Introduction to Membrane Traffic

Broadly, membrane trafficking can be of three categories. Cargo can be transported within the cell from one organelle to another using the secretory pathway or, into the cell using endocytosis, and out of the cell using exocytosis.

In the secretory pathway, substances produced inside the cell are packaged into protein-coated, membrane-bound carriers called vesicles that can be transported from one organelle to another.

The SNARE family of proteins dock the vesicle to the target membrane and catalyze the fusion of the vesicle membrane to deliver the cargo.

If the vesicles fuse with the plasma membrane, the cargo is released to extracellular space, and the process is called exocytosis. Typically, substances that need to be exported are waste products, membrane proteins, or signaling molecules required for cellular communication.

Conversely, in endocytosis, substances not produced in the cell, such as vitamins, cholesterol, and micronutrients, are imported into the cell.

In pinocytosis, a type of endocytosis, a cell membrane surrounds the extracellular fluid, including water and dissolved nutrients.

Another type of endocytosis, called phagocytosis, occurs when the cell surface receptors encounter a foreign particle, usually an invading microorganism or cell debris from damaged tissue.

The cell extends to engulf the particle, and the membranes fuse, trapping the particle inside.