi.
- Lysosomes: Some proteins modified by the Golgi apparatus are sent to lysosomes for degradation of cellular waste.
Function of the Golgi Apparatus
Overview
The Golgi apparatus is vital for protein processing and trafficking within cells. Its functions can be categorized into several processes.
1. Modification of Proteins
Once proteins reach the CIS Golgi, they undergo essential modifications:
- N-linked Glycosylation: This process involves adding a carbohydrate chain to the asparagine amino acid of a protein. It occurs in the rough ER before proteins reach the Golgi.
- O-linked Glycosylation: Occurs in the Golgi, this involves adding sugars to the hydroxyl groups of serine and threonine amino acids.
- Phosphorylation: Specifically, adding a phosphate group to mannose sugar residues which directs proteins to lysosomes.
2. Sorting and Packaging
Once proteins have undergone modifications, the Golgi apparatus sorts them and determines their final destination. This is vital for cellular function:
- Destination A: Membrane Proteins—Proteins destined for the cell membrane are often packaged in vesicles coated with clathrin which directs their transport.
- Destination B: Exocytosis—Some proteins are exported out of the cell (exocytosis) as part of cellular communication.
- Destination C: Lysosomal Enzymes—Proteins that have the phosphorylated mannose signal are tagged to become lysosomal enzymes.
3. Recycling of Proteins
The Golgi apparatus also helps recycle proteins that are no longer needed:
- Endocytosis: This process involves internalizing worn-out proteins in vesicles, forming endosomes which can be sent back to the Golgi for recycling and modification before they are sent out again.
Detailed Processes of Protein Modification
Functions and Mechanisms
- Trimming: Involves the removal of sugar residues from glycoproteins which can activate or deactivate the protein.
- Additions: Besides O-linked glycosylation, phosphorylation of mannose is crucial for lysosome-targeted proteins.
Summary of Protein Types and Modifications
After processing through the Golgi apparatus:
- Glycoproteins: Involved in signaling and cell recognition will either be excreted, integrated into the cell membrane, or become part of lysosomes.
- Final Destination Tags: Each processed protein carries a tag that indicates where it needs to go, similar to how packages are sent in a delivery system.
Conclusion
The Golgi apparatus is an indispensable organelle in cellular function that works tirelessly to ensure proteins are correctly modified, sorted, and sent to their designated locations. Understanding the Golgi apparatus’s structure and function provides critical insights into the complexity of cellular activities that sustain life. Whether it's processing proteins for secretion or sending enzymes to the lysosomes, the Golgi apparatus is central to the functioning of living cells. For those venturing deeper into cell biology, recognizing the Golgi's role can pave the way for further exploration into the intricate web of life at the cellular level.
the Golgi apparatus we'll go over its structure and function before we get started if you guys like this video it
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you to our website where you guys can check all that stuff out but Golgi apparatus what is the structure so it's
a really cool structure it's one of the organelles inside of the cell so at this point we've obviously talked
about the nucleus we talked about the endoplasmic reticulum those are all organelles the Golgi apparatus is a
really cool organelle now when we talk about this I like to think about its structure as though it's kind of like
this folded membrane right so it's this kind of like folded membrane if you will and there's a couple different anatomic
or structural points that you need to know so here we have the nucleus right so here is going to be the nucleus we
already talked about this in great detail here we have our rough endoplasmic reticulum we talked about
that in great detail and here you're going to have the Golgi apparatus so again it does have this kind of like
convoluted or kind of like coiled membrane if you will and again it's a phospholipid bilayer around that but one
of the big things is on one Edge so the edge facing the rough endoplasmic reticulum is a very specific size so
this is called the CIS Golgi so the CIS Golgi is the part of the Golgi which is near the rough endoplasmic reticulum the
basic concept behind this that we'll talk about a little bit later is that whenever the rough endoplasmic reticulum
is done modifying and synthesizing particular proteins what it'll do is it'll butt off a little vesicle that
contains its particular proteins that have been modified in a particular way to go to the golgiene that goes on the
that it's destined to go to the Golgi is that we put specific types of proteins on its surface that really kind
of says hey buddy it's time for you to specifically go to the Golgi on the side you guys remember what that protein was
called here these are called your Cop 2 proteins and they're really helpful in being able to send this vesicle from the
rafiar to the Golgi so that's one side of the Golgi the CIS Golgi then you have obviously inside of the Golgi the Lumen
that's where some of the reactions are going to take place that we'll talk about in a little bit but on the other
side of the Golgi this side that is going to be facing towards the cell membrane so here's your cell
think about in two ways one is after all the modifications occur inside of the Golgi to that protein what it does is it
does something really cool where it'll Bud off the modified protein in a particular way where it's then sent to
either go to the cell membrane and from here it can be excreted or become a part of the membrane or it can also do
something else where it turns into we'll give it a different color so we understand it's a different thing it can
get turned into lysosomes so that's the other aspect of the Golgi and again inside it it'll have this
protein that'll undergo a bunch of different types of modifications so that's really kind of when you talk
about the Golgi the basic structure of it is there's this convex let's actually write that down as well the convex side
also known as the CIS Golgi also known as the side of the Golgi facing the rough endoplasmic reticulum or the trans
Golgi this is the concave side this is the con cave side also known as the trans Golgi also known as the Golgi that
faces the cell membrane or those lysosomes that it's nearby all right so before we move on and start talking
about the function of the Golgi apparatus I want you guys to watch a quick little animated video by our
friends nucleus medical media they got a really awesome video that will give you a quick little recap but in a cool
animated way check that out and then we'll talk about the function of the Golgi apparatus in more detail proteins
where the Golgi apparatus sometimes called the Golgi body receives them as proteins move through the Golgi body
they're customized into forms that the cell can use the Golgi body does this by folding the
now so one of the big things that I want you guys to understand is Golgi apparatus with the function of it it's a
really cool structure and we have a little bit of an idea here that it has three particular types of functions one
of the biggest functions is it modifies proteins right so it seems those vesicles we already kind of talked about
this a little bit to recap it again because it's always good for repetition inside of the nucleus is your DNA DNA
and then bind onto all these ribosomes that are placed on the rough endoplasmic reticulum and then from here it'll then
do what what we know is it can get translated and when it gets translated it synthesizes some specific molecules
like proteins which are going to draw with these like Globs here these are my proteins now what happens is the
proteins go through a particular process in the rough ER that we already talked about called in-linked glycosylation so
they'll run through the rough ER and then after they run through the rough ER we'll just quickly recap here
what did they have what happened here in the rafiar again well when it's going through the rough ER we said that they
add on a sugar residue and oligosaccharide and they added on to if you guys remember they added on to
asparaging that's why we call it the in-linked glycosylation so if you guys remember just as a quick recap in the
rough endoplasmic reticulum the particular reaction that took place was what's called n linked
a sugar molecule here's your sugar molecule you're going to add it on to a protein but on this protein there's a
specific amino acid and this amino acid here which we're going to kind of show like this is
molecule really asparagine has a amino group on it and there's the N that's linked to our sugar residue that's why
we call in-linked glycosylation but protein's been synthesized at the rough ER modified a little bit in the rough ER
via in-link glycosylation and then packaged into a vesicle and then once it's packaged into this vesicle it says
all right baby let's send you on to the next Point here and the next thing I'm going to do is
I'm going to send you over to the gold Gene do you guys remember which part of the Golgi face the rough endoplasmic
reticulum it was the CIS Golgi or the convex side of the Golgi and in it it's got this Protein
that's undergone some small modifications you guys also remember what was the protein on the surface of
Cop 2 protein and that will determine its direction of saying hey you need to go to the Golgi and then what it'll do
is it'll fuse with the actual membrane of the Golgi because they're both phospholipids and lipid will dissolve in
lipid we know that so when it fuses here it'll fuse with this Golgi membrane and then release its
contents which is this glycoprotein where right into the Golgi apparatus And Then There She Goes to get further
there's a reason I mentioned this Cop 2 protein so cop2 protein allows for One Direction this direction from rough ER
to the Golgi well there's probably a cop one protein in there yeah you know it's funny
sometimes if I want certain proteins from the Golgi to go back to the rough ER for whatever reason maybe more
modification processes need to occur whatever it may need to happen sometimes the Golgi will Bud off
particular molecules here it'll butt off little vesicles and these vesicles will then actually contain some specific
I want to send back a protein to the rough endoplasmic reticulum maybe for some particular reason to get modified
fashion where let's say that they move from Golgi to Golgi all the way from the trans side to the CIS side and then from
here it buds off and then tries to move its way backward towards the rough endoplasmic reticulum
where it can then actually do what maybe it fuses and releases its contents back in this is another
particular protein that has to regulate this do you guys know what this protein is that actually is on the outer surface
that determines it going backwards from the trans Golgi to the Cisco due to the rough ER because what that is this
protein on the surface is called cop1 this is called cop1 so what I want you to know is cop1 allows for the transport
of vesicles containing glycoproteins from the trans Golgi to the Cisco Golgi to the rough endoplasmic reticulum cop2
controls the movement of the glycoprotein from the rough ER to the CIS Golgi that's what I want you guys to
now once this is continuing to move anterior grade towards the the end direction which is cell membrane
lysosomes that's the whole point once it gets into this part here so we're going to release from this vesicle our
so there's a couple different things that these enzymes that are present inside the Golgi will do you know what
they'll do they'll do a bunch of different modifications believe it or not so it's
going to run through this like a football player in practice and undergo a bunch of different modifications and
by the time it's completely modified what it's going to have at the end of it now is the fully packaged modified
protein that now has a very specific destination for it so okay now here's my protein and maybe
I've modified it a little bit more here's my protein and then on it let's just say I have here is a little bit of
different I got all these different things that are added onto it what are the reactions that occur here in the
Golgi that are a little bit different let's write these down the first thing is I take this protein
right and let's say here I've already had this process occur I've already had let's say
trim a little bit of this I can trim the sugar residues off of this in link glycosylation so that's one thing I can
protein a little bit more specific to be activated so now if I were to have this protein maybe instead
to activate this particular protein so now all I did was I just trimmed it a little bit and now in this state Maybe
it's active this is an active Protein that's ready to be released or incorporated into the membrane or become
a part of lysosomal enzymes the second thing I could do that's one thing I could do one thing I could do is
I could trim it the second thing I could do is I could add another sugar residue but not to an amine group I could add it
to a hydroxy group the oxygen group and so if I do that we actually call since in linked was I was adding a sugar to
the amine group of an amino acid well o linked glycosylation which only occurs in the Golgi
acid so you see notice how there's no kind of sugar residue now if I draw this one here look at this puppy here so now
I got this next one here this has gotten a mean group here's I got our nh2 group and maybe it
this I want to add a sugar residue onto this one so now I have a sugar residue on the hydroxy group of two
particular amino acids you don't have to go crazy but the two amino acids here that it can add
threonine for the amine group this one here is called we're going to abbreviate asparagine
Inland glycosylation add a sugar to the hydroxy carbon the O the oxygen carb the oxygen molecule of the protein the
glycoprotein and the third thing I can do which is really important is I could do what's called phosph
that Sugar residue called mannose so you see how on this there's a sugar residue let's just say
on that one of the sugars I'm gonna put is mannose so maybe on this one there's a little
Mano sugar maybe on this one there's a little Mano sugar same kind of Concepts I'm going to add a
here you have hydroxy group let's say that there's a sugar molecule on either one of these it doesn't really
there is a mano sugar here and a mannose sugar here on that Sugar residue okay if I were to put a phosphate on
these it activates them very very specifically so I put a phosphate group onto this and when I phosphorylate the
mannose of these sugar residues on the glycoprotein it activates it makes it only disestant for one particular point
this is active and it will only be active for lysosomes it'll only these proteins will only go
to become lysosomes super important so this is the protein modification one is I can trim it the sugar on the
in-linked component one is I can add a sugar on the oxygen of serine threatening the third thing is I could
phosphorylate mannose on the sugar residues and make it only particular where these proteins only become a part
of the proteins of lysosomes so now all this is occurring in here what types of things again trim
and then the most important one is phosphorylation phosphorylation and this phosphorylation
little sugar residues on there on different points right or maybe I add a phosphate group somewhere on that man of
sugar all these different things have occurred after it's done it's then said today okay well I've done
that have been you know completely activated in a very specific way and they're ready whether it be trimming
whether it be in-link glycosylation o-link glycosylation they're done from here we can do two things with these
one is I could fuse them with the cell membrane second thing is they could fuse with the cell membrane and be excreted
they could be excreted right we've already talked about this a couple times so now this could actually be
excreted out of the cell or it could become what else lysosomes and the only way that we know
that you'll make lysosomal enzymes is you have the glycoprotein undergo a very specific type of reaction it'll have the
inland glycosylation it'll have the oling glycosylation but on that Sugar residue there's one
lysosomes so the destination the Golgi says I know where you need to go I'm going to send you to become a part of
the membrane I'm going to send you to be excreted or I'm going to send you to become lysosomes that's another function
a really cool function of the Golgi apparatus and so from here it can actually determine the three
or I can send you to become a part of the become a membrane protein so maybe this glycoprotein is going to
be a channel protein maybe it's going to be an ion maybe it's going to be a carrier who knows what the heck it's
and then determines their destination because you know what's really cool is when it modifies it in a particular way
it modifies it to where it gives this thing a specific signal sequence so these have like a specific signal
sequence so it's kind of like a little like a like an Amazon thing like when you go to Amazon they have to package
particular things and determine where they're going to go they put a little tag on these proteins these
glycoproteins that says hey here's where you need to go and then it'll help to guide them towards these particular
locations so if I want this to go to the cell membrane I can actually use specific proteins let's represent this
one with let's do it here with pink why not I can put little proteins on this that coat these and make sure that
they're destined to go towards the cell membrane to either be excreted or to also be again a part of the cell
okay and they'll coat these vesicles and say hey you need to go to the cell membrane to either be released or do
fuse with the cell membrane the last particular mechanism of the Golgi apparatus is it can recycle
that it's done its job it's completely finished its life cycle and says hey you're done buddy what I want you to do
is so let's say it's this protein let's say this let's say that the Golgi manufactured this particular protein
right here and it did its job so it helped to be able to modify it determine its destination incorporates into the
to actually be and we got to kind of fix you up a little bit take you back in what it'll do is it'll invaginate this
and whenever it gets broken down what it may do is it may release these proteins back into the Golgi to be further
modified repackaged and sent back out to go back to the cell membrane so that's another
first thing is it's involved in protein modification receives proteins from who the rough ER going to which side the CIS
Golgi how does it know to go there cop two if there's proteins that are determining it to go back to the rough
ER for maybe further modifications how does it need to go back there from trans to CIS to rough ER cop one is a protein
released into the Golgi the Golgi modifies it by which ways Cuts some of the sugar off of the end
linked point of the amino acid asparagine or add some sugar residues on the hydroxyl or the oxygen group of an
and finds one specific sugar mannose and adds a phosphate group onto it phosphorylation reactions
once it does that that's the three ways that we will activate the proteins and then determine their destination once
we've done this we can then say okay I can send these to become lysosomes membrane proteins are being excreted if
I coat them with clathrons they're usually destined for the membrane or to be excreted if it has this mano6
are maybe worn down and need to undergo recycling we need to fix them up a little bit we can bring them in and the
endocytosis this process here going from here to here is a endocytosis and we'll make this thing called a endosome
and endosomes technically they get broken down a little bit by something called a lysosome which you already knew
taken back up by the Golgi and get recycled and then if they get recycled we can kind of do some modifications to
them repackage them and send them back out so that's a cool concept all right my friends this video now
covers the Golgi apparatus all the structure and the functional components I hope it made sense I hope that you