* Simple Diffusion: Movement of a substance from an area of high concentration to an area of low concentration. This doesn't require any energy, as the movement is driven by the concentration gradient itself. Think of a drop of food coloring spreading out in a glass of water.
* Facilitated Diffusion: Movement of a substance across a membrane with the help of a transport protein. While this still doesn't require the cell to use energy, the transport protein acts as a facilitator, allowing the substance to cross the membrane more easily.
* Osmosis: The movement of water across a selectively permeable membrane from an area of high water concentration to an area of low water concentration. This is driven by the difference in water potential across the membrane.
Key Points to Remember:
* No energy is required: Passive transport relies on the natural movement of molecules down their concentration gradient or the movement of water down its potential gradient.
* Downhill movement: Substances move from an area of higher concentration/potential to an area of lower concentration/potential.
* No direct cellular energy required: The cell does not expend ATP to move substances via passive transport.
Let me know if you'd like more details about any of these forms of passive transport!
