Endocytosis Notes

Endocytosis

• Endocytosis: The uptake of extracellular material into the cell.

  • The membrane folds inward, eventually pinching off to form an endocytic vesicle.

  • Endocytic vesicles contain material from outside the cell.

Importance of Endocytosis

• Ingestion of nutrients by single-celled organisms.

• Defense mechanism (e.g., macrophages phagocytosing pathogens).

• Recycling and reuse of molecules deposited in the plasma membrane during exocytosis.

  • Balances membrane addition from exocytosis.

  • Retrograde transport moves membrane back into the cell.

• Recycling old proteins by digesting them and replacing them with new ones via exocytosis.

Membrane Recycling

• Pancreatic cells recycle their entire cell membrane within 90 minutes.

• Cultured macrophages recycle their entire membrane in 30 minutes.

• This indicates a high rate of both exocytosis and endocytosis in cells.

Types of Endocytosis (Based on Material Pulled In)

• Phagocytosis:

  • Uptake of large molecules or particles.

  • Performed by white blood cells (macrophages, neutrophils).

  • Can involve engulfing bacteria or cellular debris.

• Pinocytosis:

  • Uptake of liquids and small molecules from the extracellular fluid.

  • Mechanism for transporting protein hormones:

    • Hormones in the bloodstream are taken up by endothelial cells via pinocytosis.

    • Vesicles transport hormones through the cell.

    • Hormones are released on the other side via exocytosis to reach target cells.

Mechanisms of Endocytosis

• Receptor-mediated endocytosis

• Non-receptor-mediated endocytosis