Inside your cells, membranes stack up like a spiral parking garage
Zoom into the rough endoplasmic reticulum and you find flat double membranes piled on top of each other, joined by twisting helical ramps. The resemblance to a multistorey car park is striking. Known as Terasaki ramps, these spirals, turning either left or right, let a sprawling protein factory fold into a tiny corner of the cell.
The endoplasmic reticulum, whose name means little net within the cytoplasm, is a web of membrane sacs and tubes present in most complex cells, though red blood cells and sperm lack it. Its membranes connect directly to the outer layer of the nuclear envelope. There are two versions. The rough kind is studded with ribosomes and sits mostly near the nucleus; the smooth kind has none and lies closer to the cell's edge. Cells adjust the balance between them as their workload shifts.
Ribosomes do not stay permanently attached. A free ribosome begins making a protein, and if its first 5 to 30 amino acids spell out a signal meaning destined for export, a particle grabs that tag, pauses production and docks the ribosome onto a channel in the rough membrane. The growing protein is threaded inside, the tag is clipped off, and sugars may be attached as it folds. Liver cells are especially rich in this rough form.
Finished cargo travels on to the Golgi apparatus in small bubbles called vesicles, since the two organelles are not joined by continuous membrane. Coat proteins act like address labels: COPII sends vesicles forward to the Golgi, while COPI brings them back. Elsewhere, the reticulum presses close to other organelles at contact sites, passing lipids and small molecules directly across.
The smooth version makes lipids and steroid hormones and helps detoxify substances, and it is plentiful in mammalian liver and gonad cells. Charles Garnier saw the structure under a light microscope in 1897, calling it ergastoplasm. Its lacy membranes were first revealed by electron microscope in 1945, by Keith Porter, Albert Claude and Ernest Fullam.
Source: Endoplasmic reticulum
