Here's why:

* Passive transport refers to the movement of substances across a cell membrane without requiring the cell to expend energy. This movement occurs naturally due to differences in concentration or pressure.

* Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. This movement doesn't require any energy input from the cell.

* Osmosis is a specific type of diffusion that involves the movement of water molecules across a semipermeable membrane from a region of high water concentration to a region of low water concentration. Like diffusion, this process doesn't require the cell to expend energy.

Therefore, both diffusion and osmosis are passive transport mechanisms because they occur without the cell expending energy.