DON FAWCETT / SCIENCE PHOTO LIBRARY DON FAWCETT / SCIENCE PHOTO LIBRARY
Endoplasmic reticulum. Coloured transmission electron micrograph (TEM) of a section through rough endoplasmic reticulum (red) in a human cell. Endoplasmic reticulum (ER) is a network of folded membranes forming sheets, tubes or flattened sacs in the cell cytoplasm (yellow). It is the site of the cell's membrane production. On the surface of the ER membranes are ribosomes (dark grey spheres). Ribosomes are involved in protein synthesis. Their granular appearance gives rise to the name rough ER. ER membranes with no ribosomes are called smooth ER. Magnification unknown.
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