The capacity of an organism or a specific tissue to react to changes in its internal or external environment is a fundamental characteristic of life. This reaction involves the detection of a signal, followed by a physiological or behavioral modification. For example, a plant bending toward a light source or a bacterium moving toward a nutrient gradient illustrates this fundamental process.
This ability is essential for survival and adaptation. It allows organisms to maintain homeostasis, avoid danger, and secure resources. From single-celled organisms to complex multicellular beings, the proficiency to perceive and react has been a driving force in evolution, shaping the diversity and complexity of life forms observed today. Early scientific observations recognized these reactions as key indicators of life itself.
