Diffusion describes the net movement of particles from an area of higher concentration to an area of lower concentration. This physical process occurs naturally in gases, liquids, and solids, driven by the inherent kinetic energy of the particles. It facilitates the distribution of substances without requiring external work.
Key Characteristics of Diffusion
Diffusion is a passive process, meaning it does not require cellular energy, such as adenosine triphosphate (ATP). Individual particles move randomly and continuously, but the net movement is from higher to lower concentration. This directional flow is described as movement “down” a concentration gradient.
The driving force behind this net movement is the random thermal motion of individual particles. These particles collide, resulting in their displacement. Diffusion continues until the particles are evenly distributed throughout the available space, a state known as equilibrium. At equilibrium, the net movement of particles ceases, but the individual particles continue their random motion in all directions.
Elements that Influence Diffusion Rate
The speed at which diffusion occurs is influenced by several physical properties. Higher temperatures increase the kinetic energy of particles, causing them to move more rapidly and collide more frequently. This accelerated movement leads to a faster rate of diffusion across a given area.
A steeper concentration gradient, a larger difference in particle concentration, also accelerates the diffusion rate. Conversely, a shallow gradient results in a slower rate of net movement.
The available surface area through which diffusion can occur significantly impacts its speed. A larger surface area allows more particles to move across simultaneously, thus increasing the overall rate. Particle size and mass also play a role; smaller, less massive particles diffuse more rapidly than larger, heavier ones. Shorter distances over which particles must travel also facilitate faster diffusion.
Diffusion’s Role in Living Organisms
Within living organisms, diffusion is a widespread mechanism for substance transport. In the lungs, oxygen diffuses from the air sacs, called alveoli, into the bloodstream, where its concentration is lower. Carbon dioxide, a waste product, diffuses from the bloodstream into the alveoli to be exhaled.
The absorption of digested nutrients in the small intestine also relies on diffusion. Simple sugars and amino acids, for instance, move from the intestinal lumen into the bloodstream, where their concentration is lower. Cells throughout the body use diffusion to take in necessary substances and expel waste.
Waste products, such as urea, diffuse from individual cells into the surrounding fluid and then into the blood. The circulatory system transports these substances to organs like the kidneys for excretion. Diffusion facilitates the constant exchange of materials for cellular function and maintaining internal balance.