Endoplasmic reticulum
The endoplasmic reticulum (ER) is a maze of membrane tubes and sacs continuous with the nuclear envelope. Rough ER, studded with ribosomes, folds and packages proteins; smooth ER makes lipids and detoxifies drugs.
- 1NucleusOrganelle · double membrane
- 2MitochondrionOrganelle · double membrane
- 3RibosomeMolecular machine · no membrane
- 4Endoplasmic reticulumOrganelle · single membrane network
- 5Golgi apparatusOrganelle · single membrane stacks
- 6LysosomeOrganelle · single membrane
- 7Cell membraneBoundary · phospholipid bilayer
- 8CentriolesMicrotubule pair · organizes the spindle
Animal cell: what you are looking at
A flexible, roughly rounded cell 10–30 µm across, bounded only by its membrane. It has a pair of centrioles near the nucleus and many lysosomes, but no wall, no chloroplasts, and no large central vacuole.
Centrioles. A pair of short microtubule cylinders at right angles; they organize the spindle during cell division.
The endoplasmic reticulum is outlined in coral; it occurs in animal cells, plant cells, fungi, protists. Hover or tab through the key to light up the other structures, and click any of them to open its page.
Compare all eight organelles ↗Know its structure
| Found in | Animal cells, Plant cells, Fungi, Protists |
| Membranes | One continuous membrane enclosing a single internal space, the lumen |
| Typical size | Tubules about 30–100 nm wide; the network can account for more than half of all membrane in a cell |
| Number per cell | One interconnected network whose balance shifts: liver cells are rich in smooth ER, antibody-secreting cells in rough ER |
| Structure | Rough ER: flattened cisternae dotted with ribosomes. Smooth ER: branching tubules without ribosomes |
| Discovered | Seen by Keith Porter, Albert Claude, and Ernest Fullam in 1945 with one of the first electron microscopes |
What it does
- Folds and modifies proteins. Rough ER threads new polypeptides into its lumen, attaches sugar chains (N-linked glycosylation), and folds them with chaperones
- Builds lipids. Smooth ER makes phospholipids and cholesterol and, in gonads and adrenal glands, steroid hormones
- Detoxifies. Liver smooth ER carries cytochrome P450 enzymes that oxidize drugs, alcohol, and other toxins
- Stores calcium. In muscle, the sarcoplasmic reticulum holds Ca²⁺ and releases it to trigger contraction
Endoplasmic reticulum: common questions
Wondering about the why? Start here.
What does the endoplasmic reticulum do?
The ER is a network of membrane tubes and sacs continuous with the nuclear envelope. Rough ER, studded with ribosomes, threads new proteins into its lumen, folds them, and attaches sugar chains; smooth ER makes phospholipids, cholesterol, and steroid hormones, detoxifies drugs in liver cells, and stores calcium. It can account for more than half of all the membrane in a cell.
Read: transcription and translationWhat is the difference between rough and smooth ER?
Rough ER has ribosomes on its surface and is arranged in flattened sacs; smooth ER has none and forms branching tubules. They are regions of one continuous membrane, not separate organelles, and the balance shifts with the cell’s job: an antibody-secreting plasma cell is packed with rough ER and releases about 2,000 antibodies per second, while liver cells exposed to drugs can double their smooth ER within days.
Meet the ribosomeWhere do proteins go after the ER?
Vesicles bud from the ER and fuse with the cis face of the Golgi apparatus, where sugar chains are trimmed and rebuilt and sorting tags are added. At the trans face, new vesicles carry each protein to its destination: the cell membrane, a lysosome, or outside the cell. A typical protein takes about 20 minutes to cross a Golgi stack.
Continue to the Golgi apparatus