Vesicular Transport: Endocytosis and Exocytosis

Flashcards covering key concepts of vesicular transport, endocytosis, and exocytosis from the lecture notes.

What is vesicular transport?

Movement of materials into, out of, or within a cell enclosed in membrane-bound vesicles during endocytosis, exocytosis, or intracellular trafficking.

What is endocytosis?

Plasma membrane engulfs extracellular material and internalizes it in vesicles.

What is phagocytosis?

Cell eating; engulf large particles into a phagosome; example: macrophages engulfing bacteria.

What is pinocytosis?

Cell drinking; uptake of extracellular fluid and dissolved molecules via small vesicles; example: nutrient absorption in intestinal cells.

What is receptor-mediated endocytosis?

Selective uptake; ligands bind receptors to trigger vesicle formation, often clathrin-coated (LDL uptake).

What is caveolae-mediated endocytosis?

Small flask-shaped plasma membrane invaginations used for signaling and transport; involved in transcytosis in endothelial cells.

What is constitutive exocytosis?

Continuous, non-regulated secretion of vesicle contents to the plasma membrane (e.g., secretion of extracellular matrix proteins).

What is regulated exocytosis?

Vesicles stored and released in response to a stimulus (e.g., Ca2+); e.g., insulin release from pancreatic beta cells.

What is lysosomal exocytosis?

Fusion of lysosomes with the plasma membrane to secrete enzymes or repair membrane (bone resorption by osteoclasts).

What is vesicle-mediated transcytosis?

Vesicle transports molecules across a cell from one side to the other and fuses with the opposite membrane (e.g., transport of antibodies across epithelial cells).

What is exosome release?

Small vesicles formed inside multivesicular bodies are released when these fuse with the plasma membrane (intercellular communication, signaling).

What is exocytosis (general)?

Fusion of intracellular vesicles with the plasma membrane to release contents outside the cell.

What is a phagosome and how is it processed?

Vesicle formed around a phagocytosed particle; fuses with lysosome to form phagolysosome for digestion.

What are the steps of phagocytosis?

Particle recognition → attachment to receptor → pseudopodia formation → phagosome formation → fusion with lysosome → phagolysosome → digestion → residual body expulsion.

Who are professional phagocytes?

Neutrophils, monocytes, macrophages, dendritic cells, mast cells, eosinophils.

What is neutrophil function?

Phagocytose antigens coated with antibodies and complement; intracellular killing; inflammation and tissue damage.

What is the role of monocytes?

Precursors of macrophages and dendritic cells.

What is macrophage function?

Phagocytosis (intracellular and extracellular killing); antigen presentation; tissue repair.

What is dendritic cell function?

Phagocytosis and antigen presentation; acts as APC; initiates adaptive immunity.

What is eosinophil function?

Immunity against parasites; phagocytose parasites; eosinophilic granules; levels rise in parasitic infestation.

What are the steps of pinocytosis?

Membrane invagination forms vesicles; internalization of extracellular fluid and solutes; non-selective uptake.

What are the steps of receptor-mediated endocytosis?

Ligand binds receptor; receptors cluster in coated pits; invagination; coated vesicle formation; uncoating; early endosome; sorting; receptor recycling; fusion with lysosome; ligand degradation.

What is clathrin-mediated endocytosis?

A type of receptor-mediated endocytosis using clathrin-coated pits to internalize ligand-receptor complexes.

How is LDL uptake linked to familial hypercholesterolemia?

LDL binds LDL receptor in clathrin-coated pits; defective receptor impairs receptor-mediated endocytosis of LDL, causing high plasma LDL.

What is the role of dynamin?

GTPase required for scission/pinching off of clathrin-coated vesicles during receptor-mediated endocytosis.

Differentiate endocytosis and exocytosis.

Endocytosis internalizes materials; exocytosis releases materials; constitutive exocytosis is continuous; regulated exocytosis requires stimulus.

Describe LDL processing after endocytosis.

Endosome acidifies; LDL dissociates from receptor; receptor recycles; LDL goes to lysosome; cholesterol released; high intracellular cholesterol downregulates LDL receptor synthesis.

What is transcytosis and provide an example?

Transport of vesicles across a cell and fusion with the opposite membrane; example: IgA transport across mucosal epithelium.

How can basolateral exocytosis of zymogen granules cause pancreatitis?

Enzymes released into interstitium instead of lumen cause autodigestion and inflammation; basolateral trafficking disrupts normal apical secretion.

What are zymogen granules?

Secretory granules storing digestive enzyme precursors at the apical pole; fuse with apical membrane to release into lumen.

What regulates zymogen granule exocytosis in the pancreas?

Typically regulated exocytosis triggered by secretagogues and Ca2+ signaling; release into duct lumen.

What are the clinical implications of vesicular transport defects?

Defects can cause diseases such as familial hypercholesterolemia (LDL receptor defect) and pancreatitis due to enzyme mistrafficking; can affect insulin secretion and ECM protein secretion.