Diffusion is the net movement of atoms, ions or molecules from a region of higher concentration to a region of lower concentration, driven by random thermal motion and a gradient in chemical potential. The process continues until the concentration becomes uniform throughout the space.
Explanation
Particles are constantly moving due to kinetic energy. When there is a concentration difference, there are more particles in one region than another, so random collisions lead to a net movement down the gradient. This tendency is described mathematically by Fick’s laws, which relate diffusion flux to the concentration gradient. The rate of diffusion depends on the size and mass of the particles, temperature, viscosity of the medium, and the magnitude of the gradient. In gases, diffusion is fast because particles have high kinetic energy and are far apart, while in liquids and solids, it is slower due to greater intermolecular interactions. Diffusion is a passive process that does not require external energy; in biological systems, it underlies passive transport across membranes. Small nonpolar molecules like oxygen and carbon dioxide diffuse directly through lipid bilayers, while ions and polar molecules use channel proteins or carriers in facilitated diffusion. Osmosis is the diffusion of water across a semipermeable membrane driven by solute concentration differences. Diffusion operates over short distances; beyond a few millimetres, it becomes inefficient because the time required increases with the square of the distance. This limitation influences the size and shape of cells and the evolution of circulatory and respiratory systems.
Biological relevance
Diffusion is essential for gas exchange in the lungs, where oxygen diffuses from alveoli into blood and carbon dioxide diffuses out. It governs the movement of nutrients and waste products between cells and their surroundings, such as glucose entering red blood cells via GLUT transporters. The spread of a drop of ink in water or the smell of perfume in a room are everyday examples of diffusion. In soil and aquatic environments, diffusion supplies microorganisms with oxygen and dissolved nutrients. Pharmaceutical formulations consider diffusion when designing drug delivery systems to ensure proper release rates. Researchers model diffusion to understand morphogen gradients during embryonic development and the movement of signalling molecules in tissues. In materials science, diffusion affects alloy formation, corrosion and semiconductor doping.
Diffusion is a fundamental physical process that underlies many natural phenomena and helps organisms exchange substances with their environment.
Related Terms: Osmosis, Concentration gradient, Passive transport, Fick’s law, Brownian motion