Endoplasmic Reticulum

[golgiapparatus88]

Question from TPR hyperlearning workbook:

"The ER lumen corresponds most closely to which of the following compartments?
a) interior of the nucleus
b) the cytoplasm
c) the extracellular environment
d) the intermembrane space in mitochondria

The answer is the extracellular environment by this reasoning

"The interior of the ER (the lumen) corresponds to the interior of the Golgi, the interior of secretory vesicles, and the extracellular environment in turn"

can someone help me understand this!?

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[matth87]

Proteins that are destined for excretion to the extracellular environment are translated through the ER membrane into the ER lumen. The ER Lumen is "continuous with the extracellular environment". This is because:

Protein's in the ER lumen get transported to the Golgi which get secreted to the extracellular environment via exocytosis. So if you think about it these three environments are all continuous with each other. Thats why it "corresponds most closely with the extracellular environment".

Proteins in the ER Lumen will never end up in the cytoplasm, or the nucleus, or the mitochondria.

Hope this helps.