Transmembrane Transport of Macromolecules and Particles PDF

involves the uptake of extracellular fluid and...

involves the uptake of extracellular fluid and solutes in solution via small vesicles (~150 nm in diameter **Pinocytosis** non-specific process: Occurs in all cells continuously (constant invagination of the membrane) specific mechanism where ligands (molecules) bind to receptors on the cell surface, triggering the formation of vesicles. 1. The clathrin-coated pits on the plasma membrane play a critical role here. Once the ligand binds to the receptor, the membrane invaginates and pinches off, forming a coated vesicle. 2. Membrane invaginates and pinches off to **Steps:** form a vesicle responsible for the specificity of vesicle targeting. Ensuring that the vesicle fuses with **Receptor-mediated Endocytosis** 3. The vesicle is coated with clathrin for the correct compartment. stability and trafficking **v-SNARE** (on vesicle) binds specifically to **t-SNARE** (on target membrane) **SNARE Proteins** Membrane Fusion This type of endocytosis occurs in specific regions of the plasma membrane, called clathrin pits, which are supported by networks Example: **Botulinum toxin** inhibits SNARE- organized into structures of hexagons and of proteins (e.g., adaptin) mediated fusion, preventing neurotransmitter pentagons to form a coat around the vesicles. release and causing paralysis Clathrin helps to maintain the shape of the Occurs via clathrin-coated pits **Clathrin Structure:** vesicle and directs it to its destination. Waste disposal Endocytosis **Types of Endocytosis** Triskelion (3 heavy + 3 light chains) Transmembrane LDL binds LDL receptor (LDL-R) Transport of macromolecules It is essential for: Transport of Membrane maintenance, to balance the plasma membrane added during endocytosis. Macromolecules and LDL-R internalized into vesicle LDL particule Continuous secretion of materials Particles In the bloodstream, cholesterol is transported in Low-Density Lipoproteins (LDL). Each LDL The vesicle loses its clathrin coat and fuses with early endosome particle consists of cholesterol, phospholipids, example of receptor-mediated endocytosis: and a protein called apolipoprotein B100. The Present in all cells **Constitutive Exocytosis** cholesterol import LDL moves to lysosome, the receptor separates LDL particles bind to LDL receptors on the cell from LDL and is recycled back to the surface, triggering vesicle formation for membrane (changes in the acidic environment Transports macromolecules to the plasma internalization. within the early endosome (ph5)) , cholesterol membrane or outside the cell released **Types of Exocytosis** The reverse of endocytosis: It allows cells to Occurs in specialized cells (e.g., neurons, export materials, such as hormones, hormone-secreting cells) Exocytosis LDL-R recycled to plasma membrane: LDL is neurotransmitters, and digestive enzymes, by transported to lysosomes, where cholesterol is fusing vesicles with the plasma membrane. -Binding site of LDL and LDL-R freed for cellular use. Triggered by external signals (like the release of **Regulated Exocytosis** hormones or neurotransmitters) is specialized in the engulfment of large particles like dead cells or microorganisms. Histamine release causes vasodilation, edema, Example: the release of histamine by mast cells and redness in response to allergic reactions. in response to immune signals. involves phagosomes, large vesicles that form around the target. **Clathrin-coated vesicles**: Transport from plasma membrane or Golgi to endosomes or lysosomes. Phagocytosis can be triggered by immune **Phagocytosis** system markers like antibodies, lectins, or complement proteins **COP I vesicles**: Transport between Golgi compartments or from Golgi to ER Different types of coated vesicles: **Vesicle Transport** (Endoplasmic Reticulum) 1. Adhesion of the particle **COP II vesicles**: Transport from ER to Golgi Phagocytosis occurs in stages: 2. Ingestion via membrane extension **Retromer vesicles**: recycling from early 3. Digestion within the phagosome Forms large vesicles (phagosomes) endosomes to Golgi The process is driven by the actin cytoskeleton, which reorganizes itself to support vesicle formation