What is a key characteristic of living organisms?

A fundamental characteristic of living organisms is that they are composed of cells. This feature is essential because cells are considered the basic units of life; all living things, from the simplest bacteria to complex multicellular organisms like humans, are made up of one or more cells. This cellular structure is the foundation for all biological processes, including metabolism, growth, and response to environmental stimuli.

While movement may be observed in many organisms, it is not universal across all life forms. For example, plants do not move in the same way that animals do, yet they are undoubtedly alive. Similarly, the ability to reproduce is essential for the continuation of species, but some life forms, particularly certain bacteria, can reproduce under very specific conditions and might not necessarily need nutrients in the same direct manner as more complex organisms do.

The idea of having a fixed structure does not accurately reflect the dynamic nature of living organisms, as many can change in structure over time (for example, growth or adaptation). Therefore, the defining characteristic that encompasses all forms of life is their cellular composition.