Vesicle

A vesicle is a tiny bubble of membrane, made of a phospholipid bilayer like the plasma membrane, that encloses a little fluid and cargo. Vesicles bud off one membrane and fuse with another, which lets a cell move proteins and other materials between compartments without spilling them into the cytosol. They are smaller than vacuoles and, unlike most vacuoles, merge readily with other membranes.

Different vesicles do different jobs. Transport vesicles carry newly made proteins and lipids from the endoplasmic reticulum to the Golgi apparatus, and secretory vesicles leave the Golgi and empty their cargo at the cell surface by exocytosis. Endocytosis works the other way, folding part of the plasma membrane inward to form a vesicle that brings material in. At nerve endings, synaptic vesicles store neurotransmitters until a signal arrives, and lysosomes can be thought of as specialized vesicles full of digestive enzymes. Coat proteins such as clathrin help bend the membrane as a vesicle buds, and proteins called SNAREs make sure each vesicle fuses with the correct target membrane.

In medicine the word has wider senses: a small fluid-filled blister on the skin, as in chickenpox or herpes, or an anatomical sac such as the seminal vesicle.