When are endocytic vesicles formed?

Endocytic vesicles are formed when the plasma membrane surrounds a particle outside the cell, a process known as endocytosis. This mechanism allows cells to take in external substances by enclosing them within a membrane-bound vesicle. The plasma membrane invaginates, or folds inward, around the material, which can include nutrients, signaling molecules, or other particles. Once the membrane surrounds the particle, it pinches off to form an endocytic vesicle, allowing the substance to be brought into the cell for various purposes such as nutrient uptake or receptor-mediated signaling.

The other choices do not accurately describe the formation of endocytic vesicles. Cell division involves processes that do not directly relate to the uptake of external material. The breakdown of a molecule pertains more to degradation processes within the cell, such as those carried out by lysosomes, rather than the formation of vesicles for uptake. Lastly, the movement of ions through cell membranes typically involves channel proteins or transporters and does not involve vesicle formation.