5.14
Pinocytosis
Cells use energy-dependent bulk transport mechanisms to move large particles, or substantial quantities of small particles, into or out of the cell. D…
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.
Q1: What is pinocytosis and how does it work in cells?
Pinocytosis is a type of endocytosis where cells engulf extracellular fluid containing water and dissolved nutrients, often called cellular drinking. Structural proteins like clathrin triskelions assemble on the cytosolic side of the plasma membrane, forming curved pits that pinch off into coated vesicles. These vesicles transport the fluid and its contents throughout the cell for processing.
Q2: How does pinocytosis differ from phagocytosis?
Pinocytosis and phagocytosis are distinct endocytic processes. Phagocytosis, or cell-eating, engulfs large particles and produces large vesicles. Pinocytosis transports fluid along with smaller particles in substantially smaller vesicles. While phagocytosis targets specific large materials, pinocytosis is non-selective, taking in surrounding extracellular fluid and any particles it contains.
Q3: Why is pinocytosis considered non-specific transport?
Pinocytosis is non-specific because it lacks receptors to bind particular substances on the cell surface. Instead of selectively targeting specific molecules, the cell indiscriminately takes in surrounding extracellular fluid along with whatever particles it contains. This makes pinocytosis an equal-opportunity importer that engulfs material without molecular discrimination.
Q4: What role do structural proteins play in pinocytosis?
Structural proteins such as clathrin triskelions are essential for pinocytosis. These proteins assemble and coat the cytosolic side of the plasma membrane, creating the curved pits necessary for vesicle formation. As the membrane pockets deepen, the proteins help stabilize the structure until the vesicle pinches off and separates from the membrane.
Q5: Where in the body does pinocytosis occur and why?
Pinocytosis occurs in many cell types throughout the body. In the small intestine, microvilli use pinocytosis to absorb nutrients from food. Egg cells use pinocytosis to obtain nutrients before fertilization. This process allows cells to efficiently uptake fluid and dissolved nutrients from their surrounding environment.
Q6: How do vesicles form during pinocytosis?
During pinocytosis, sections of the cell membrane sink inward, creating tear-shaped pockets that surround the fluid and molecules being taken into the cell. As the membrane continues to deepen and curve, it eventually reconnects, causing the vesicles to pinch off and separate from the membrane. The enclosed substances then enter the cell within these newly formed vesicles.
Q7: How does pinocytosis compare to receptor-mediated endocytosis?
Unlike receptor-mediated endocytosis, pinocytosis is non-selective and does not require specific receptors to recognize target molecules. Receptor-mediated endocytosis uses receptors to bind particular substances before internalization, whereas pinocytosis takes in surrounding extracellular fluid indiscriminately. Both processes use vesicles to transport material into the cell, but differ in their selectivity mechanisms.