Endoplasmic Reticulum and Golgi Apparatus

The endoplasmic reticulum (ER) is an extensive, interconnected network of membrane-bound tubules, vesicles and flattened sacs called cisternae that extends throughout the cytoplasm of a eukaryotic cell, typically continuous with the outer membrane of the nuclear envelope. By dividing the space within a cell into two effectively separate compartments -- the luminal space enclosed within the ER's own membranes, and the surrounding extra-luminal space (the general cytoplasm, or cytosol, outside the ER) -- the endoplasmic reticulum allows the cell to carry out certain chemical processes in a physically separate, specialised environment from the rest of the cytoplasm.

The endoplasmic reticulum occurs in two structurally and functionally distinct forms. Rough endoplasmic reticulum (RER) is studded on its outer, cytoplasm-facing surface with large numbers of ribosomes, giving it a granular or "rough" appearance under the electron microscope, from which it takes its name. Because ribosomes attached to the RER are actively engaged in synthesising proteins that will be secreted from the cell, inserted into membranes, or delivered to other organelles, the RER is especially abundant and well developed in cells specialised for protein secretion, such as the cells of glands. Smooth endoplasmic reticulum (SER), by contrast, entirely lacks attached ribosomes and appears smooth-surfaced under the electron microscope. Free of the protein-synthesising machinery found on the RER, the SER instead specialises in the synthesis of lipids, including steroidal hormones, and is correspondingly abundant in cells that manufacture large quantities of these substances, such as the cells of certain endocrine glands.

Material synthesised within the endoplasmic reticulum -- particularly newly made proteins destined for secretion or for delivery elsewhere in the cell -- is not simply released at random but is instead packaged into small, membrane-bound transport vesicles that bud off from the ER and travel through the cytoplasm to the Golgi apparatus, the next major station in the endomembrane system.

The Golgi apparatus, first described by the Italian scientist Camillo Golgi in 1898, is built from a stack of flattened, membrane-bound sacs called cisternae, arranged one above another somewhat like a stack of hollow discs or saucers, and often curved into a shallow cup shape. Because the Golgi apparatus in most cells is not a single isolated structure but occurs as several such stacks distributed through the cytoplasm, often interconnected, the entire collection within a cell is sometimes referred to as the Golgi complex, typically situated close to the nucleus.

The Golgi stack has two structurally and functionally distinct faces. The cis face (sometimes called the forming face) is convex and lies closest to the endoplasmic reticulum; it is here that transport vesicles arriving from the ER fuse with the Golgi membrane and deliver their contents into the Golgi stack. The trans face (the maturing face) is concave and lies on the opposite side of the stack, facing away from the ER, typically toward the plasma membrane; it is from here that mature vesicles eventually bud off and depart, carrying their finished cargo to its final destination, whether that is another organelle, the plasma membrane, or release outside the cell by exocytosis. …