Rough endoplasmic reticulum. Transmission electron micrograph (TEM) of a section through a cell, showing membranes of rough endoplasmic reticulum (ER, dark lines), an organelle involved with protein synthesis. The flattened stacks of membranes are studded with ribosomes (small dots) hence the term rough ER. Amino acids in the cytoplasm are assembled and joined by the ribosomes to form elongating peptide chains accumulated in the luminal space between the membranes. Additional biosynthesis by other cell organelles results in the formation of functional proteins used for cell metabolism or export as a secretory product from the cell. Magnification: x33,000, when printed 10 centimetres wide.