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Exam 3 study guide Lecture 12 and 13 Endomembrane system: ER, golgi, endosomes, and lysosomes (not peroxisome) 1.Smooth ER: drug detoxification -Hydroxylation: addition of OH groups. Improves solubility of hydrophobic drugs- for excretion -Hydroxylation of organic acceptor molecules is catalyzed by Cytochrome P-450 (abundant in hepatocyte (liver cells) -Dangers: some oxidized products are more toxic than the original ones. Increased tolerance to some drugs and a decrease in the effectiveness of other drugs The smooth ER of hepatocytes is also involved in the enzymatic breakdown of stored glycogen, by the enzyme glucose-6-phosphatase:-Hydrolyzes the phosphate group from glucose 6 phosphate to form free glucose and inorganic P (Pi) -Because membranes are generally impermeable to phosphorylated sugars, the glucose-6-phosphate must be converted to free glucose by glucose-6-phosphatase in order to leave the celland enter the bloodstream.-glucose 6 phosphate activity is not significant in muscle or brain cells bc they retain it to meet their own energy needs The sarcoplasmic reticulum of muscle cells is an example of smooth ER that specializes in calcium storage-Lumen contains high concentrations of calcium-binding proteins-Calcium ions are pumped into the ER by calcium ATPases-Calcium ions are released when needed for muscle contractionHigh expression of enzymes involved in cholesterol synthesis and in the synthesis of steroid hormones: cortisol- producing cells of the adrenal gland-the Leydig cells of the testes, which produce testosterone-the cholesterol- producing cells of the liver-The follicular cells of the ovaryMembrane biosynthesis Enzymes that synthesize phospholipids are integral membrane ER proteins with their active sites facing the cytosol. -Newly synthesized phospholipids inserted into cytosolic leaflet bc biosynthesis of fatty acids for membrane phospholipid molecules occurs in the cytoplasm-Some are later flipped into the opposite leaflet by flippases: catalyze the translocation of phospholipids through ER membraneEven though membrane phospholipids are synthesized in the ER, membranes of different organelles can have different lipid compositions. Why?
Retrograde transport: is the flow of vesicles from Golgi cisternae back to the ER (away from plasma membrane into the ER)