Receptor-mediated endocytosis
A highly selective form of endocytosis that begins when receptors on a cell's surface bind specific substances, triggering the plasma membrane to form a vesicle around them, drawing them into the cell.
Examples Vitamins, antibodies, hormones, low-density lipoproteins (LDLs), and the iron-transporter, known as transferrin, are all substances taken up into cells by receptor-mediated endocytosis.
Phagocytosis
A process by which specialized cells, known as phagocytes, engulf, and dispose of large, solid particles such as dead cells and bacteria or viruses, helping to protect the body from invading microbes.
It is a specific form of endocytosis, resulting only in the ingestion of much larger solid particles that have not previously been broken down.
Examples Macrophages and neutrophils are examples of phagocytes.
Pinocytosis
Also known as bulk-phase endocytosis, pinocytosis is carried out by most cells of the body. It is the process by which small amounts of extracellular fluid along with any dissolved solutes are taken up into the cell.
Pinocytosis is a non-specific process, where all solutes within the extracellular fluid are transported.
Examples Cells that undergo pinocytosis include cells of the kidney, epithelial cells of the intestines, cells of the liver, and capillary epithelial cells.
Receptor-mediated endocytosis
Receptor-mediated endocytosis occurs in the following steps:
1. A ligand binds to a specific receptor on the cell's plasma membrane, forming a ligand-receptor complex. Each receptor is associated with a protein, known as a clathrin, on the membrane's cytoplasmic side.
The regions of the plasma membrane in which these receptors and their associated clathrin molecules are expressed are known as clathrin-coated pits.
2. The clathrin-coated pit sinks into the cell, forming a vesicle that contains the ligand-receptor complexes. This vesicle becomes detached from the plasma membrane and enters the cytoplasm.
3. Once in the cytoplasm, the clathrin molecules coating the outer edge of the vesicle leave, and associate with new receptors on the plasma membrane.
4. The uncoated vesicle fuses with an endosome, and the ligands and receptors separate, collecting at opposite ends of the endosome.
5. Sections of the endosome containing unbound receptors pinch off, forming transport vesicles that return the receptors to the plasma membrane.
6. The remaining vesicles, which now contain free ligands, fuse with a lysosome containing digestive enzymes.
7. Finally, the lysosome's digestive enzymes break the ligands down into smaller molecules, which are then released into the cytoplasm of the cell for use in a number of cell processes.
