Living things are physical systems that maintain their own internal order by taking in energy from their surroundings, carrying instructions for their own construction, and producing copies of themselves. That description sounds simple, but pinning down exactly where “living” starts and “non-living” ends has challenged scientists for over a century. No single definition has universal agreement, yet biologists converge on a core set of properties that all known life shares.
The Properties All Living Things Share
Rather than a single magic ingredient, life is defined by a collection of abilities working together. Biologists generally agree that all living organisms can grow, reproduce, maintain a stable internal state, respond to their surroundings, and carry out chemical processes to obtain energy. Remove any one of these and you’re looking at something that behaves more like a machine or a mineral than an organism.
A framework published in Science by biochemist Daniel Koshland breaks this down further into seven “pillars” captured by the acronym PICERAS: Program, Improvisation, Compartmentalization, Energy, Regeneration, Adaptability, and Seclusion. Program refers to the organized plan, encoded in DNA or RNA, that directs what a cell builds and when. Improvisation is the capacity to respond to new situations rather than follow a rigid script. Compartmentalization means every living thing is enclosed in some kind of container (a membrane, a cell wall) that keeps its internal chemistry concentrated and protected. Energy captures the fact that life is never at rest; it constantly moves molecules around and must take in fuel to do so. Regeneration covers both repair (healing a wound) and reproduction (making offspring). Adaptability is evolution over generations. And Seclusion describes how thousands of chemical reactions run efficiently inside tiny cells because each pathway is kept somewhat separate from the others, receiving only the signals it needs.
You don’t need to memorize the acronym. The key insight is that life isn’t one trait. It’s a package deal.
Why Energy Is Non-Negotiable
Every living cell runs on metabolism: the sum of all chemical reactions that break things down and build things up. Breakdown pathways convert complex molecules from food into simpler ones, releasing energy in the process. The most familiar version turns glucose into ATP, a small molecule cells use like a rechargeable battery. Building pathways do the opposite, using energy to assemble proteins, membranes, and DNA from raw materials.
This constant chemical activity is what separates a living cell from, say, a crystal. A crystal can grow, and it has an ordered structure, but it doesn’t harvest energy to maintain itself. In 1944, physicist Erwin Schrödinger framed this idea in thermodynamic terms: everything in the universe tends toward disorder, yet organisms stay highly organized. They manage this, he wrote, by “continually sucking orderliness from their environment.” Cells eat, absorb sunlight, or consume chemicals, and use that imported order to push back against decay. Stop feeding an organism and it dies, because it can no longer resist the slide toward disorder.
Information That Copies Itself
Life on Earth runs on nucleic acids: DNA and RNA. These long chain molecules store the instructions for building proteins, which do most of the work inside cells. More importantly, nucleic acids can guide the formation of exact copies of their own sequence through a process called complementary base pairing, where each unit in the chain dictates which unit lines up next to it on the new strand.
This is the basis of heredity. When a cell divides, it passes a copy of its DNA to each daughter cell. When organisms reproduce, they pass genetic instructions to offspring. Without a self-copying information molecule, there’s no inheritance, and without inheritance, there’s no evolution. NASA’s working definition of life leans heavily on this point: life is “a self-sustained chemical system capable of Darwinian evolution.” If a system can’t pass information to the next generation and have that information occasionally change (mutate), it can’t adapt over time, and by this definition, it isn’t alive.
Cells as the Basic Unit
All known living things are made of cells or are themselves single cells. Cell theory, first proposed in the 1800s and refined ever since, states that cells are the smallest independent units of life. Each cell is a compartment enclosed by a membrane, containing the molecular machinery to read genetic instructions, harvest energy, and reproduce. Larger organisms, from mushrooms to whales, are communities of specialized cells cooperating, but even the most complex human tissue can be traced back to individual cells doing these fundamental jobs.
The requirement for a compartment is more than a technicality. Diluting the contents of a cell kills it, even if every chemical remains active, because the concentrations drop too low for reactions to proceed efficiently. The membrane isn’t just a bag. It’s a selective barrier that controls what enters and leaves, keeping internal chemistry in the narrow range that sustains life.
Staying Stable in a Changing World
Living systems regulate themselves. Your body temperature hovers near 37°C whether it’s freezing outside or sweltering. Blood sugar rises after a meal and then drops back to baseline. These are examples of homeostasis, and they depend on feedback loops: sensors detect a change, signals trigger a correction, and the system returns to its set point. Feedback loops operate at every scale, from individual cells deciding when to grow and when to stop, to organs coordinating across an entire body.
This self-regulation is one of the clearest markers of life. A rock doesn’t adjust when its environment changes. A bacterium does, shifting which genes it activates, which fuels it burns, and how fast it divides.
Where the Boundaries Get Blurry
Viruses are the most famous challenge to any tidy definition of life. They carry genetic information (DNA or RNA), they evolve, and they reproduce. But they cannot generate their own energy, they lack ribosomes (the machinery that builds proteins), and they cannot replicate without hijacking a living host cell. By the standard checklist, viruses fail on metabolism and independent reproduction. Most biologists classify them as non-living entities that exploit living systems, though the debate never fully closes because viruses clearly share some hallmarks of life.
At the other end of the spectrum, scientists have created synthetic cells by assembling genetic material and membranes in the lab. Whether these qualify as “alive” depends on how strict your criteria are. The loosest definitions accept any technology that mimics key aspects of biological systems. The strictest require a system that operates, makes decisions, and sustains itself independently. No artificial system yet meets that bar, but the question forces biologists to keep sharpening their definitions.
Life at the Extremes
One of the most surprising discoveries of the past few decades is just how far life can push into environments once considered lethal. Microorganisms have been found 6.7 kilometers deep inside Earth’s crust and more than 10 kilometers below the ocean surface, at pressures over a thousand times atmospheric. Some thrive at pH 0 (essentially battery acid) or pH 12.8 (stronger than bleach). Others grow in hydrothermal vents at 122°C or in frozen seawater at −20°C.
Tardigrades, tiny eight-legged animals barely visible to the naked eye, push these limits even further when they enter a dormant state. In that state they have survived temperatures from −272°C (one degree above absolute zero) to 151°C, the vacuum of outer space, pressures of 6,000 atmospheres, and blasts of X-rays and gamma rays. These extremes don’t redefine what “living” means, but they dramatically expand where life can exist, which matters for the search for life beyond Earth.
A Working Answer, Not a Final One
If you need a single sentence: living things are organized systems enclosed in compartments that use energy to maintain themselves, carry heritable genetic information, and evolve over generations. That covers every organism we’ve ever found, from deep-sea microbes to blue whales. It also neatly excludes fire (no genetic program), crystals (no metabolism or evolution), and computers (no self-sustaining chemistry). The edges remain genuinely fuzzy, especially around viruses and future synthetic systems, but for everything you’ll encounter in biology class, medicine, or daily life, those core criteria hold.