Plant Physiology - Musienko M. M. 2001

Chemical and molecular composition, structure, and functions of the plant cell
Endoplasmic reticulum and membrane biogenesis

The Endoplasmic reticulum (reticulum — network) is a complex three-dimensional membrane system, the shape and extent of which are determined by The Cell type and its stage of differentiation. In three dimensions, it has a lamellar Structure and consists of numerous membrane-bound sacs called cisternae. Electron Microscopy reveals numerous granules up to 30 nm in diameter on their surface—Ribosomes, where Protein Synthesis takes place. This is the so-called rough ER. Cells specialized in lipid synthesis feature a highly branched, smooth tubular endoplasmic reticulum. Both types of ER can coexist within the same cell and are interconnected. The Functions of the rough ER are associated with The transport of Proteins synthesized by the ribosomes on its surface.

At the onset of protein synthesis, the initial segment of the synthesized polypeptide chain constitutes the so-called "signal sequence," whose conformation corresponds to a specific receptor on the ER membrane. The receptor forms a channel through which the protein enters the ER cisternae for subsequent transport. During translocation, the protein undergoes significant modifications, such as phosphorylation and conversion into Glycoproteins.

The endoplasmic reticulum plays a leading role in the Biogenesis of Cellular membranes, as it serves as the primary site for the synthesis of both Membrane Proteins and cellular Lipids. The final stages of membrane lipid synthesis, particularly Glycolipids and Phospholipids, are localized within the ER. The latter provide for The formation of Mitochondrial and Chloroplast membranes. In addition, the ER is the site of steroid Biosynthesis and the synthesis of all Unsaturated Fatty acids: plant-specific fatty acids—such as linoleic, linolenic, and arachidonic—are synthesized precisely here. The membranes of vacuoles, spherosomes, Microbodies, and other Organelles are derivatives of ER membranes. The reticulum is directly connected to the nuclear envelope. Through the membrane System of the Golgi apparatus, it participates in the Synthesis of Plasma membrane components. The transition of membranes into various types of organelles has been termed "membrane flow." METABOLISM/2.html">THE CONCEPT OF the endomembrane system describes the interaction of cellular membrane components. According to this concept, "membrane flow" explains the functional continuity of membranes and their involvement in the vital activity of all cell organelles.

Thus, the endoplasmic reticulum is an extensive membrane system that divides the interior of a Introduction/5.html">Eukaryotic Cell into compartments and channels. It forms a single continuum with the outer membrane of the nuclear envelope. The rough ER is responsible for the synthesis and trans-membrane transport of proteins, whereas the smooth ER handles lipid synthesis and other biosynthetic processes.