Endocytosis is the cellular process by which cells internalize external material by enveloping it with a portion of the plasma membrane, forming vesicles that carry the cargo into the cell. It is a form of active transport and encompasses several distinct pathways, including phagocytosis, pinocytosis, and receptor-mediated endocytosis. Key forms and features
- Phagocytosis (cell eating): Large particles or whole cells are engulfed by the cell. The inward-facing membrane area invaginates and expands to surround the target, forming a vesicle that is internalized. The vesicle then fuses with a lysosome for degradation. In immune cells like neutrophils and macrophages, this process targets pathogens.
- Pinocytosis (cell drinking): The cell membrane folds inward to form vesicles that capture extracellular fluid and dissolved solutes. This process continuously samples the extracellular environment and is less selective than receptor-mediated pathways.
- Receptor-mediated endocytosis: Highly specific uptake driven by binding of ligands to cell-surface receptors, which cluster and invaginate the membrane to form a vesicle containing the bound cargo. This pathway allows cells to efficiently internalize specific molecules such as cholesterol via low-density lipoprotein (LDL) particles.
- Vesicle formation and trafficking: In endocytosis, the plasma membrane forms a membrane-bound vesicle that is transported into the cytoplasm. The vesicle may fuse with endosomes and later merge with lysosomes for cargo processing or recycle back to the plasma membrane.
Context and mechanisms
- Endocytosis is driven by the cytoskeleton and involves coordinated membrane remodeling components. The process is energy-dependent and relies on signaling pathways to regulate vesicle formation, cargo selection, and trafficking. Disruptions in endocytic pathways are linked to various diseases and disorders, including immune dysfunction and neurodegenerative conditions.
- Beyond nutrient uptake, endocytosis also modulates receptor availability at the cell surface, influences signal transduction, and participates in antigen presentation in immune cells.
Representative sources for further reading
- General overview and definitions, including phagocytosis, pinocytosis, and receptor-mediated endocytosis.
- Bulk transport and receptor-mediated endocytosis with emphasis on specificity and endosomal trafficking.
- Comprehensive reviews and foundational explanations of endocytosis mechanisms.
If you’d like, I can tailor this into a concise study sheet or compare endocytosis with exocytosis, highlighting when each process occurs and their energy requirements.
