5.16
Exocytosis
Exocytosis is used to release material from cells. Like other bulk transport mechanisms, exocytosis requires energy.
While endocytosis takes particles…
Exocytosis is the process by which molecules produced inside a eukaryotic cell are secreted out of the cell through the fusion of membrane-bound vesicles with the plasma membrane.
For example, secretory proteins destined for exocytosis first leave the endoplasmic reticulum. They are then packaged into vesicles that contain specific membrane proteins, which guide vesicle targeting and fusion.
This cargo moves through the cisternae of the Golgi apparatus. Vesicles repeatedly bud and fuse until the contents reach the trans-Golgi network.
Here, the cargo buds off into vesicles once again. These vesicles then move toward their target, the plasma membrane, where the vesicular membrane fuses with it and releases the protein into the extracellular space. Vesicular membrane components are retrieved and recycled within the cell.
Q1: What is exocytosis and how does it work in eukaryotic cells?
Exocytosis is the process by which substances produced inside a eukaryotic cell are secreted out through fusion of membrane-bound vesicles to the plasma membrane. Cargo like peptide hormones leaves the endoplasmic reticulum in transport vesicles, moves through the Golgi apparatus, and finally fuses with the plasma membrane, releasing contents into the extracellular space.
Q2: What are the main differences between regulated and constitutive exocytosis?
Regulated exocytosis requires an external signal and is used to release neurotransmitters and secrete hormones. Constitutive exocytosis is carried out by all cells without requiring a signal, releasing components of the extracellular matrix or incorporating proteins into the plasma membrane. Regulated exocytosis has five major steps, while constitutive exocytosis has four.
Q3: How do motor proteins facilitate vesicle movement during exocytosis?
Motor proteins actively move vesicles along cytoskeletal tracks composed of microtubules and filaments during the vesicle trafficking step. This active transport mechanism ensures vesicles carrying cargo reach the plasma membrane efficiently, positioning them for subsequent tethering and docking stages of exocytosis.
Q4: What happens during vesicle priming in regulated exocytosis?
Vesicle priming occurs only in regulated exocytosis after the vesicle docks but before it releases its contents. This step includes modifications that prepare vesicles for fusion with the plasma membrane, ensuring they are ready to respond to external signals and release their cargo appropriately.
Q5: What is the difference between complete fusion and kiss-and-run fusion?
In complete fusion, vesicles entirely collapse and become part of the plasma membrane, expelling their contents from the cell. In kiss-and-run fusion, the vesicle only temporarily fuses with the plasma membrane, releases its contents, and returns to the cell's interior for recycling.
Q6: What are the main functions of exocytosis in cells?
Cells use exocytosis to release signaling molecules like neurotransmitters, insert proteins such as ion channels into their cell membranes, secrete proteins for use in the extracellular matrix, and release waste. Like other bulk transport mechanisms, exocytosis requires energy to move material out of the cell.
Q7: How do vesicle tethering and docking prepare vesicles for fusion?
Vesicle tethering links vesicles to the plasma membrane, establishing initial contact. During docking, the vesicle membrane attaches to the plasma membrane and the two membranes begin to merge. These sequential steps position vesicles correctly and bring their membranes into close proximity before the final fusion event occurs.