TRPH - Is ER lumen contiguous with extracellular space?

[Monkeymaniac]

TRPH review booklet explains that

Since the membranes of these organelles commuicate through the traffic of vesicles, the interior of the ER, the Golgi, lysosomes, and the extracellular environment are in a sense contiguous.

One of the review question states

Disulfide bridges are found in extracellular proteins because the cytoplasm is a reducing environment that changes cystine to two cysteins. Given this fact, does it make sense that disulfide bridges are formed in the ER lumne?

The answer is,

Yes. Remember, the ER lumen is equivalent to (contiguous with ) the extracellular space.

I understand that some vesciles get transferred from ER to Golgi to extracellular environment. But does the traffic in the opposite direction also exist? That is, I don't understand how ER lumen can have environment similar to that of the outside the cell if no material/liquid is transferred from outside the cell to ER.

Thanks in advance!

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[RogueUnicorn]

yes, endocytosed vesicles are also processed via lysosome, ER, etc

 

[sizillyd]

The ER can have an environment similar to extracellular space because it is where the folding of proteins take place and the manner in which the proteins are folded in the ER is the manner in which they will exist in the extracellular space. There also exists transport as you stated that goes backwards. This is called retrograde transport that goes in the direction of the trans Golgi to the cis Golgi. I hope this was able to help you. It was a little difficult for me to understand the question.

 

[dingyibvs]

I don't think it is, at least not technically. I think what your book is trying to convey is that since the ER, the Golgi, etc. and the extracellular space transfer material through vesicles all the time, which requires that both the their contents and membranes to merge with each other all the time, that you can essentially consider them to be all contiguous. Think about it, when a vesicle takes material from the ER to the Golgi, for example, some contents within the ER lumen will be dumped into the Golgi lumen, and some portion of the ER membrane will merge with the Golgi lumen. So, it makes sense that the Golgi membranes, the ER membranes, the cell membrane...etc., the Golgi lumen, the ER lumen, and the extracellular space...etc., would be composed of similar material.

Based on what I understand, only the ER and the Nuclear Membranes are actually physically contiguous.

 

[badmintondr]

For the MCAT would it be more correct to go with no because it is not physically continuous with the ER?

I often have the trouble of wording and implications when I do questions because certain books word/expect different things from the same statement.

 

[MegaSpectacular]

BR is trying to say that they are topologically equivalent. I'm in cell bio, they explain this pretty well.

They are not contiguous. That is why it says "in a sense". They have the same topology. Which means the area in the golgi / ER/ lysosomes, all of these areas and the outside of the cell all have a similar area, as opposed to the other area which is the cytoplasm.

It is an important idea to understand though.

CONTIGUOUS is a horrible word to convey the idea, because it is wrong. The textbook "molecular biology of the cell" explains the idea much better. Topologically equivalent.

people are stretching the idea to say they are "composed of similar material". Not really. The environment in the golgi/ER/lysosomes can be radically different, and is. E.g. the pH allows proteins to act differently in each of these environments. Yet they do share the same fluids but once they arrive the proteins and transmembrane proteins can set up completely different environments.

Saying they are the same or contiguous is helping to kind of understand something that really isn't true. (I.e. outer membrane of nucleus IS contiguous with ER; a contig in DNA sequencing does overlap; etc)

 

[youngrace]

A bit of a necropost, but a similar thing confused me when I was reading the other day.

Q: "Disulfide bridges are found in extracellular proteins because the cytoplasm is a reducing environment that changes cystine to two cysteines. Given this fact, does it make sense that disulfide bridges are formed in the ER lumen?"

A: "Yes. Remember, the ER lumen is equivalent to (contiguous with ) the extracellular space."

...Uhh, what? I don't see why they added that bit about the cytoplasm being a reducing environment, since that doesn't really fit in anywhere. Since the lumen is functionally quarantined off from the cytoplasm, why are we even bringing it up?

 

[dmf2682]

It's quarantined off but the environment is similar. See previous posts. Contiguous is a bad word to use. I liked topologically equivalent.

 

[Rhino1000]

I just worked on this problem, and obtained a wrong answer for similar reasons. I agree with @dingyibvs; the gap between the nuclear inner and outer membrane is contiguous with the endoplasmic reticulum, which, I believe, is even further contiguous with the cytoplasm because of the nuclear pores on the nuclear membrane. I think both nucleus and cytoplasm would be greater answers than the answer that the book provides.

In other words, I think it is more correct to say that the interior of the endoplasmic reticulum is contiguous with either the cytoplasm or the nucleus than saying it is contiguous with the extracellular environment. Someone correct me if I seem confused or incorrect.

 

[Czarcasm]

I just worked on this problem, and obtained a wrong answer for similar reasons. I agree with @dingyibvs; the gap between the nuclear inner and outer membrane is contiguous with the endoplasmic reticulum, which, I believe, is even further contiguous with the cytoplasm because of the nuclear pores on the nuclear membrane. I think both nucleus and cytoplasm would be greater answers than the answer that the book provides.

In other words, I think it is more correct to say that the interior of the endoplasmic reticulum is contiguous with either the cytoplasm or the nucleus than saying it is contiguous with the extracellular environment. Someone correct me if I seem confused or incorrect.

While it's true that they are all in some sense interconnected due to nuclear pores, a key fact presented even in Biochemistry classes is that both the interior of the ER lumen as well as the extracellular environment are both reducing (and therefore, a suitable environment for the formation disulfide bridges). I'm sure there's some valid reason for it (ie. reducing agents present in lumen that cannot diffuse through the nuclear pore or cytosol) or something more scientifically sound -- but for the purpose of the MCAT, it's just a fact you should commit to memory.

 

[Rhino1000]

While it's true that they are all in some sense interconnected due to nuclear pores, a key fact presented even in Biochemistry classes is that both the interior of the ER lumen as well as the extracellular environment are both reducing (and therefore, a suitable environment for the formation disulfide bridges). I'm sure there's some valid reason for it (ie. reducing agents present in lumen that cannot diffuse through the nuclear pore or cytosol) or something more scientifically sound -- but for the purpose of the MCAT, it's just a fact you should commit to memory.

Thanks for the reply! However, I still believe that TPRH made some sort of error here, because the definition of contiguous is basically the same thing as sharing the same space. See the merriam-webster definition: "used to describe things that touch each other or are immediately next to each other." Even though their interiors may share similar properties, that seems hardly qualified to qualify the two spaces as "contiguous." So while they may (mistakenly, no doubt) believe that the extracellular space is "in a sense contiguous with the ER lumen," the fact is that the cytoplasm as well as the nuclear interior is by definition contiguous (with the ER lumen), and is, then, reasonably considered indistinguishable and therefore infinitely similar to, the cytoplasm and the nuclear interior; the end result being that either the cytoplasm or the nuclear interior would be the greater answer compared to theirs.

While their explanation may be incorrect, for the reasons mentioned above, I believe you may be correct in their reasoning for the answer. The question was :
The ER lumen corresponds most closely to which of the
following compartments?
I would have thought that because the nuclear interior as well as the cytoplasm was literally the same exact thing as the ER lumen, that that would be the greatest answer (I still believe dogmatically that this to be the case, because they are identical), their answer used other logic that compares the properties of the spaces, in a way that makes the extracellular space their favorite answer. I understand their confused reasoning, but I can't help but think that that answer is logically not so sound.

 

[DeathandTaxes]

Czarcasm, I disagree with you on one point:

I believe the extracellular environment should be oxidizing. Making disulfide bridges (going from S-H to S-S) means you're taking away bonds from hydrogen (which by definition is oxidizing)

http://www.ncbi.nlm.nih.gov/pubmed/18159092

 

[Czarcasm]

Czarcasm, I disagree with you on one point:

I believe the extracellular environment should be oxidizing. Making disulfide bridges (going from S-H to S-S) means you're taking away bonds from hydrogen (which by definition is oxidizing)

http://www.ncbi.nlm.nih.gov/pubmed/18159092

I'm glad you caught that. You're absolutely correct. I'm not sure why I said reducing.