EK BIO CH 4 Substance can reach ER lumen via endocytosis???

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dreamer67

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Page 76 says a substance must cross the membrane to reach the cytosol, which makes sense because if endocytosis occured, the substance would be surrounded by a membrane. But it says that a substance can reach the ER lumen via endocytosis, without ever crossing a membrane. However, earlier in the chapter it says endocytotic vessels go to the golgi to recycle the cell membrane. So, how exactly are substances reaching the ER lumen via endocytosis?

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Page 76 says a substance must cross the membrane to reach the cytosol, which makes sense because if endocytosis occured, the substance would be surrounded by a membrane. But it says that a substance can reach the ER lumen via endocytosis, without ever crossing a membrane. However, earlier in the chapter it says endocytotic vessels go to the golgi to recycle the cell membrane. So, how exactly are substances reaching the ER lumen via endocytosis?

lolwut? this is why EK bio is kind of bleh.

endocytotic vesicles usually get combined with a lysosome from the golgi, and the stuff that was taken up via endocytosis is digested using pH sensitive enzymes. also, this is actually probably not that important, but things get transported in between the golgi and ER all the time, forward and backwards. i think you're just getting stuck on the wording. receptor mediated endocytosis can result in digestion of stuff, recycling stuff by sending it back to the golgi which can send stuff back to the ER. it doesn't "cross" a membrane because its carried by a "membrane"/vesicle.
 
Page 76 says a substance must cross the membrane to reach the cytosol, which makes sense because if endocytosis occured, the substance would be surrounded by a membrane. But it says that a substance can reach the ER lumen via endocytosis, without ever crossing a membrane. However, earlier in the chapter it says endocytotic vessels go to the golgi to recycle the cell membrane. So, how exactly are substances reaching the ER lumen via endocytosis?


First to the above poster, EK bio book is the best book out there and its very clear. As to you dreamer67, you need to read more clearly and carefully. Each sentence of EK book is very valuable because they are very condence.

The answer is substances can reach ER lumen without crossing a membrane simply because the ER lumen is extended from the cell membrane to the cytoplasm to get along the nuclear envelope. In other words, the ER lumen is contiguous or in actual contant with the plasma membrane and therefore substances that cross the cell membrane can just get into the ER lumen via endocytosis without crossing another membrane since the cell membrane and ER lumen are connected. Read in top of page 75; it says, "In my places, the ER is contiguous with the cell membrane and the nuclear envelope. The ER lumen is contigous in places with the space between the double bilayer of the nuclear envelope." This is exactly what I said before. So in short, there is only one membrane to cross, which is the cell membrane !
 
Think about it in the other direction, exporting a polypeptide hormone. A cell manufactures a protein into the rough ER. Then the ER blebs a membrane-bound vessicle into the golgi. The golgi blebs a membrane-bound vessicle into a secretory vessicle. Then via exocytosis the secretory vessicle merges with the cytoplasm and the contents are ejected into the extracellular environment.

How many membranes did the protein cross in its journey? Zero. Sometimes its membrane pinched off behind it. Sometimes its membrane merged with another and they mixed their contents. But never did the protein have to cross a phospholipid bilayer.

Now just run the movie in reverse.
 
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First to the above poster, EK bio book is the best book out there and its very clear. As to you dreamer67, you need to read more clearly and carefully. Each sentence of EK book is very valuable because they are very condence.

The answer is substances can reach ER lumen without crossing a membrane simply because the ER lumen is extended from the cell membrane to the cytoplasm to get along the nuclear envelope. In other words, the ER lumen is contiguous or in actual contant with the plasma membrane and therefore substances that cross the cell membrane can just get into the ER lumen via endocytosis without crossing another membrane since the cell membrane and ER lumen are connected. Read in top of page 75; it says, "In my places, the ER is contiguous with the cell membrane and the nuclear envelope. The ER lumen is contigous in places with the space between the double bilayer of the nuclear envelope." This is exactly what I said before. So in short, there is only one membrane to cross, which is the cell membrane !

This doesn't answer OP's question.

OP, imagine a bubble, and some stuff inside a bubble. when the bubble can merge with another bubble, the stuff has effectively transferred locations. however, the stuff never had to cross the barrier of the bubble. its the same idea here, vesicle transport is when part of the membrane (bubble) pinches off and goes inside, then the membrane (bubble) fuses with another membrane. go blow some bubbles and you'll see what i mean.
 
This doesn't answer OP's question.

OP, imagine a bubble, and some stuff inside a bubble. when the bubble can merge with another bubble, the stuff has effectively transferred locations. however, the stuff never had to cross the barrier of the bubble. its the same idea here, vesicle transport is when part of the membrane (bubble) pinches off and goes inside, then the membrane (bubble) fuses with another membrane. go blow some bubbles and you'll see what i mean.

Yea sure it doesn't answer it...How is your answer different than mine? I said that the ER lumen is contiguous with the cell membrane meaning they both kind of fuse together so substances that come and cross the membrane of the cell are transferred into the ER lumen if they have to without crossing another membrane, exactly like the bubble idea but instead, I made it realistic of what is happening inside the cell. Please don't judge my posting if it answer the question or no. Let OP answer that himself or herself. No disrespect here, and thank you!
 
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