Saccharomyces cerevisiae yeast grow mainly on the surface of solid or liquid substrates. The life cycle of such microbial populations is substantially influenced by diurnal ambient temperature variation. The response of an unrestricted S. cerevisiae yeast culture to temperature changes has been studied in experiments. With the absence of environmental constraints on the operation of the cell chemiosmotic energy system, the culture remains heat-resistant when the growth temperature is switched from 12–36°C to 37.5–40°C. As a result, the yeast reproduction rate remained 1.5–2 times higher than under stationary conditions for one to four turnover cycles after the transition. This observation is explained by the adaptive potential of the chemiosmotic system acquired in the course of evolution. When the adaptive resource is exhausted, heat resistance is restored at 12–36°C within one generation with restricted or unrestricted nourishment. The adaptive significance of heat resistance is obvious: it keeps the reproduction rate high when the ambient temperature reaches a brief maximum shortly after noon.