Explain how exocytosis occurs in a cell.

Exocytosis is a process where cells transport molecules out of the cell via vesicles that merge with the plasma membrane.

Exocytosis is a vital cellular process that allows cells to secrete substances out of the cell. This process is particularly important in cells that are specialised for secretion, such as those in the pancreas that release insulin, or nerve cells that release neurotransmitters. However, all cells are capable of exocytosis as it is also used to replace lipids and proteins in the plasma membrane.

The process of exocytosis begins in the endoplasmic reticulum, where proteins are synthesised. These proteins are then packaged into transport vesicles, which bud off from the endoplic reticulum and are transported to the Golgi apparatus. Here, the proteins are modified and sorted for transport to their final destinations. The proteins destined for exocytosis are packaged into secretory vesicles, which move towards the plasma membrane.

The secretory vesicles then dock at the plasma membrane, a process that is mediated by specific proteins known as SNAREs. These proteins help to ensure that the vesicle is correctly positioned at the membrane and ready for fusion. The vesicle and plasma membranes then begin to merge, a process known as membrane fusion. This is facilitated by the action of another set of proteins, known as fusion proteins.

Once the vesicle and plasma membranes have fused, the contents of the vesicle are expelled out of the cell. This is achieved by the opening of a pore in the plasma membrane, through which the vesicle contents can be released. This pore then closes up again, ensuring that the integrity of the plasma membrane is maintained.

In summary, exocytosis is a complex process that involves the transport of molecules from their site of synthesis in the endoplasmic reticulum, through the Golgi apparatus, and out of the cell via vesicles that fuse with the plasma membrane. This process is essential for the secretion of many important substances and for maintaining the composition of the plasma membrane.