Endomembrane System Components PDF

Understand the various compartments of the endomembrane system and how the compartments and their functions are inter-related to support the coordinated function of the system Endoplasmic Reticulum (ER) Structure: The ER is a continuous network of flattened sacs, tubules, and vesicles dist...

Understand the various compartments of the endomembrane system and how the compartments and their functions are inter-related to support the coordinated function of the system Endoplasmic Reticulum (ER) Structure: The ER is a continuous network of flattened sacs, tubules, and vesicles distributed throughout the cytoplasm. It is divided into two main regions: rough ER (RER) and smooth ER (SER). The RER is studded with ribosomes on its cytosolic surface, giving it a "rough" appearance. The SER lacks ribosomes, has a smooth appearance, and forms a network of interconnected tubules. Functions: Rough ER (RER): ○ Protein Synthesis: Ribosomes on the RER synthesize membrane-bound and secretory proteins. These proteins are translocated into the ER lumen or inserted into the ER membrane during synthesis. ○ Protein Folding and Modification: Newly synthesized proteins undergo proper folding, disulfide bond formation, and initial glycosylation (addition of carbohydrate side chains) in the RER. ○ Quality Control: Misfolded or improperly assembled proteins are recognized and targeted for degradation via the ER-associated degradation (ERAD) pathway. Smooth ER (SER): ○ Lipid Synthesis: The SER is involved in the synthesis of lipids, including phospholipids, cholesterol, and steroid hormones. ○ Detoxification: Enzymes in the SER, such as cytochrome P450, metabolize and detoxify drugs and harmful substances by adding hydroxyl groups, increasing their solubility. ○ Carbohydrate Metabolism: The SER contains glucose-6-phosphatase, which is involved in the hydrolysis of glucose-6-phosphate to glucose, an essential step in glycogenolysis. ○ Calcium Storage: The SER sequesters calcium ions and releases them in response to specific signals, playing a crucial role in muscle contraction and other cellular processes. Golgi Complex Structure: The Golgi complex consists of a series of flattened membrane-bound sacs called cisternae, organized into stacks.  It has two distinct faces: the cis face (cis-Golgi network, CGN), which is oriented towards the ER, and the trans face (trans-Golgi network, TGN), which faces the plasma membrane. The medial cisternae lie between the cis and trans faces. Functions: Protein and Lipid Modification: Glycoproteins and glycolipids undergo further modifications in the Golgi. This includes the addition, removal, or modification of sugar moieties, phosphorylation, and sulfation. Sorting and Packaging: The Golgi sorts proteins and lipids and packages them into vesicles for transport to their specific destinations, such as lysosomes, the plasma membrane, or secretory vesicles. Vesicle Formation: The TGN is the primary site where transport vesicles bud off, carrying processed molecules to various cellular destinations. Endosomes Structure: Endosomes are membrane-bound vesicles formed via endocytosis. They mature through stages: early endosomes, late endosomes, and recycling endosomes. Functions: Sorting of Endocytosed Material: Early endosomes receive endocytosed material and sort it, directing some to recycling endosomes and others to late endosomes for degradation. Recycling: Recycling endosomes return membrane receptors and other molecules back to the plasma membrane. Maturation: Late endosomes fuse with lysosomes, where their contents are degraded. Lysosomes Structure: Lysosomes are membrane-bound organelles containing hydrolytic enzymes. They maintain an acidic pH optimal for enzyme activity. Functions: Degradation of Biomolecules: Lysosomes degrade macromolecules, including proteins, lipids, nucleic acids, and carbohydrates. Autophagy: They digest damaged or unnecessary cellular components through autophagy, contributing to cellular renewal and homeostasis. Pathogen Destruction: Lysosomes can fuse with phagosomes containing pathogens, leading to their degradation. Vacuoles (primarily in plant cells) Structure: Vacuoles are large membrane-bound compartments filled with cell sap, a solution of water, ions, sugars, amino acids, and other substances. Functions: Storage: Vacuoles store nutrients, waste products, and other substances. Turgor Pressure: They provide structural support by maintaining turgor pressure, essential for plant cell rigidity. Detoxification: Vacuoles can sequester harmful substances, detoxifying the cell. Vesicular Trafficking Process: Vesicular trafficking is the transport of materials within the cell via membrane-bound vesicles. Formation and Budding: Vesicles form by budding from donor membranes, incorporating specific cargo proteins and lipids. Transport: Vesicles are transported along the cytoskeleton by motor proteins, such as kinesins and dyneins on microtubules and myosins on actin filaments. Tethering and Docking: Vesicles are tethered to and docked at target membranes by tethering proteins and SNARE complexes. Fusion: The vesicle membrane fuses with the target membrane, releasing its contents into the target compartment. Interrelation of Compartments: ER-Golgi Transport: Proteins and lipids synthesized in the ER are transported to the Golgi for further processing and sorting. Golgi-Endosome-Lysosome Pathway: The Golgi sorts and packages proteins and lipids into vesicles directed to endosomes and lysosomes. Endosomes mature and deliver their contents to lysosomes for degradation. Secretory Pathway: The Golgi packages proteins into secretory vesicles, which fuse with the plasma membrane to release their contents outside the cell (exocytosis). Endocytic Pathway: The plasma membrane internalizes extracellular materials into endocytic vesicles, which fuse with early endosomes. Early endosomes sort the material, directing some to recycling endosomes and others to late endosomes and lysosomes.