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Endoplasmic reticulum and Golgi bodies

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During protein synthesis, proteins meant for use within the cell are translated by free ribosomes. Proteins meant to be embedded in the cell membrane or used outside the cell are translated by ribosomes attached to the rough endoplasmic reticulum. These proteins are then transported to the Golgi body for further maturation and sorting before being released. Created by Sal Khan.

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  • starky seedling style avatar for user Irene
    Just a quick question. What dictates whether or not a strand of mRNA attaches to a free-floating ribosome as opposed to one attached to the Rough E.R.? Or is it just a "luck of the draw" sort of thing?
    (163 votes)
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    • leafers seedling style avatar for user laura white
      In eukaryotes, free ribosome's actually assemble around the mRNA then will begin reading the code. Fairly early on a certain sequence in the RNA may be recognised which causes the whole translation process to halt while the mRNA-ribosome complex is all transferred to the endoplasmic reticulum, translation will then begin again with the growing polypeptide chain entering the ER lumen. The process is known as cotranslational localisation. Generally speaking, proteins that are destined for lysosomes, membranes or to be secreted from the cell are made on the RER.
      (138 votes)
  • starky tree style avatar for user Jaiia Cerff
    does each cell have only one E.R, and Golgi body?
    (55 votes)
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    • leaf blue style avatar for user Peterson
      As I am sure you know, the ER (endoplasmic reticulum) is a network of cisternae around the nucleus. Thus, there is only one network of ER in a cell, but within that is a number of sacs and tubes that make up the entire ER. As for the Golgi apparatus (or body), is to is made up of multiple sections of cisternae (just like the ER, yet in a different formation), and there are often multiple complexes of Golgi to be found in a single cell, with the maximum being ~20.
      (66 votes)
  • starky sapling style avatar for user Cas
    If the proteins take part of the membrane of the ER when they turn into vesicles, how does the ER reattach? Does the ER become smaller and smaller every time? Does it grow to keep up with how much of its membrane is lost?
    (44 votes)
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  • piceratops tree style avatar for user sahasavi
    at what is chromatin?
    (18 votes)
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    • blobby green style avatar for user hms420
      Chromatin is a combination of DNA and protein, and is what makes a Chromosome. It has many functions, but mainly a way to efficiently pack in as much DNA as possible, and improve the strength of the structure.
      DNA strand wraps around protein histones, to form a nucleosome, it looks like 'beads on a string'. This in turn wraps around on itself to create a '30 nm structure', and wraps around on itself again.
      If you google image 'Chromatin' you will see what I mean, there are some good diagrams illustrating it.
      (34 votes)
  • primosaur ultimate style avatar for user T-TAS
    Does the vesicle have two membranes around after it has ripped through the membrane of the E.R. and the Golgi body ?
    (19 votes)
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    • leafers sapling style avatar for user Peter Collingridge
      No. When a vesicle comes in contact with a membrane in the ER or Golgi body, the membrane does not wrap around it to create another membrane. Instead, the two membrane's fuse, so the contents are released into the lumen of the ER or Golgi body (as at ). Then, when the contents leave again, a new vesicle will form around them.
      (22 votes)
  • duskpin ultimate style avatar for user secretspyman2
    I have heard that the smooth ER gets rid of toxins. What toxins are these exactly and how would they get in the cell at all?
    (16 votes)
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    • blobby green style avatar for user kierstyn.marker
      One example of a toxin is alcohol. Liver cells of alcohol drinkers have a lot of smooth ER because it helps to detoxify the alcohol. If the smooth ER were absent or dysfunctional, the cells would shrivel up and die. This can lead to cirrhosis. SER also protects cells from harsh effects of drugs. There are also toxins produced by cellular processes that the SER take care of. Diffusion, Facilitated diffusion, active transport, and endocytosis are some ways that toxins enter the cell.
      (20 votes)
  • starky tree style avatar for user Jaiia Cerff
    What is the E.R lumen and what purpose does it serve?
    (12 votes)
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  • starky sapling style avatar for user newnoogler
    Okay, so I get the basic idea but, exactly what is the difference between transcription and translation.
    (11 votes)
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  • leaf yellow style avatar for user adamzalaquett
    Why is it that a protein has to go through the Golgi Body after exiting the ER, Sal mentions that it goes through a maturation process, but what does that actually do? Why can't the vesicle made from the ER transport its protein straight to its final destination?
    (8 votes)
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    • piceratops ultimate style avatar for user abhinavmohapatra2016
      Great qustion!
      You canthink of the Golgi bodies as the post office of the cell. Before anything goes out, the golgi body sort of packages it with materials which will be easily given access to move through the cell membrane. This process is called glycolysation. If there were no golgi, then the protein may not have been given access to go through the plasma membrane due to the phospholipids. The vesicle made by the ER is designed to only go about the cytoplasm, not the plasma membrane.
      Hope I helped.
      (5 votes)
  • leaf green style avatar for user Eve Koller
    probably a silly question but: how fast is this process taking place?
    (6 votes)
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    • piceratops ultimate style avatar for user Rayce Wiggins
      This is actually an excellent question! Generally speaking, protein synthesis occurs pretty quickly. The rate depends on the type of cell. For example: prokaryotes synthesize faster than eukaryotes. To give you an idea of how fast this is happening, some prokaryotic cells are known to synthesize proteins at a rate of 20 amino acids per second.

      Now in this video there are other processes going on besides just synthesis like transportation. But since these processes involve newly manufactured proteins you can imagine that they take place at relatively fast rate as well.
      (4 votes)

Video transcript

We've already talked about the process from going from DNA to messenger RNA. And we call that process transcription. And this occurs in the nucleus. And then that messenger RNA makes its way outside of the nucleus, and it attaches to a ribosome. And then it is translated into a protein. And so you could say that this part right over here, this is being facilitated by a ribosome. Or it's happening at a ribosome. With that high-level overview, I now want to think a little bit in more detail about how this actually happens, or the structure of things where this happens inside of a cell. And so I'm going to now draw the nucleus in a little bit more detail so that we can really see what's happening on its membrane. So this right over here is the nucleus. Actually, let me draw it like this. And instead of just drawing the nucleus with one single line, I'm going to draw it with two lines. Because it's actually a double bilipid membrane. So this is one bilipid layer right over here. And then this is another one right over here. And I'm obviously not drawing it to scale. I'm drawing it so you can get a sense of things. So each of these lines that I'm drawing, if I were to zoom in on this-- so if I were to zoom in on each of these lines, so let's zoom in. And if I got a box like that, you would see a bilipid layer. So a bilipid layer looks like this. You have the circle is a hydrophilic end and those lines are the fatty hydrophobic ends. So that's our bilipid layer. So that's each of these lines that I have drawn, each of them are a bilipid layer. So the question is, well, how does the mRNA-- obviously you have all this transcription going on. You have the DNA, you have the mRNA. It's all in here, this big jumble of chromatin inside the nucleus. How does it make its way outside of this double bilipid layer? And the way it makes its way out is through nuclear pores. So a nuclear pore is essentially a tunnel. And there are thousands of these. It's a tunnel through this bilipid layer. So the tunnel is made up of a bunch of proteins. So this right over here-- and this is kind of a cross section of it. But you could almost imagine it if you're thinking of it in three dimensions, you would imagine a tunnel. A protein-constructed-- a tunnel made out of proteins that goes through this double bilipid membrane. And so the mRNA can make its way out and get to a free ribosome, and then be translated into a protein. But this right over here is not the complete picture. Because when you translate a protein using a free ribosome, this is for proteins that are used inside the cell. So let me draw the entire cell right over here. This is the cell. This right over here is the cytosol of the cell. And you might be sometimes confused with the term cytosol and cytoplasm. Cytosol is all the fluid between the organelles. Cytoplasm is everything that's inside the cell. So it's the cytosol and the organelles and the stuff inside the organelles is the cytoplasm. So cytoplasm is everything inside of the cell. Cytosol is just the fluid that's between the organelles. So anyway, the free ribosome over here, this translation is good for proteins used within the cell itself. The proteins can then float around the cytosol and used in whichever way is appropriate. But how do you get protein outside of the cell, or even inside the cellular membrane? Not within it, within the cell, but embedded in the cell membrane or outside of the cell itself. And we know that cells communicate in all sorts of different ways and they produce proteins for other cells or for use in the bloodstream, or whatever it might be. And that's what we're going to focus on in this video. So contiguous with this what's called a perinuclear space right over here, so the space between these two membranes-- So you have this perinuclear space between the inner and outer nuclear membrane. Let me just label that. That's the inner nuclear membrane. That's the outer nuclear membrane. You could continue this outer nuclear membrane, and you get into these kind of flaps and folds and bulges. And this right over here is considered a separate organelle. So you get this thing that looks like this, and I'll just do it the best that I can draw it. And this right over here is called the endoplasmic reticulum. So this right here is endoplasmic reticulum, which I've always thought would be a good name for a band. And the endoplasmic reticulum is key for starting to produce and then later on package proteins that are either embedded in the cellular membrane or used outside of the cell itself. So how does that happen? Well, the endoplasmic reticulum really has two regions. It has the rough endoplasmic reticulum. And the rough endoplasmic reticulum has a bunch of ribosomes. So that's a free ribosome right over here. This is an attached ribosome. These are ribosomes that are attached to the membrane of the endoplasmic reticulum. So this region where you have attached ribosomes right over here, that is the rough endoplasmic reticulum. I'll call it the rough ER for short. Perhaps an even better name for a band. And then there's another region, which is the smooth endoplasmic reticulum. And the role that this plays in protein synthesis, or at least getting proteins ready for the outside of the cell, is you can have messenger RNA-- let me do that in that lighter green color-- you can have messenger RNA find one of these ribosomes associated with the rough endoplasmic reticulum. And as the protein is translated, it won't be translated inside the cytosol. It'll be translated on the other side of the rough endoplasmic reticulum. Or you could say on the inside of it, in the lumen of the rough endoplasmic reticulum. Let me make that a little bit-- let me draw that a little bit better. So let's say that this right over here, that right over here is the membrane of the endoplasmic reticulum. And then as a protein, or as a mRNA is being translated into protein, the ribosome can attach. And let's say that this right over here is the mRNA that is being translated. Let's say it's going in that direction right over here. Here is the membrane of the ER. So ER membrane. This right over here-- and actually, the way I've drawn it right over here, this is just one bilipid layer. So let me just draw it like this. I could do it like this. And this is actually, this bilipid layer is continuous. It's continuous with the outer nuclear membrane. So let me just make it like that so you get the picture. And then at some point in the translation process, the protein can be spit out on the inside. As it's being translated, it can be spit out on the inside of the endoplasmic reticulum. So this is the lumen. This is the ER lumen right over here. So we're inside the endoplasmic reticulum here. Here we're outside in the cytosol. So that way you get the protein now, inside the ER. Inside the endoplasmic reticulum, and it can travel through it. And at some point, it can bud off. So let's say, imagine the protein is right over here. And the smooth endoplasmic reticulum has many functions, and I won't get into all the depth of how it's involved. But at some point that protein can bud off. So let me draw a budding off protein. So let's say this is the membrane of the endoplasmic reticulum. And a protein, let's say, ends up right over here. And then it can bud out. So it could go from that to-- let me do that same color. It could go from that to that-- I think you see where this is going-- to that, and then to that. And then it could go to something like this. Now it has budded out. And when you have a protein, or really you have anything that's being transported around a cell with its own little mini membrane, we call this a vesicle. So now it'll bundle up, and now it is a vesicle. Now, this vesicle can then-- let me draw some of these vesicles holding some proteins, so let me draw that-- can then go to the Golgi apparatus, which I'll drawn in blue right over here as best as I can. So the Golgi apparatus. This is not-- obviously there could be better drawings of something like this. And then they can essentially do the reverse process, and they can attach themselves to the Golgi, oftentimes the Golgi body, named after Mr. Golgi who discovered this. And then the proteins, once they get into the inside of the Golgi body, then they essentially go into a maturation process so that they're ready for transport outside of the cell, or maybe to be embedded into the cellular membrane. So this right over here is the Golgi body, or a Golgi body or Golgi apparatus. And then once they're done with that process, then this is kind of the fully-manufactured protein ready to be used. And actually, maybe I'll make it a slightly different-- well, I'll just use that same color. This is the fully-manufactured protein. And now it can transport to the cell membrane. And that protein can either be transported outside of the cell, or it can be embedded within the membrane itself.