Which of the following statements about the endomembrane system is/are correct? (select all that apply)

a. secreted proteins are inserted into the er as they are being made by ribosomes.
b. the nucleus is surrounded by two phospholipid bilayers.
c. free ribosomes attach to the endoplasmic reticulum.
d. secreted proteins are moved from the er to the golgi by ribosomes

Correct statements are: A, B and C.

Explanation:

Ribosomes are the primary structure for protein synthesis. They can be found in the rough endoplasmic reticulum or floating in the cytosol.

Free ribosomes are not attached to any cytoplasmic structure or organelle. They synthesize proteins only for internal cell use. Other ribosomes are attached to the membrane of the endoplasmic reticulum and they are in charge of synthesizing membrane proteins or exportation proteins. Free and attached ribosomes are identical and they can alternate their location. This means that although free ribosomes are floating in the cytosol, eventually, they can get attached to the endoplasmic reticulum membrane.

Synthesis of proteins might start in the cytoplasm with the production of a molecule portion known as a signal sequence. This portion leads the synthesizing protein and associated ribosome to a specific region in the Rough endoplasmic reticulum where it continues the protein building. Here it also happens the protein folding and the initial stages of glycosylation.

Once proteins are synthesized in the endoplasmic reticulum, they are packaged and sent to the Golgi complex in vesicles, where it happens the final association of carbohydrates with proteins.  

The membrane of the rough endoplasmic reticulum continues with the nucleus´ external membrane, which also has ribosomes attached to it. Together they form the endomembrane system.

The nucleus is also surrounded by an internal membrane, a lipidic bilayer that is attached to the chromatin.

Between these two membranes, there is a space called perinuclear space and certain pores that allow selective transport between the nucleus and the cytoplasm.

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