The endoplasmic reticulum is an organelle that is responsible for the synthesis of lipids and the modification of proteins. It is part of the larger endomembrane system, which is the set of membrane-bound organelles that are involved mainly in the modification and transportation of proteins. This includes the nuclear envelope, Golgi apparatus, endoplasmic reticulum, vesicles and lysosomes.
The endoplasmic reticulum (ER) is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids. However, these two functions are performed in separate areas of the ER: the rough ER and the smooth ER. The hollow portion of the ER tubules is called the lumen. The membrane of the ER, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope.
The rough endoplasmic reticulum (RER) is so named because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope. Proteins that will leave the cell (transmembrane, nuclear and secreted, mainly) require specific modifications that occur in the RER. The mRNAs for these proteins contain a specific signal sequence that directs them to dock onto the ribosomes on the RER. Ribosomes then transfer the newly synthesized proteins into the lumen of the RER where they undergo structural modifications, such as folding or the acquisition of side chains. These modified proteins will be incorporated into cellular membranes—the membrane of the ER or those of other organelles —or secreted from the cell (such as protein hormones, enzymes). The RER also makes phospholipids for cellular membranes. If the phospholipids or modified proteins are not destined to stay in the RER, they will reach their destinations via transport vesicles that bud from the RER’s membrane.
The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface. Functions of the SER include synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; and storage of calcium ions.
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Key Points
• If the endoplasmic reticulum (ER) has ribosomes attached to it, it is called rough ER; if it does not, then it is called smooth ER.
• The proteins made by the rough endoplasmic reticulum are for use outside of the cell and it also makes phospholipids for cellular membranes.
• Functions of the smooth endoplasmic reticulum include synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; and storage of calcium ions.
Key Terms
Endoplasmic reticulum: an organelle that is responsible for the synthesis of lipids and the modification of protein
Endomembrane system: set of organelles involved in the modification, packaging and transport of proteins and other macromolecules
Rough: have ribosomes attached, make proteins for outside of the cell and phospholipids for cellular membranes
Smooth: makes carbohydrates, lipids, hormones, detoxification, storage of calcium ions
Lumen: the cavity or channel within a tube or tubular organ
Electron microscope: a microscope that uses a beam of accelerated electrons as a source of illumination
Signal sequence: a specific sequence found in the mRNA of proteins that are synthesized in the RER
Phospholipids: are a class of lipids that are a major component of all cell membranes. The structure of the phospholipid molecule generally consists of two hydrophobic fatty acid “tails” and a hydrophilic “head” consisting of a phosphate group.
Ribosomes: made of RNA and produce proteins