Diffusion describes the net movement of particles from an area of higher concentration to an area of lower concentration. This process continues until particles are evenly distributed. For example, food coloring in water gradually spreads to tint the entire volume. Similarly, perfume sprayed in a room eventually fills the space as fragrance molecules disperse.
The Driving Force Behind Diffusion
Diffusion occurs due to the constant, random motion of individual molecules, known as Brownian motion. This movement results from the thermal energy in all matter. Particles continuously collide, moving in unpredictable directions. The collective outcome in an uneven system is a net flow from higher to lower concentration.
Imagine a crowded room connected by an open doorway to an empty room. People move randomly, but statistically, more individuals will move from the crowded room to the empty one. This net movement continues until people are roughly equally distributed, reaching equilibrium. Random movement persists, but there is no further net change in distribution. Robert Brown observed this jittery motion in 1827 while studying pollen grains, leading Albert Einstein to explain its link to molecular diffusion.
Factors That Influence Diffusion Speed
Several factors influence diffusion speed.
Temperature
As temperature rises, particles gain kinetic energy, moving faster and colliding more frequently. This accelerates spreading, much like sugar dissolving more rapidly in hot tea compared to cold tea.
Concentration Gradient
A steeper gradient, meaning a larger concentration difference, results in a faster rate of net particle movement. The rate naturally slows as distribution approaches equilibrium.
Particle Mass
Smaller, lighter particles possess higher average velocities, diffusing more rapidly than larger, heavier ones. This explains why gases diffuse faster than liquids, and liquids faster than solids.
Medium Density
Medium density also affects diffusion rates. Diffusion is fastest in gases, where particles are widely spaced and encounter less resistance. It is slower in liquids due to increased molecular collisions and restricted movement, and slowest in solids where particles are tightly packed. For example, ink spreads faster in air than in water because air is less dense.
Diffusion in Biological Systems
Diffusion is a process across cell membranes in living organisms. The cell membrane acts as a selectively permeable barrier, regulating substance entry and exit.
Simple Diffusion
This involves direct movement of small, nonpolar molecules, such as oxygen (O2) and carbon dioxide (CO2), through the lipid bilayer. An example is gas exchange in the lungs, where oxygen diffuses from alveoli into the bloodstream, and carbon dioxide moves from blood into alveoli for exhalation.
Facilitated Diffusion
For larger or charged molecules that cannot easily pass through the lipid bilayer, facilitated diffusion provides an alternative pathway. This process moves substances down their concentration gradient but requires specific membrane proteins. Channel proteins form pores for ions like sodium (Na+) or potassium (K+), while carrier proteins bind to molecules such as glucose and change shape to transport them.
Osmosis: The Diffusion of Water
Osmosis is a specialized form of diffusion involving the net movement of water molecules across a selectively permeable membrane. Water moves from an area of high water concentration to an area of lower water concentration, or from a solution with low solute concentration to high solute concentration.
Tonicity and Animal Cells
The effect of osmosis on cells depends on the tonicity of the surrounding solution, its ability to cause water to move into or out of a cell. When an animal cell, such as a red blood cell, is placed in a hypotonic solution (lower solute concentration than the cell’s interior), water rushes into the cell. This influx of water causes the cell to swell and potentially burst (lysis), because animal cells lack rigid cell walls.
Conversely, if an animal cell is in a hypertonic solution (higher solute concentration than the cell’s interior), water moves out of the cell. This loss of water causes the cell to shrivel (crenation). In an isotonic solution, where the solute concentration outside the cell is equal to that inside, there is no net movement of water, and the cell maintains its stable volume.
Plant Cells and Osmosis
Plant cells respond differently to tonicity due to their rigid cell walls. In a hypotonic solution, water enters the plant cell, causing the cell membrane to press against the cell wall, generating turgor pressure. This turgid state provides structural support. If a plant cell is in a hypertonic solution, water leaves the cell, causing the plasma membrane to pull away from the cell wall (plasmolysis).