Rough endoplasmic reticulum. Coloured high resolution scanning electron micrograph of rough endoplasmic reticulum (ER) in a pancreatic acinar cell. ER is a network of folded membranes forming sheets, tubes or flattened sacs in the cell cytoplasm. It is the site of specific enzymically controlled biochemical reactions. On the surface of the ER membrane are numerous ribosomes (tiny spheres). Ribosomes are involved in protein synthesis. They give the ER its granular appearance and give rise to the name "rough" ER. ER (not seen) has no ribosomes and is concerned with fat metabolism.
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