TBR Biology 2, Chapter 1, Passage 2, # 7

[drechie]

TBR Biology Book 2, Chapter 1, Passage 2, Question #7, page 59.

The exterior of a rough microsome is equivalent to the:
A. cytosol.
B. lumen of the nucleus.
C. lumen of the rough ER.
D. lumen of the Golgi.


I answered B, the lumen of the nucleus, because i thought ribosomes were made in the nucleus (or, atleast they have some origin in the nucelus?) and because ribosomes are on the exterior of the rough microsome. My answer and thought process for this question was egregiously incorrect. Please help on clarifying TBR's answer so i can think through their reasoning. Thanks!

A is correct. In the passage, it is stated that the rough microsome always has on its outside surface the ribosomes. This is implying that the topology of the ER is conserved. In other words, the outside of the rough ER in an intact cell is the cytosol. The ribosomes that are attached to the ER are attached on the cytoplasmic face of the ER membrane. This case is no different for the microsomes. They have not been turned inside out. Therefore, it becomes clear that the exterior of a rough microsome is equivalent to the cytosol. Consider the other choices. The lumen of the nucleus is not correct. The lumen of the nucleus is enclosed by the nuclear membrane, and it is not continuous with any other cellular fluid. The lumen of the ER is of course equivalent to the inside of the microsome. That is why we take advantage of these mini-ER systems for experiments. Finally, it is evident that the lumen of the Golgi has little to do with the exterior of the microsome. Choice D can be eliminated. The correct choice is A.

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[Lunasly]

I just finished this passage.

I chose A. The question stem says that the EXTERIOR of the microsome is equivalent or consistent with which of the following organelles. According to the passage, the microsomes in this passage are supposed to represent rough ER. Furthermore, the passage claims that all microsomes, of course, have ribosomes on their outer surface. What the question is basically asking is: What other part of the cell also has ribosomes? That would be the cytosol. The nucleus does not contain ribosomes. If I remember correctly, the nucleus is only responsible for transcribing the DNA in rRNA. I can't remember if some protein components are formed in the nucleus, but I am almost certain that the entire ribosome is NOT formed in the nucleus itself. That is irrelevant, though because the structure of the lumen of the nucleus would not be equivalent or constant with the external surface of the microsome. This eliminates choice B. Choices C and D can also be eliminated for obvious reasons. This leaves the best answer as choice A.

 

[thezilchplatypus]

In Eukaryotes the ribosomal subunits are actually produced in the Nucleolus of the Nucleus. However, the two subunits then leave the Nucleolus and enter the cytoplasm where they fuse to generate a functional ribosome.