diffusible

Diffusible is a term used in science to describe substances that can spread through a medium by diffusion, moving from regions of higher concentration to regions of lower concentration without the input of cellular energy.

Diffusion is governed by concentration gradients and is influenced by temperature, the viscosity of the medium, and properties of the diffusing molecule. Lipid-soluble, small, uncharged molecules diffuse readily across membranes, while charged, large, or highly polar molecules diffuse more slowly unless facilitated by specific transport mechanisms or channels.

In biology and medicine, diffusible substances include gases such as oxygen and carbon dioxide that move across respiratory membranes, neurotransmitters and paracrine signaling molecules that diffuse through extracellular space to reach target cells, and many metabolites secreted by microbes that spread through culture media to affect neighboring organisms. The term may also apply to drugs or dyes that spread passively through tissues or gels.

In microbiology, diffusible factors may influence microbial interactions, either promoting growth or inhibiting rivals, and diffusion limits can shape community structure. The concept contrasts with non-diffusible or immobilized substances, which remain localized. Related concepts include diffusion coefficients and permeability, which quantify how readily a substance diffuses under given conditions.