Passive transport

Passive transport moves substances across a cell membrane without using cellular energy. Unlike active transport, which requires energy, passive transport is driven by the second law of thermodynamics. Substances follow Fick's first law, moving from areas of high concentration to low concentration because this increases the system's entropy. The rate of this process depends on the membrane's permeability, which is influenced by the structure of its lipids and proteins. The four main types are simple diffusion, facilitated diffusion, filtration, and osmosis.

Diffusion is the net movement of material from a region of higher concentration to one of lower concentration. The difference in concentration is called the concentration gradient, and diffusion continues until the gradient disappears. This movement is described as going "down the concentration gradient," in contrast to active transport, which moves substances "against the gradient." However, in some cases, like passive drug transport, the driving force is not simply the concentration gradient. If solutions on either side of the membrane have different equilibrium solubilities, the difference in saturation—not just concentration—drives the transport. This is especially relevant for supersaturated solutions used in drug delivery.

Simple diffusion and osmosis are similar. Simple diffusion moves solutes from high to low concentration until the solute is evenly distributed and equilibrium is reached. Osmosis is like simple diffusion but specifically describes the movement of water across a selectively permeable membrane until water and solute concentrations are equal on both sides. Neither process requires ATP energy.

For passive diffusion, the law of diffusion states that the mean squared displacement is ⟨r²⟩ = 2dDt, where d is the number of dimensions and D is the diffusion coefficient. The time to diffuse a distance x is roughly x²/2dD, and the average speed is about 2dD/x. This means diffusion is fast over short distances but slow over long distances. In cells, prokaryotes are small enough for diffusion to handle internal material transport. Larger eukaryotic cells either have very low metabolic rates to cope with slow diffusion or use active transport systems, like kinesin moving along microtubules.

A biological example is gas exchange in human respiration. When we inhale, oxygen diffuses across the alveoli membrane into the pulmonary capillaries. At the same time, carbon dioxide diffuses from the capillaries into the alveoli to be exhaled. This movement follows concentration gradients: oxygen moves from high concentration in the lungs to low concentration in the blood, and carbon dioxide moves from high concentration in the blood to low concentration in the lungs. Cellular respiration creates these gradients. Because the gases are small and uncharged, they pass directly through the cell membrane without special proteins, and no energy is needed.

Facilitated diffusion, also called carrier-mediated osmosis, moves molecules across the cell membrane using special transport proteins embedded in the plasma membrane. These proteins actively take up or exclude ions. No energy is required for the molecules to pass through. Active transport of protons by H+ ATPases can alter the membrane potential, allowing for the facilitated passive transport of ions like potassium down their charge gradient through high-affinity transporters and channels. An example is glucose absorption into cells via the GLUT2 transporter.