Diffusion is the passive movement of substances down their concentration gradients—requiring no expenditure of cellular energy. Substances, such as molecules or ions, diffuse from an area of high concentration to an area of low concentration in the cytosol or across membranes. Eventually, the concentration will even out, with the substance moving randomly but causing no net change in concentration. Such a state is called dynamic equilibrium, which is essential for maintaining overall homeostasis in living organisms.
Diffusion plays an integral role in biological processes such as respiration, the process by which organisms exchange gases with their environment. After breathing in air, the concentration of oxygen in the alveoli, air sacs of the human lung, is higher than the oxygen concentration in the blood. Consequently, oxygen diffuses down its concentration gradient into the blood. In order to get into body tissue, oxygen and other nutrients carried in the blood must diffuse into tissues down their concentration gradients. Metabolic waste such as carbon dioxide diffuses from tissues into capillaries where the carbon dioxide concentration is less than that inside body tissues. Blood carrying carbon dioxide is then pumped to the lungs where carbon dioxide readily diffuses into alveoli that have a lower concentration of the gas than blood. Carbon dioxide is then exhaled out of the body from the alveoli.
Diffusion is also responsible for gas exchange in plants. The carbon dioxide needed for photosynthesis diffuses into plant leaves from the air through small pores on leaves called stomata. Conversely, oxygen produced as a byproduct of photosynthesis diffuses out of leaves and into the air through stomata.
Factors such as temperature, molecular mass, solvent density, solubility, and the magnitude of a molecule’s concentration gradient influence diffusion rates. For instance, in solution, each substance has its own concentration gradient that is independent of the concentration gradient of other substances. A larger concentration difference between compartments leads to faster diffusion rates. Consequently, the closer a system is to equilibrium, the slower the rate of diffusion.
The rate of diffusion across a membrane depends mostly on the molecules’ relative hydrophobicity. Specifically, the more lipid soluble and nonpolar molecules are, the more readily they will diffuse through the membrane. This includes small gases such as oxygen and carbon dioxide, as well as larger substances like vitamins. Other uncharged but polar molecules, such as water and larger ones like glucose will pass through, although at a much slower rate. In contrast, charged ions—no matter their size—and non-lipid soluble proteins are repelled by the lipid bilayer and require other mechanisms to cross.
Simple diffusion occurs when substances are able to directly diffuse across membranes along their concentration gradients without assistance. However, facilitated diffusion takes place when substances require the use of membrane-embedded transport proteins to traverse membranes without expending energy.
