Living things are organisms that can grow, reproduce, use energy, respond to their environment, and maintain stable internal conditions. Every living thing on Earth, from single-celled bacteria to blue whales, shares these core traits. NASA offers a concise working definition: “Life is a self-sustaining chemical system capable of Darwinian evolution.” That definition captures the two essentials: living things run on chemistry (metabolism) and they change across generations (evolution).
Seven Characteristics All Living Things Share
Biologists generally agree on seven properties that separate living organisms from nonliving matter. An entity needs all of them, not just one or two, to qualify as alive.
- Organization: All living things are made of cells, from a single bacterium to a trillion-cell human body.
- Metabolism: They take in energy and raw materials, then use chemical reactions to build what they need and break down what they don’t.
- Growth: They increase in size or complexity over time by adding new cells or enlarging existing ones.
- Reproduction: They produce offspring, either on their own or with a partner.
- Response to stimuli: They detect and react to changes in their surroundings.
- Homeostasis: They regulate their internal conditions to stay within a livable range.
- Evolution: Populations of living things change genetically over generations through natural selection.
Cells: The Basic Unit of Life
Every known living thing is built from at least one cell. Cell theory, one of the foundational ideas in biology, rests on three principles: all plants and animals are made of cells, cells possess all the attributes of life (including growth and reproduction), and all cells arise from the division of preexisting cells. No new cell appears from scratch. A cell is simultaneously the structural unit (it forms the body), the functional unit (it carries out life processes), and the reproductive unit (it divides to create new cells).
Some organisms consist of a single cell that handles everything: feeding, waste removal, and reproduction. Others, like humans, contain trillions of specialized cells organized into tissues and organs, each type handling a different job.
How Living Things Use Energy
Metabolism is the sum of every chemical reaction happening inside an organism’s cells. It splits into two broad categories. Catabolism breaks large, complex molecules down into simpler ones, releasing energy in the process. When your body digests food into carbon dioxide, water, and simpler compounds, that’s catabolism at work. Anabolism goes the other direction, using energy to build complex molecules like proteins, fats, and DNA from smaller building blocks.
These two processes run constantly and depend on each other. The energy released by breaking things down fuels the construction of new materials. Without metabolism, a cell can’t maintain itself, grow, or do anything else on the list of life’s characteristics.
Responding to the Environment
Living things don’t just sit passively in their surroundings. Animals pull their hand away from a hot surface. Plant roots grow downward in response to gravity, while stems bend toward light. The bending happens because growth hormones concentrate on the shaded side of the stem, making those cells elongate faster and curving the plant toward the light source. Some plants open their leaves during the day to collect sunlight and close them at night to conserve water. When a plant detects a pathogen, cells around the infected tissue often die deliberately to wall off the infection, and the plant may produce chemical compounds to fight the invader. Willow trees, for example, produce a compound (the natural precursor to aspirin) that kills bacteria.
Even single-celled organisms respond to stimuli. Bacteria swim toward nutrients and away from toxins. The ability to sense and react to conditions is what allows living things to survive in changing environments.
Maintaining Internal Stability
Homeostasis is the process of keeping internal conditions relatively constant even when the outside world fluctuates. Your body temperature stays near 37°C (98.6°F) whether you’re in a snowstorm or a sauna. When you get too warm, sweat glands activate to cool you down. When you get too cold, muscles shiver to generate heat. A region of the brain monitors temperature continuously and triggers these responses at the slightest deviation.
Temperature is just one example. Living things also regulate the balance between acidity and alkalinity in their fluids, control water and salt levels, and manage blood sugar. Without homeostasis, the chemical reactions that sustain life would slow down, speed up uncontrollably, or stop altogether.
Passing on Genetic Information
Every living organism carries genetic instructions encoded in DNA (or, in some simple organisms, RNA). These instructions guide how the organism develops, functions, and reproduces. When a cell divides, it copies its DNA so each new cell gets a complete set of instructions. When organisms reproduce, they pass genetic material to their offspring.
This inheritance isn’t perfect. Small, random changes (mutations) in DNA create variation within a population. Over many generations, individuals with traits better suited to their environment tend to survive and reproduce more successfully. This is natural selection, the engine behind evolution. As Charles Darwin recognized, evolution requires only two ingredients: heritable variation and environmental pressure that favors some traits over others. Over vast stretches of time, this process has produced the enormous diversity of life on Earth.
The Three Domains of Life
All known living things fall into three large groups called domains. Bacteria and Archaea are both single-celled organisms without a nucleus, but they are as fundamentally different from each other as either is from animals or plants. Carl Woese demonstrated this in the 1970s by comparing a key molecule found in the protein-building machinery of cells. His analysis revealed that methane-producing microbes and similar organisms form their own distinct lineage, Archaea, separate from all other bacteria. The discovery was one of the most important in twentieth-century biology.
The third domain, Eukarya, includes every organism whose cells contain a nucleus: animals, plants, fungi, and a vast array of single-celled life like amoebas and algae. Despite their incredible diversity, all three domains share the same basic toolkit: cells, DNA-based heredity, and metabolism.
Living Things vs. Nonliving Things
In ecology, the living components of an environment are called biotic factors (from a Greek word meaning “with life”), while nonliving components are abiotic (“without life”). Biotic factors include every organism in an ecosystem: plants, animals, fungi, and microorganisms. Abiotic factors are things like water, sunlight, temperature, air, minerals, and landforms. Both categories interact constantly. Plants need sunlight and water (abiotic) to grow, and animals need plants or other animals (biotic) for food.
The distinction sounds obvious, but it gets blurry at the edges. A fallen log is no longer alive, yet it provides habitat for fungi and insects. Soil is nonliving, but it teems with billions of living microorganisms per handful. The boundary between living and nonliving is clearest at the molecular level: if it has cells, runs a metabolism, and carries genetic instructions, it’s alive.
Where Viruses Fit In
Viruses are the most famous borderline case. They carry genetic material, they evolve over time, and they can replicate in enormous numbers. By those measures, they seem alive. But viruses fail several key tests. They have no cells, no metabolism, and no ability to generate their own energy. They cannot reproduce on their own. A virus particle is essentially inert until it enters a living host cell and hijacks that cell’s machinery to make copies of itself. Viruses also lack ribosomes, the molecular structures every cell uses to build proteins.
Because of these limitations, most biologists classify viruses as nonliving under a strict definition. They occupy a gray zone: not quite alive, but not simply inert chemistry either. They depend entirely on living cells to do anything, which is why they’re sometimes called obligate intracellular parasites.