ER lumen corresponds to extracellular env. Why?

[brood910]

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TPRH science workbook passage 23 Q7:

The ER lumen corresponds most closely to which?

1. interior of nucleus
2. cytoplasm
3. extracellular env
4. intermembrane space in mitochondria.

Answer is 3.
Why is this?
Their answer key says that interior of ER = interior of golgi = interior of secretory vesicle = extracellullar env

My reasoning so far is that secreted proteins move from ER lumen to golgi to get packaged and then get into secretory vehicle + then get released into extracellular env. So they all have high [proteins]. Is this why?

 

[QuinnTheEskimo]

TPRH science workbook passage 23 Q7:

The ER lumen corresponds most closely to which?

1. interior of nucleus
2. cytoplasm
3. extracellular env
4. intermembrane space in mitochondria.

Answer is 3.
Why is this?
Their answer key says that interior of ER = interior of golgi = interior of secretory vesicle = extracellullar env

The ER lumen, the inside of vesicles and the inside of the golgi are all part of the endomembrane system. Vesicles can bud off and fuse from one to the other. Vesicles from the ER cannot bud off and fuse to the nucleus, the cytoplasm, or the mitochondria.