Diffusion always results in the net movement of particles from an area of higher concentration to an area of lower concentration. This physical process is a form of passive transport, meaning it does not require cellular energy to occur. Diffusion is the natural tendency of atoms, ions, or molecules to spread out into the available space until they are evenly distributed. This spontaneous spreading is fundamental to life, governing how substances like oxygen and nutrients move across cell boundaries.
The Driving Force: Random Molecular Motion
The underlying mechanism that drives diffusion is the constant, random motion of individual molecules. All particles in liquid or gas states possess kinetic energy, which causes them to move and collide continually. This constant, chaotic movement, often referred to as Brownian motion, powers the process of diffusion.
When a substance is introduced into a space, the difference in quantity between two regions is called the concentration gradient. The net movement from high to low concentration is purely a statistical outcome of the random motion. Because there are more molecules on the high-concentration side, it is more probable that one will randomly move toward the low-concentration side. This results in a net shift of molecules down the concentration gradient until the substance is uniformly distributed, reaching a state of dynamic equilibrium.
Movement Through Selective Barriers
In biological systems, diffusion often occurs across a selectively permeable barrier, such as a cell membrane. This transport mechanism is categorized into two main types: simple diffusion and facilitated diffusion. Both rely on the concentration gradient but differ in how the particles cross the lipid bilayer.
Simple diffusion is the direct passage of small, non-polar molecules through the hydrophobic core of the cell membrane. Gases like oxygen (O₂) and carbon dioxide (CO₂) are examples, as they are small and lack a charge, allowing them to dissolve easily through the lipid environment. This direct movement occurs without assistance from membrane proteins.
Facilitated diffusion is required for molecules that are too large or too polar to pass directly through the membrane, such as glucose, amino acids, and ions like sodium (Na⁺). These substances still move down their concentration gradient, but they require the assistance of specific transmembrane proteins. These proteins act either as channels, which allow fast flow, or as carrier proteins, which bind to the molecule and change shape to shuttle it across the membrane.
Diffusion Versus Osmosis
While diffusion refers to the net movement of any particle from high to low concentration, osmosis is a specific case involving the movement of solvent molecules, nearly always water, across a selectively permeable membrane. Diffusion describes the movement of the solute particles (like salt or sugar) to equalize their concentration. Osmosis, by contrast, describes the movement of the solvent (water) to equalize the solute concentrations.
Water moves from an area where the solute concentration is low (meaning the water concentration is high) to an area where the solute concentration is high (meaning the water concentration is low). This movement is often described using the terms hypotonic (lower solute concentration) and hypertonic (higher solute concentration). A cell placed in a hypotonic solution, for example, will experience a net inflow of water as the water moves to balance the higher solute concentration inside the cell. Both processes are driven by the fundamental principle of moving down a concentration gradient toward equilibrium.