The primary mechanism used by cells to control their internal environment is through homeostasis. Homeostasis maintains a stable internal condition despite external fluctuations, ensuring optimal functioning of cellular processes. Cells regulate their internal environment using various mechanisms, including the selective permeability of their cell membranes, ion channels, and transport proteins. These systems carefully manage factors such as temperature, pH, ion concentrations, and nutrient levels. Homeostasis is critical for the survival of cells and, by extension, the entire organism, as it allows adaptation to environmental changes while preserving essential physiological functions.