* Diffusion: The movement of molecules from an area of high concentration to an area of low concentration. This is driven by the concentration gradient itself and doesn't require cellular energy. Examples include:
* Simple diffusion: Movement of small, uncharged molecules across the cell membrane (e.g., oxygen, carbon dioxide).
* Facilitated diffusion: Movement of larger or charged molecules across the cell membrane with the help of transport proteins (e.g., glucose).
* Osmosis: The movement of water across a semi-permeable membrane from an area of high water concentration (low solute concentration) to an area of low water concentration (high solute concentration). This is driven by the difference in water potential and doesn't require cellular energy.
* Filtration: The movement of water and small solutes across a membrane driven by pressure differences. This is common in the kidneys for filtering blood.
* Passive transport: A general term for the movement of molecules across a membrane without the use of cellular energy. This includes diffusion, osmosis, and filtration.
Important Note: While these processes don't directly use cellular energy (ATP), they still require the cell to maintain a proper membrane structure and potentially produce specific transport proteins. This indirectly requires energy expenditure for cell maintenance and function.
