The physiological changes occurring in an organism in response to a change in a particular environmental factor (e.g. temperature). Thermal acclimation studies reveal how such properties as metabolic rate, muscle contractility, nerve conduction, and heart rate differ between cold- and warm-acclimated members of the same species. These changes occur naturally during acclimatization and equip the organism for living in, say, cold or warm conditions according to the seasons. Metabolic acclimation is explained mainly by changes in concentration and/or activity of crucial enzymes. For example, different isozymes may be expressed at different times of the year. Changes in composition of membrane lipids, particularly their degree of saturation, also occur, helping to maintain membrane stability in changing conditions. Heat-shock proteins help to protect and repair proteins damaged by thermal stress, and their expression increases under such conditions. Plants too show better tolerance of environmental extremes following previous exposure to such conditions (see cold hardening).