Cell Biology Course (Bio-1101) Lect. 6: Cell Organelles II PDF

Cell Biology Course (Bio-1101) // Lect. 6: Cell Organelles II Course coordinator Dr.Radwa.H.Lutfy Single-bound membrane organelles Such as... 1- Endoplasmic reticulum 2- Golgi apparatus 3- Lysosomes 4- Vacuoles  The endomembrane system...

Cell Biology Course (Bio-1101) // Lect. 6: Cell Organelles II Course coordinator Dr.Radwa.H.Lutfy Single-bound membrane organelles Such as... 1- Endoplasmic reticulum 2- Golgi apparatus 3- Lysosomes 4- Vacuoles  The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, various kinds of vesicles and vacuoles, and plasma membrane.  I- Endoplasmic reticulum (ER) * Structure: The endoplasmic reticulum (ER) is an extensive network of membranes or intercommunicating channels that are called cisternae and accounts for more than half of total membranes in many eukaryotic cells. (endoplasmic means “within cytoplasm,” and reticulum means “net.”) It extends from outer nuclear membrane throughout most of cytoplasm to plasma membrane. * General function of ER: It moves molecules through cisternal space (lumen or cavity ) from one part of the cell to another throughout cytoplasm. * Types: 1- Rough Endoplasmic Reticulum (RER) with ribosomes on its surface. It is rich in cells specialized for protein secretion, such as pancreatic acinar cells (making digestive enzymes), fibroblasts ( making collagen), hepatocytes (secreting albumins) and plasma cells (making immunoglobulins). 2- Smooth Endoplasmic Reticulum (SER) without ribosomes on its surface. It is rich in Leydig cells of testis (secreting testosterone), hepatocytes (for detoxification) and other cells. * Function of RER: The RER is a place where proteins that are synthesized, packaged, and then secreted. * Function of SER: 1- Enzymes in the SER synthesize phospholipids (major component of cellular membranes) and steroids or sex hormones (e.g. testosterone from leyding cells of testis). 2- SER enzymes of hepatocytes ( e.g. cytochrome P450 family) allow detoxification of harmful molecules (e.g. alcohol, toxins, drugs,.. etc.). In liver cells, SER enzymes process components of bile. Jaundice is a disease with yellowish color of skin due accumulation in extracellular bilirubin and other pigments, which are normally metabolized by SER enzymes in liver cells and excreted as bile. 3- Muscle contraction and relaxation involve the release and recapture of Ca2+ by the SER in skeletal muscle cells, called the sarcoplasmic reticulum.  II- Golgi apparatus The Golgi apparatus appears as a series of flattened, membranous sacs, or cisternae. It receives vesicles containing proteins recently produced by the RER through its cis face. Proteins undergoes modification inside it. Golgi apparatus has: a- Cis-face receives incoming transport vesicles (vesicles containing proteins ) from RER. b- Cisternae in which protein modifications occur. c- Trans-face release secretory vesicles (with modified vesicles). Protein modifications within the Golgi Complex include: 1- Glycosylatlon- Addition of carbohydrate. 2- Sulfation-Addition of sulfur. 3- Phosphorylation-Addition of phosphate. 4- Proteolysis- Cleavage of peptide bonds.  III- Lysosomes Synthesis of lysosomal enzymes occurs in the RER, with packaging in the Golgi apparatus. A lysosome is a membranous sac of hydrolytic enzymes that many eukaryotic cells use to digest (hydrolyze) macromolecules. Lysosomal enzymes work best in the acidic pH of lysosomes. If a lysosome breaks open or leaks its contents, the released enzymes are not very active because the cytosol has a near- neutral pH. However, excessive leakage from a large number of lysosomes can destroy a cell by self-digestion.  Types of lysosomes 1- Primary Lysosomes: They are the newly secreted from Golgi apparatus and not fuse with any other vesicle. 2- Secondary lysosomes (= Heterolysosomes): They are formed by fusion of primary lysosomes with other vesicles. So , the ingested material will be degraded. A- Phagolysosomes are formed by fusion of a phagocytic vacuole (or phagosomes) with lysosome. B- Multivesicular bodies are formed by fusion of endocytic vesicle (containing endosome) with a lysosome. C- Autophagolysosomes are formed by fusion of an autophagic vacuole (formed by autophagy in which worn out e.g. mitochondrion or nonfunctional organelles are surrounded by a vacuole) with a lysosome. 3- Tertiary Lysosomes: They are those lysosomes in which only indigestible food materials (residual bodies) have been left. These particles are secreted from cell by exocytosis.  IV- Vacuoles Vacuoles are large vesicles derived from the endoplasmic reticulum and Golgi apparatus. Types: 1- Food vacuoles formed by phagocytosis (engulfment of food particle by pseudopodia) in phagocytic cells e.g. Macrophage & some protists e.g. Amoeba. 2- Many protists live in fresh water have contractile vacuoles that pump excess water out of the cell. 3- In plants: A- some small vacuoles that store organic compounds.g. proteins in seeds. B- Some Vacuoles protect plant against animals by storing poisonous or unpalatable compounds. C- Some vacuoles contain pigments, such as red and blue pigments of petals that help attract pollinating insects to flowers. D- Mature plant cells generally contain a large central vacuole Which contains cell sap rich in minerals and other stored compounds.