Understanding the Endoplasmic Reticulum: Structure and Functions Explained
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Introduction
Welcome, Ninja nerds! In this lesson, we are diving deep into the endoplasmic reticulum (ER), a crucial component of cellular biology. The endoplasmic reticulum is an intricate structure that serves multiple essential functions within the cell. We will explore the two types of endoplasmic reticulum, the rough ER and smooth ER, examining their structures, functions, and the key roles they play in cellular processes.
Types of Endoplasmic Reticulum
The endoplasmic reticulum is primarily categorized into two distinct types:
1. Rough Endoplasmic Reticulum (RER)
The rough ER is characterized by its ribosome-studded surface, giving it a rough appearance under a microscope. It is flanked by tubular structures and is continuous with the outer membrane of the nuclear envelope. The ribosomes present on its surface are pivotal in protein synthesis.
Functions of Rough ER:
- Protein Synthesis: The primary role of the rough ER is to synthesize proteins. After transcription in the nucleus, messenger RNA (mRNA) travels to the ribosomes on the rough ER where it is translated into polypeptide chains.
- Modification of Proteins: Proteins synthesized in the rough ER undergo several modifications, such as N-linked glycosylation, where sugar molecules are added, transforming them into glycoproteins necessary for cell surface chemistry and signaling.
- Vesicular Transport: Following synthesis and modification, proteins are packaged into vesicles tagged with specific proteins (COPII) that direct them to the Golgi apparatus for further processing.
2. Smooth Endoplasmic Reticulum (SER)
The smooth ER lacks ribosomes, giving it a more tubular and smooth appearance. It is also continuous with the nuclear envelope but engages in different metabolic processes.
Functions of Smooth ER:
- Lipid Synthesis: The smooth ER is crucial for synthesizing lipids, including fatty acids, phospholipids, and steroids. It utilizes acetyl-CoA as a substrate for biosynthesis.
- Biotransformation (Detoxification): This organelle plays a critical role in detoxifying harmful substances, such as drugs and alcohol, primarily in liver cells. The cytochrome P450 enzyme system within the smooth ER helps metabolize and convert these substances into more water-soluble forms for easier excretion.
- Calcium Storage and Regulation: In muscle cells, a specialized form of smooth ER, known as the sarcoplasmic reticulum, stores calcium ions, which are essential for muscle contraction and relaxation processes.
- Glucose Release: The smooth ER functions in gluconeogenesis, the process of synthesizing glucose from non-carbohydrate sources, by facilitating the conversion of glucose-6-phosphate to glucose using the enzyme glucose-6-phosphatase, allowing glucose to exit the hepatocyte.
In-Depth Functions of the Endoplasmic Reticulum
Protein Synthesis and Modification in Rough ER
The rough ER is particularly abundant in cells with high levels of protein synthesis. Notable examples include:
- Pancreatic Exocrine Cells: These cells produce digestive enzymes that are secreted into the gastrointestinal tract.
- Plasma Cells: These immune cells are responsible for producing large quantities of antibodies.
- Goblet Cells: Found in the lining of the intestines, they produce mucins, crucial for lubrication and protective barriers.
Lipid Biosynthesis in Smooth ER
The smooth ER is fundamental in several biosynthetic pathways:
- Fatty Acids and Triglycerides: Synthesis of fatty acids occurs via a series of enzymatic reactions. Fatty acids can subsequently combine with glycerol to form triglycerides.
- Phospholipids: Crucial for membrane integrity and formation, phospholipid synthesis occurs predominantly in the smooth ER.
- Cholesterol and Steroid Hormone Production: The smooth ER synthesizes cholesterol, which serves as a precursor for steroid hormones such as testosterone and estrogen, especially in gonadal and adrenal gland cells.
Biotransformation and Detoxification Processes
The liver's smooth ER is rich in the cytochrome P450 enzyme family that facilitates:
- Metabolism of Drugs and Toxins: Acetaminophen and alcohol are examples of substances metabolically transformed into less harmful products due to this pathway.
- Synthesis of Bile Acids: Essential for digestion, bile acids are derived from cholesterol in hepatocytes.
Conclusion
In summary, the endoplasmic reticulum is a vital organelle that is divided into the rough and smooth types, each with distinct structural features and functions. The rough ER specializes in protein synthesis and modification, while the smooth ER is pivotal for lipid synthesis, detoxification, glucose regulation, and calcium storage in muscle cells. Understanding the endoplasmic reticulum's structure and functions is essential for grasping complex biochemical processes within the cell. Remember, it's about understanding these processes rather than rote memorization!
Thank you for watching, and as always, until next time!
the endoplasmic reticulum we'll go over the structure and the function of it all right so first thing before we get
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to go check that out all right let's start talking about the endoplasmic reticulum and there's two types so we
have to basically go over their structure which thank goodness it's not too bad their functions a lot more
involved though but there's two types one is the rough endoplasmic reticulum the rough endoplasmic reticulum okay and
I'm going to write rough endoplasmic reticulum right if you guys need it it's right there but the rough endoplasmic
reticulum the big thing to know is that this is a again a tubular structure that is continuous with the outer
membrane of the nuclear envelope but the big difference here is that again there's another structure here this one
here in this kind of like maroonish color is called the smooth ER or endoplasmic reticulum it's also a
and they're both having their membranes made up of phospholipid bilayers the big difference between the two is you see
these red dots that are kind of like studded on the outside of the rough ER that's why we call it rough it has many
difference and that's it let's not make it too complicated for the structure of the endoplasmic reticulum it's a
membranous tubular structure made up of a phospholipid bilayer that is continuous on the outer membrane of the
nuclear envelope rough ER started with ribosomes smooth ER no ribosomes studied on the surface after ribosomes leave the
nucleus they will have the important job of synthesizing or making proteins outside the nucleus the ribosomes and
sometimes abbreviated as ER there are two types of er rough ER has ribosomes attached to it and smooth ER
doesn't have ribosomes attached to it the endoplasmic reticulum is a membrane-enclosed passageway for
transporting materials such as the proteins synthesized by ribosomes proteins and other materials emerge from
where the Golgi apparatus sometimes called the Golgi body receives them let's now dig right into the functions
right let's not waste any time endoplasmic reticulum functions particularly pertaining to the rough ER
so we're going to talk about that one first the one that started with ribosomes what is his job
really the big thing is that all the time when you're in school you get stuck with having to memorize these like lines
oh it's involved in approaching synthesis and modification and sending no no don't memorize understand
so first thing here as we have here the nuclear the nucleus if you will inside the nucleus you have
the DNA now DNA is really cool in the sense that what it can do is it can make something called RNA via a process
called transcription so here the particular one that we care about is called mRNA and no RNA is basically
pores it goes and it binds onto these ribosomes that are studded on the surface of the rough ER here's again
another mRNA and they're going to bind onto this the ribosomes in combination with TRNA
it's going to synthesize proteins and what we're just going to do is we're going to show a couple of them
here so here what it's going to do is the MRNA will be translated by the ribosome the MRNA it'll be translated by
the ribosomes and make proteins here's my proteins this protein once it's pushed into the
the endoplasmic reticulum and along the way it undergoes modification so this protein is not fit to be excreted yet we
got to modify it a little bit and tag it so you know we do here's the protein right here's the
protein here's this enzyme this enzyme is going to do something very interesting where in one hand it's
going to have the protein it's going to bind onto the protein but it binds onto a very specific amino acid on the
something called oligosaccharides which is basically sugar molecules and what it's going to do is it's going
asparagine it's a very specific one and then what it's going to do is it's going to take this sugar we call this
oligo that's a heck of a name saccharide so I just like to put sugar it's going to take this and add that on to the
what's going to be linked onto this oligosaccharide molecule this was the protein non-modified this
is the protein modified you know what they call this and this is why I'm kind of focusing on it because there's a
glycosylation glycosylation this is one of the primary ways that the rough ER modifies son of a gun glycosylation
modifies specific types of proteins now if you think about it you're taking a protein and adding a
sugar residue onto it you know what you're really making it like glycoprotein and glycoproteins are
really interesting in the sense that when we make glycoproteins there really only is a couple destinations that are
glycoprotein that I've kind of helped to synthesize and modify a little bit so here I'm going to draw some
oligosaccharides that are coming off it there's my glycoprotein I'm going to butt it off of the rough
endoplasmic reticulum into like a little vesicle if you will all right so here's this vesicle that comes off of the rough
endoplasmic reticulum and inside of it it's going to have my protein and then connect it to it it's going to
tags this vesicle and says hey you have to go to the Golgi apparatus you know what this protein is called
basically a tag that says you need to go from the rough endoplasmic reticulum to the Golgi apparatus and then from here
it moves nicely towards the Golgi this is your Golgi and there's two ends the CIS end which is coming from the rafiar
with this structure here it'll fuse beautifully with the Golgi apparatus and then what it'll do is it'll say here
you go Golgi here's this protein that I synthesized and I modified a little bit by doing
but then after that I want you to package him and I want you to send him to three locations that you feel
appropriate so we're not going to go through the mechanisms of the Golgi right now we'll cover that in the Golgi
to send this glycoprotein to one of them is I can send it to the cell membrane to be incorporated into the cell membrane
right so now what I can do is I can take this vesicle here and I can have it Go fuse with the cell
membrane and if it fuses with the cell membrane maybe this will become some type of channel protein or Carrier
protein some type of protein here that is incorporated into the cell membrane that's one thing
this could be a lot of different things we already talked about what membrane proteins do this is a glycoprotein that
maybe is involved in carrying maybe some type of Channel maybe it's a peripheral protein that acts as an enzyme maybe it
links different things together it's a membrane protein that's one so one thing I can do is as I can send
this to the membrane second thing I can do is I can actually cause this to go to the cell membrane
this particular protein so now what I'm going to do is I'm going to release this protein out into the wild to be free
and this glycoprotein maybe it's a hormone maybe it's a neurotransmitter but this is a secretory protein so one
second thing is it'll become a secretory protein maybe this will become like a hormone of
that really is a nasty nasty organelle and it can actually become very helpful in breaking down other different things
this could be a part of lysosomes this could be a part of lysosome so the third thing it could become is
this is the job of the rough endoplasmic reticulum so when you get a hint of it it is heavily involved in the synthesis
of the proteins push into Lumen modified via inland glycosylation makes vesicles and tags them to send
them to the Golgi Golgi does some more modifications and then says okay rough ER told me to do three things with this
incorporate into the cell membrane secret it or turn it into lysosomal proteins that's it nothing crazy to it
so now instead of memorizing words we understand this but the next step that we have to take here is if we say that
what type of cells in the body if you were asked this on a test are rich in rough ER because they synthesize lots of
exocrine portion of the pancreas so the pancreas particularly the exocrine portion because these men whoo they'd be
pumping out those pancreatic digestive enzymes so many digestive enzymes that need to be released to break down all
the food that we eat these ones plasma cells you know what plasma cells are responsible for these
things pump out antibodies antibodies are glycoproteins my friends so because they need to make tons and tons of
antibodies these should have a very heavy type of rough ER another one is called goblet cells you
find these a lot in like your tracheal lining your stomach lining and they produce massive amounts of mucin which
is a type of protein and that is another one lots and lots of mucine proteins to coat the lining of the cells
and the last one is neurons neurons are a big one and we actually give this specific ER that's found in the Soma or
the cell body of the neuron a very specific name we called a nissl body the missile body all it is you take
what's called initial stain so basically the rough ER right it has like this charge to it so what you do is you give
them a particular die and the die will kind of bind with the proteins of the rough ER and give it like this kind of
stained color and that's what we call the nissl body but basically neurons they need to have lots of these because
so that's the big concept that I need you guys to understand so we know that rough ER is involved heavily in the
synthesis of proteins the modification of proteins the packaging of them to send them to the Golgi and then from
there they are destined for three particular end points that's the big thing to take away and then which cells
are rich in them the ones that make lots of proteins all right now let's talk about the smooth ER all right my friends
smooth endoplasmic reticulum the functions what does it do really the big thing that I want you to remember okay
we talked about how the rough ER is a site of protein synthesis modification of proteins package and sends them to
the Golgi from the gold you can become membrane excretory or lysosomal we talked about that what I want you to
remember for smooth ER is it's really the site of lipid synthesis so what do you mean here okay watch this
you know uh we can make a couple different types of lipids and really it all starts with this very special
molecule that we have within ourselves called acetyl COA you guys probably remember that right we take like
something like glucose and we can convert this into acetyl COA certain amino acid can become acetyl COA a lot
of different things will actually convert into acetyl-coa but acetyl we know we can actually go into the Krebs
into the smooth endoplasmic reticulum and within the smooth endoplasmic reticulum what happens is the acetyl COA
can go through various different types of reactions which are carried out by multiple multiple enzymes but I'm not
going to go through here I'm just going to give you that acetyl COA it can go through many different enzymatic steps
and what can happen is is we can take acetyl-coa and start linking these two carbon chains together and then what you
can make out of this is maybe you make a fatty acid right so maybe from this one of the things that you make is a fatty
acids and usually the enzymes that are really heavily involved in this particular process here if you really
wanted to remember it it could be done via what's called fatty acid synthase right that's one thing that we could do
the second thing is we can do is we can take a seat up we can take these fatty acids that you make from acetyl-coa and
combine them with phospholipids so I can add on like a phosphate group on to that I can add
phospholipids now the question that you probably have from this is okay what would I do so I know it's synthesizing
fatty acids I know it's synthesizing phospholipids what can we do with those fatty acids
you can do a lot of things with fatty acids with fatty acids I could I could make things like triglycerides
I could do things like utilizer for ATP production right so I could break down fatty acids via beta oxidation to make
ATP I could combine them with glycerol and make triglycerides I can do a lot of different things with this you know what
else I could do with it I could make phospholipids you know phospholipids could actually become phospholipids are
great for being a part of what your cell membrane we already talked about this so many different things it's really
helpful that phospholipid bilayer you know what else we could do we could take the acetyl-coa through
multiple multiple steps we can make this big kind of like ring structure here that you could have it's super cool this
thing could be cholesterol so what else could we synthesize we could synthesize cholesterol
now cholesterol is really really cool because cholesterol we already know can do so many things it can be incorporated
into your cell membrane you can use it to make bile acids which is helpful for being able to emulsify
or other types of steroid hormones so that's a really really important thing to be able to take away from this
the smooth endoplasmic reticulum they help to be able to take all of these things like acetyl-coa convert into all
these other intermediates and substrates and then form fatty acids phospholipids or cholesterol
fatty acids can be utilized for ATP via beta oxidation they can be used to make triglycerides of a combined with
glycerol you can make phospholipids if you add a phosphate group and other types of residuals onto them which can
be incorporated into the cell membrane and you can make cholesterol which can help to make cell membrane components
fluidity it can also be involved in bile acids lipoproteins steroid and sex hormones let's actually dive into that a
when you know that it helps to be able to make things like sex hormones if you think about this in the testes the
testes would have a very very rich smooth endoplasmic reticulum right so the testes in the ovaries have to have a
what the pancreatic exocrine cells it's rich in the plasma cells it's rich in the goblet cells or in the neurons and
the same thing testes what do they make testosterone so actually take the cholesterol and conform it to make a
very special hormone testosterone ovaries to make things like estrogen they'll make things like progesterone
so that's another thing all of these things are derived from cholesterol so from here you can actually take
cholesterol and modify it a little bit more and when you modify it you can make these sex hormones same concept exists
we have adrenal gland and on the outer parts of the adrenal glandia the adrenal cortex they're also super rich in the
smooth endoplasmic reticulum they'll take all the acetyl-coa make a bunch of cholesterol modify the
cholesterol and make steroid hormones some of these they can make is what's called cortisol
this is made by something called the Zona fasciculata then they can make another one called
and they can also make other types of random gonadocorticoids which we can call a dehydroxy epion dostron and
epiongosterone these are gonadocorticoids but you see how they again smoothen the plasma curriculum
makes cholesterol and then they use enzymes to modify that cholesterol to make steroid hormones or modify to make
these hepatocytes hepatocytes are also really cool because they have a lot of smooth endoplasmic reticulum in them
and the reason why is they can take this cholesterol that they synthesize and modify it to make two particular things
they can modify to make something called bile acids which are really important for being able to help emulsify big fat
vldl things like HDL all of these things are helpful in that structure so that's really really important when
we think about what the actual smooth ER does for lipid synthesis it helps making fatty acids phospholipids cholesterol
and then in certain cells you can take that cholesterol and again cells that are rich and smooth endoplasmic
reticulum and make hormones such as sex hormones and intestines in the ovaries testosterone estrogen progesterone
corticosteroids or mineral corticoids present in the adrenal cortex you can take the cholesterol mixed steroid
hormones or you can make bile acids and lipoproteins in the hepatocytes so really really important there
all right what else does the smooth ER do a lot of other things but the big thing that we've covered so far is lipid
synthesis fatty acids phospholipids cholesterol from the cholesterol steroid hormone sex hormones bylasses
lipoproteins the other thing that's really really important for is in hepatocyte so this
is our liver all right this is our liver and the liver it's involved in what's called biotransformation This is
basically drug and toxin metabolism that's the fancy word for it so here we have hepatocytes these are our
hepatocytes and these are parasites we already know are rich and smooth ER for what lipoproteins bile acids but you
know what else it's rich in smoothie R for another particular thing called biotransformation so what I want to do
is I want to zoom in on these hepatocytes and take a look here's my smooth endoplasmic reticulum
in the smooth endoplasmic reticulum there is a special enzyme this is called the cyp450 the cytochrome p450 system
endoplasmic reticulum they interact with the cytochrome p450 system and the basic thing that happens out of
this is that the cytochrome p450 system will take and metabolize these drugs toxins or alcohol by adding on different
types of groups onto them maybe adding hydroxyl groups onto them maybe adding like these like these groups onto them
that make them super super water soluble so basically it takes a drug takes a toxin takes alcohol metabolizes them
excrete into the urine so it makes it easy because now the smooth endoplasmic reticulum will
metabolize the drugs the alcohol the toxins making them water soluble will enhance the ease of excretion
of these drugs Isn't that cool so a parasites are really rich and what smooth ER for what type of function one
is lipid synthesis for cholesterol because they're able to make steroid I'm sorry bile acids and also lipoproteins
they're involved in the sex organs the adrenal cortex fatty acid synthesis phospholipid
synthesis they also play a role in biotransformation biotransformation is again what taking drugs toxins alcohol
neutralizing and metabolizing them making them water soluble easy to be excreted okay so we got bio
transformation and lipid synthesis for the smooth ER what else does it do another really cool function here
is it plays a role in glucose release so again here's the hepatocytes here's your liver right
and then these hepatocytes again we're kind of noticing a trend here that the liver cells are super rich and smooth
they are so are they test these ovaries in the adrenal cortex but hepatocytes have been kind of an overarching theme
that we've noticed within the hepatocytes we know that they're rich and smooth they are
gluconeogenesis and basically this is where the liver will synthesize glucose from non-carbohydrate sources
things like lactate things like amino acids things like odd chain fatty acids things to that effect now here's what's
really important when glucose gets brought into the cell when glucose is brought into the cell in
some particular way right usually through a reaction it'll undergo this process where it gets converted
into what's called glucose 6-phosphate right and that's via a glucokinase or a hexokinase process so we add on a
it can't leave the cell so because of that it is stuck in the cell so if I want to get this glucose out of
the cell I have to go back this way the only way glucose can leave is in this fashion it can come in as glucose and it
the glucose 6-phosphate will get reacted with these enzymes called glucose 6 phosphatases and what they will do is is
they will rip off that phosphate and convert it into glucose so that we can pump the glucose
out of the hepatocyte so that is the concept that I want you to understand is that in the actual
tase and what it'll do is it'll cut this phosphate off of the six carbon of glucose liberate glucose so that it can
now be released out of the cell that's one of the jobs of the smooth ER Isn't that cool so we got lipid synthesis
we've got biotransformation which is a fancy thing for drug alcohol toxin metabolism of neutralization to be
excreted and glucose really specifically cutting off of the phosphate of the six carbon of glucose so it can be liberated
and that's only at the smooth ER the last function I want to talk about is calcium storage let's talk about that
all right so smooth ER at this point we've covered lipid synthesis biotransformation we've overcome also
glucose release the last thing is calcium storage and release now it's very interesting because
calcium is a really interesting thing that in specific type of cells so muscle cells so skeletal muscle cells cardiac
muscle cells smooth muscle cells they have a very specific like modified smooth ER so I really want to I want you
to think about this is called a modified smooth endoplasmic reticulum and you know what we actually call it because we
reticulum because it's very very close to the cell membrane of the muscle cell and what's cool about this is that let's
say here is the sarcoplasm this is the basically uh the I'm sorry the sarcolemma this is the muscle cell
that depolarization will then travel it'll make this entire cell membrane positive there's like a little
invagination in skeletal muscles it's called the T tubules and what happens is this positive charge
moves down the cell membrane it activates the sarcoplasmic reticulum which is a modified smoothie r
and inside of this sarcoplasmic reticulum is rich in calcium and if I stimulate this smooth ER I'm
going to open up channels that are kind of storing calcium inside of it to release it and then what calcium will do
our myofilaments and so what it'll do is it'll bind onto very specific molecules that are present on the myofilaments and
stimulate them to initiate the contraction process so that's another cool thing so we know
now that the smooth ER is involved in lipid synthesis we know that it's also involved in biotransformation we know
that it's involved in glucose release and we know now that there is a modified smooth ER in muscle cells that contains
Calcium and when stimulated when it's depolarized from again being stimulated by a nerve or whatever it may be it'll
release calcium into the cytoplasm to allow for this muscle cells the myofilaments to contract we don't want
calcium just sitting out here in the cytoplasm all the time so it stores it so when it's needed it can then be
released and then guess what when you're done with contraction you go to relaxation guess where the calcium has
to go back into the sarcoplasmic reticulum to be stored for the next contraction to be pushed back out
all right my friends in this video we cover the endoplasmic reticulum the structure and the function I hope it