Passive transport

Passive diffusion across a cell membrane.

Diffusion (a form of passive transport) across the phospholipid bilayer, which is semipermeable, is demonstrated through simple and facilitated diffusion. Simple diffusion does not require energy, while small, non-polar molecules move from a high to a low solute concentration. Facilitated diffusion also does not require energy, yet it needs a protein channel to move large and polar molecules.

Passive transport is a type of membrane transport that does not require energy to move substances across cell membranes. Instead of using cellular energy, like active transport, passive transport relies on the second law of thermodynamics to drive the movement of substances across cell membranes. Fundamentally, substances follow Fick's first law, and move from an area of high concentration to an area of low concentration because this movement increases the entropy of the overall system. The rate of passive transport depends on the permeability of the cell membrane, which, in turn, depends on the organization and characteristics of the membrane lipids and proteins. The four main kinds of passive transport are simple diffusion, facilitated diffusion, filtration, and/or osmosis.

Passive transport follows Fick's first law.