Pinocytosis

Pinocytosis is a type of endocytosis in which a cell takes in extracellular fluid that contains water and dissolved molecules.

This process is called cell drinking.

One common mechanism is clathrin-mediated pinocytosis. Structural proteins called clathrin triskelions assemble on the cytosolic side of the plasma membrane through adaptor proteins to form a clathrin-coated pit.

The membrane bends inward, and extracellular fluid and dissolved molecules enter the forming pocket.

The membrane then pinches off to form a clathrin-coated vesicle that carries the fluid into the cell.

The vesicle then loses its clathrin coat and fuses with early endosomes for further intracellular transport.

Cells also take in extracellular fluid through caveolae-mediated pinocytosis.

Caveolae are small, flask-shaped invaginations of the plasma membrane that contain caveolin proteins and cholesterol-rich lipid domains.

These regions bud inward and trap extracellular fluid, forming vesicles that can participate in different intracellular transport pathways, depending on the cell type and the molecules being transported.

Cells also perform clathrin- and caveolin-independent pinocytosis, in which the plasma membrane bends inward without forming clathrin coats or caveolae.

The inward fold traps extracellular fluid and dissolved molecules, forming vesicles that carry the fluid into the cell.

Another form is macropinocytosis, in which the cell membrane forms actin-driven ruffles that fold back onto the cell surface, trapping extracellular fluid and forming large vesicles called macropinosomes.

These vesicles then enter the endosomal pathway for intracellular sorting.