Question

Most proteins destined to enter the endoplasmic reticulum

Select one:

a. are completely translated on free ribosomes in the cytosol.

b. are transported across the membrane after their synthesis is complete.
    

c. remain within the endoplasmic reticulum.

d. begin to cross the membrane while still being synthesized.   

Answer 1

The site of protein synthesis in the cell is ribosome. Ribosomes are found in all the cell. A basic cell have lots of ribosomes and some of them are free floating in the cytoplasm and some are attached to the rough endoplasmic reticulum. The function of ribosome is same in both that is synthesis of protein.

Option a is incorrect.

In mammals the transport of protein is both co-translational and post translational. In endolasmic reticulum the import of protein into ER is co-translational. It means that proteins are imported inside the ER before the polypeptide chain is completely synthesized. This is known as co-translational process.

So option d is correct.

The import of proteins into mitochondria, nuclei, chloroplasts, and peroxisomes is a  posttranslational processes. It means that one end of the protein is translocated into the ER as the rest of the polypeptide chain is still in the synthesis proess. The protein is never released into the cytosol.

So option b is also correct.

Proteins are translocated in the ER.

So option c is also correct.