# Passive Transport --- **Passive transport** is types of [[membrane transport]] that do not require expenditure of [[adenosine triphosphate]] to operate, instead using phenomenon like the [[energy|kinetic energy]] of molecules and the tendency of solutes to even out a [[concentration gradient]] toward [[equilibrium]]. ## Simple Diffusion **Simple diffusion** is the process of molecules moving directly from one side of a [[plasma membrane]] to the other, towards [[equilibrium]]. It is important to note that diffusion is defined by a *difference in [[concentration gradient|concentration]] over time*. Random motion of molecules at equilibrium doesn't count. Generally speaking only small, [[polar and nonpolar|nonpolar]] molecules can pass through membranes through simple diffusion. It doesn't have to be all three: for example, water can diffuse though even though it is polar because it is so small. Still, most of the water that passes though the membrane does so through membrane channels called **aquaporins**. ## Facilitated Diffusion **Facilitated diffusion** is similar to simple diffusion in that it allows solutes pass across the membrane with the [[concentration gradient]], *without* costing energy. However, if the cell wants to transport larger or more [[polar and nonpolar|polar]] solutes at a faster rate than they can pass through the [[plasma membrane]], then it has special [[membrane transport protein|membrane transport proteins]] that facilitate the diffusion. There are two main types, of transport proteins used in facilitated diffusion: - [[channel proteins]] transport small ions - [[carrier proteins]] transport large polar molecules. Facilitated diffusion typically occurs in one direction for any given solute. Various factors effect the rate of facilitated diffusion, such as the concentration of solutes, the number of transport proteins in the membrane, the steepness of the concentration gradient, and the presence/concentration or absence of neurotransmitters or a specific electrical charge that activates a gated channel. ## Osmosis [[Osmosis]] is the phenomenon where water moves across a semi-permeable membrane to even out a concentration gradient. Water *can* pass through biological membrane, but will only *diffuse* across one under duress. In other words, it *can* but it doesn't *like to*. ## Filtration **Filtration** is the passage through a membrane through small holes, like the [[glomerular filtration]] that takes place in the kidneys. Instead of relying on a concentration gradient as with all the previous examples, filtration relies on a pressure gradient. ___