One of the main functions of plasma-membrane is transport across the membrane. Three main ways the membrane helps in transport:
1. Passive transport is of smaller molecules and it takes place along the gradient (diffusion and facilitated diffusion). There is no involvement of cellular energy.
2. Active transport is at the expenditure of cellular energy (either ATP, light, or any other form). This is across the gradient.
3. Bulk transport is of larger molecules. This needs the formation of vesicles in which the molecules which need to be transported are wrapped. Bulk transport of solid is called phagocytosis in which some solid particle is engulfed by the cell. So this is the entry of solid bulk molecules to the cell. Pinocytosis is liquid droplet entry into the cell.
This means when some larger molecules leave the cell, it is called reverse phagocytosis.