| What are endoplasmic reticulum? | They are a part of the group of membranous organelles named endomembrane system, that comprises ER, Golgi apparatus, endosomes, lysosomes, nuclear membrane (envelope) and vacuoles. This system is involved in coordinated functions, such as synthesis and maturation of carbohydrates, lipids and proteins, sorting , targeting and packing of the synthesized material, and a role in bulk transportation (endo and exocytosis) and intracellular digestion and signaling, Mitochondria and chloroplasts are not a part of this system since their membranes are independent to the other cytoplasmic membranes. |
| How are endoplasmic reticulum classified? | Two broad types that vary structurally and functionally, with some cooperate functions (lipid synthesis) and are both consisting of membranous system enclosing a lumen, and they divide the cytoplasm into cytosol and lumen, found at same rate in hepatocytic cells.
-Smooth ER ( consists of interconnected tubular structures without ribosomes, abundant in cells with active steroid synthesis (Leydig cells in testes)
-Rough ER (extensive and consists of interconnected flattened sacks (cisternae) where ribosomes are attached at the cytosolic face, continuous with nuclear envelope which also attaches ribosomes on its cytosolic face, extremely abundant in protein synthesizing cells (salivary glands, pancreatic cells). |
| List the main functions of the SER. | Extensive in skeletal muscle fibers (named sarcoplasmic reticulum), kidney tubule cells and steroid producing endocrine glands.
-Synthesis of steroid hormones and lipids
-Detoxication of xenobiotics
-Dephosphorylation of glucose-6-phosphate
-sequestering Ca ions. |
| Talk about the lipid synthesis role of SER. | They take place in endocrine glands (Leydig, adrenal cortex) requires specific enzymes. which are present in SER membrane and its lumen, Integral proteins of the hepatocytic SER are responsible for lipid production and lipoproteins circulating in blood stream (LDL...) SER is also involved in phospholipid synthesis. |
| What are xenobiotics? | Xenobiotics are foreign chemicals such as polycyclic aromatic hydrocarbons- ethanol and barbiturates- that promote the proliferation of SER in liver cells. |
| Talk about SER function in detoxication of xenobiotics. | Happens in the liver, nut many organs also contribute to this process (skin, lungs, intestine). Detoxication is transformation of toxic hydrophobic parent chemicals into less toxic hydrophilic ones, which are then eliminated by kidney or liver and through urine or bile products respectively. It is carried out by a system of oxygen transferring enzymes called oxygenases having broad specificity for a wide variety of substrates, among them there is the family of cytochromes P450 present in SER. |
| Talk about Dephosphorylation process done by SER. | Takes place in hepatocytes, storing glucose and glycogen, which are granules next to the SER membranes, when body is not supplied with glucose, through the intestine, the liver uses glycogen reserve by glycogenolysis. Phosphorylase adds phosphate to produce glucose-1-phosphate from glycogen which is then isomerized into glucose-6-phosphate, which cannot be released into blood stream, and is kept in the cytoplasm since plasma membrane is not permeable to phosphorylated sugars. SER contains glucose-6-phosphatase that removes phosphate and becomes glucose which can diffuse by pumps, transporter. |
| Talk about sequestering Ca ions in SER. | SER sequesters (traps) Ca ions inside cisternal space (lumen), which contributes in release in response to stimulation by different signals, there is presence of calcium binding proteins that help store the Ca ions making its concentration low in the cytosol. |
| Talk about the mechanism of Ca sequestering in SER. | Regulated calcium delivery into cytosol which triggers many functions (muscle contraction, nerve impulses, fusion of secretory vesicles.)
AP in muscle fibers reach SER via transverse tubules (extensions of plasma membrane touching SER membrane) which opens SER Ca channels, and enables Ca to interact with proteins (actin, myosin, tropomyosin, troponin...) which triggers contraction, when AP goes out, Ca are pumped actively into the SER lumen so that cytosol Ca concentration becomes less than lumen, which causes muscle relaxation. |
| List main RER functions. | -synthesis of proteins by membrane bound ribosomes
-Glycosylation of proteins and maturation
-Biosynthesis of lipids and proliferation of membranes
-Export of vesicles to Golgi apparatus. |
| Talk about the proteins synthesis role of RER. | Secreted proteins and proteins of the endomembrane system are synthesized by ribosomes attached to the RER cotranslationaly delivery, all other proteins in cells are synthesized by free ribosomes with posttranslational delivery.
Protein synthesis is catalyzed by a signal sequence with the help of SRP, and the translocon will be inserting the growing polypeptide, after removing signal sequence into the lumen of the RER, each protein has an address code that makes the protein go to a specific place.
Integral proteins are synthesized like other proteins, but translocon recognizes helix shaped hydrophobic aa chain, so the translocon captures it and retain it to be delivered to the lipid bilayer by lateral opening |
| Talk about folding function of RER. | Proteins that do not enter RER (nuclear, mitochondrial and chloroplastic) also fold, but RER folds proteins using chaperons that help folding to give protein their secondary...structure, also have a role in disulfide bridges (guides or monitors) may fail sometimes. |
| Talk about the N-glycosylation role of RER. | Sugar linked to N of ِside-chain Asparagine (oligosaccharide) necessary for maturation and activity (glucose, N-acetylglucosamine, mannose...) transferred to specific asparagine and synthesized by specific enzymes (oligosaccharyl transferrases, glycosyl transferrases) molecule that carries oligosaccharide before transfer is lipid called (dolichol) that accepts oligosaccharide prior to its transfer to asparagine. (all enzymes are in membrane of RER) |
| Talk about biosynthesis of membrane by RER. | Synthesized by integral enzymes using precursors to synthesize phospholipids (phosphatidyl choline...) in the cytosolic face of the RER membrane (then moved to appropriate layer by flippases and then sent to other organelles even non-endoplasmic organelles. |
| Talk about vesicle exportation to Golgi apparatus by RER. | Vesicles sent by RER to Golgi with moved lipids and proteins, synthesized by RER to Golgi apparatus (maturation) (mitochondria and chloroplasts do not get vesicles from RER) |
