Smooth ER is not involved in protein synthesis. It is the site of the synthesis of fatty acids and phospholipids. It is called “smooth” because it has no ribosomes on its surface. Smooth ER creates steroids and stores ions so that the cell can have them when it needs them.
Rough ER gets its name because its surface is studded (covered) with ribosomes, which make it look bumpy or “rough” under a microscope.
These ribosomes are said to be “bound” to the ER. The largest number of bound ribosomes is found in cells that produce secretory products.
These secretory proteins (enzymes) are manufactured here. The term secretory just means that these proteins are made for secretion, or delivery outside the cell, into the body.
Once a protein has been synthesized (made), Rough ER creates a bubble around it by pinching off a portion of its own membrane. This bubble is called a “transition vesicle.”
The transition vesicle then moves either to the cell membrane or to the Golgi Apparatus.
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