Benchmark 5C
The Living Environment: Cells
Grades 9-12, page 114

Most cells function best within a narrow range of temperature and acidity. At very low temperatures, reaction rates are too slow. High temperatures and/or extremes of acidity can irreversibly change the structure of most protein molecules. Even small changes in acidity can alter the molecules and how they interact. Both single cells and multicellular organisms have molecules that help to keep the cell's acidity within a narrow range.
 

NSES Content Standard C Life Science: Regulation and behavior Grades 5-8, page 157 All organisms must be able to obtain and use resources, grow, reproduce, and maintain stable internal conditions while living in a constantly changing external environment. NSES Content Standard C  Life Science: Regulation and behavior Grades 5-8, page 157 Regulation of an organism's internal environment involves sensing the internal environment and changing physiological activities to keep conditions within the range required to survive.