* Simple Diffusion: The movement of molecules from an area of high concentration to an area of low concentration. This happens because of the random motion of molecules, and no energy is required from the cell to facilitate this movement.
* Facilitated Diffusion: The movement of molecules across a membrane with the help of a transport protein. While a protein is involved, the movement still goes down the concentration gradient, so no energy is required from the cell.
* Osmosis: The movement of water across a semi-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, not by the cell.
* Filtration: The movement of water and small solutes across a membrane from an area of high pressure to an area of low pressure. This is not driven by the cell but by the pressure gradient.
It's important to note that even in these passive processes, the cell might be indirectly involved:
* Maintaining the concentration gradients: Cells often use energy to maintain concentration gradients that are necessary for passive processes to occur. For example, the sodium-potassium pump actively moves sodium and potassium ions across the cell membrane, creating a concentration gradient that facilitates passive diffusion of these ions.
* Creating the pressure difference: The cell might be involved in maintaining the pressure difference necessary for filtration to occur. For instance, in the kidneys, the cells in the glomerulus use energy to maintain the high pressure that drives filtration.
Therefore, while these processes technically don't require direct energy input from the cell, the cell may indirectly contribute to their function.
