characteristics of life
/ kair-ik-ter-IS-tiks uv LYFE /
What separates a living mouse from a rock, a flame, or a clever robot? There is no single magic ingredient, so biologists instead use a checklist of shared features that living things have and nonliving things lack. Something counts as alive when it shows all of them together — not just one or two. This list is how we draw the line between the living and the non-living.
The usual features are: living things are made of one or more cells; they take in and transform energy and matter (metabolism); they keep a steady internal state (homeostasis); they grow and develop; they respond to their surroundings; they reproduce, passing on heritable instructions in DNA; and, across generations, populations evolve. A fire seems to 'consume' fuel and grow, and a crystal grows too, but neither maintains homeostasis, carries heritable instructions, or evolves — so they fail the checklist as a whole.
The honest part is that the boundary is fuzzy. A virus has genes and evolves, but it has no cells, no metabolism of its own, and cannot reproduce without hijacking a host — so most biologists place it in a gray zone rather than calling it fully alive. Seeds and frozen tadpoles pause nearly all activity yet are not dead. 'Life' is best understood not as a yes/no switch but as a bundle of properties, and the edges of that bundle are exactly where the most interesting questions live.
A candle flame grows, consumes fuel, gives off heat, and even 'reproduces' if you touch it to another wick — yet it has no cells, no DNA, and no homeostasis, so it fails the checklist. That contrast is the cleanest way to see what 'alive' really demands.
A flame mimics some signs of life but fails the full checklist — no cells, no DNA, no homeostasis.
There is no universally agreed single definition of life, and the lists differ slightly between textbooks; viruses are the famous case that sits on the boundary.