What is a key characteristic of cells according to the theory developed in the 19th century?

The concept that cells are the fundamental units of life is a key characteristic established by the cell theory in the 19th century. This theory, developed by scientists like Matthias Schleiden, Theodor Schwann, and Rudolf Virchow, posits that all living organisms are composed of cells, and that these cells are not only the building blocks of life but also the basic units of function within living beings.

Cells carry out the essential processes needed to sustain life, including metabolism, growth, reproduction, and response to stimuli. Understanding that all life forms are made up of cells emphasizes the shared characteristics of life and establishes a common framework for studying biology, regardless of the organism's complexity. Thus, recognizing cells as the fundamental unit of life paved the way for advances in biology, microbiology, and medicine.