What mainly determines whether a molecule will be transported via endocytosis/exocytosis or via passive transport?

The size of the molecule

Explanation:

Some small molecules can be transported into pur out of the cell by passive transport. Passive transport is a mechanism of transport that does not require energy, instead relying on diffusion of molecules from a high concentration of that molecule to a lower concentration of that molecule (i. e. down a concentration gradient)

Molecules that can participate in passive transport include small, uncharged molecules that can cross the phospholipid bilayer of the cell membrane such as water, carbox, dioxide and oxygen.

In contrast, endocytosis/exocytosis are types of active transport that require energy. Substances that cannot freely cross the membrane due to their size are transported into/out of the cell via a vesicle system. Endocytosis involves the cell membrane engulfing the particle to bring it into the cell, exocytosis is the reverse, where vesicular transport allows large molecules to leave the cell.

Substances that undergo endocytosis/exocytosis include cell debris, complex proteins, large macromolecules and waste material