What causes rough endoplasmic reticulum to look “rough” under a microscope?

Due to the presence of ribosomes on the surface of endoplasmic reticulum

Explanation:

The ribosomes are attached to the surface of endoplasmic reticulum (ER) due to which its surface looks rough and hence these ERs are called as rough endoplasmic reticulum. While in case of smooth endoplasmic reticulum , the surface lacks the ribosomes which are protein synthesizing particles. Under the microscope , the ribosomes can be easily identified and thus rough endoplasmic reticulum( RER) can be distinguished with smooth endoplasmic reticulum (SER)

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it takes hundreds of years to become stable.

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