What Is Life? Characteristics of Living Organisms

A living organism is an individual entity that carries out the processes of life: it takes in matter and energy, grows, responds to its surroundings, reproduces and is made of one or more cells. In brief: biologists decide whether something is alive by checking a set of characteristics together, not a single one. A dog, an oak tree and a bacterium meet all of them, while a rock or a candle flame does not.

The question looks simple, but it has no single agreed answer in science, and the edge cases are interesting. This guide explains the characteristics of living things, how to separate living from nonliving things with a clear method, what every organism needs to stay alive and where definitions become uncertain.

What is a living organism?

An organism is a single living individual, such as one bird, one tree or one yeast cell. It is organized in a way that lets it maintain itself, and it exchanges matter and energy with its environment instead of simply sitting in it.

The key word is organized. Every known organism is built from cells, and each cell is a small, ordered system with its own chemistry. Some organisms, such as bacteria, consist of a single cell. Others, such as humans, consist of trillions of cells that work together.

No single sentence defines life in a way that every biologist accepts. Textbooks therefore describe life by listing characteristics that living things share. This approach is practical and works well for nearly all everyday cases.

What are the characteristics of living things?

Most school courses name seven or eight characteristics. The exact list varies slightly between textbooks, but the ideas are the same. The table below uses a common version.

Why one characteristic is never enough

Each characteristic on its own can be found in nonliving things. A crystal grows when more mineral is added, and a flame consumes fuel and releases energy. A thermostat responds to temperature. Only living organisms show all of the characteristics together.

A second point matters just as much: an individual does not have to show every characteristic at every moment. A mule cannot reproduce, and a dormant seed shows almost no activity. Both are still alive, because the characteristics apply to the kind of organism and its life as a whole.

How do you tell living from nonliving things?

Biologists usually separate three groups: living things, things that were once alive and nonliving things that never were. Wood, paper and a dead leaf belong to the middle group. They come from organisms but no longer carry out life processes.

A simple four-step test

1. Look for cells. If the object is made of cells, it is alive or was once alive.
2. Look for metabolism. A living organism takes in energy and transforms it, for example through eating or photosynthesis.
3. Look for reproduction. The kind of organism must be able to produce offspring, even if this individual cannot.
4. Look for regulation. The organism responds to its environment and keeps its internal state within limits.

If an object passes all four steps, it is a living thing. If it fails the first step, it is not an organism, however active it may look.

Worked example: is a mushroom alive, and is fire?

Take a mushroom first. It is made of cells, so step 1 is passed. It absorbs nutrients from the material it grows on and breaks them down for energy, which satisfies step 2. It releases spores that grow into new fungi, so step 3 is passed. It grows toward moisture and reacts to light and temperature, which covers step 4. A mushroom is a living organism, and it belongs to the kingdom Fungi, not to the plants.

Now take a campfire. It seems to grow, it consumes fuel, it releases heat and it spreads, which makes it resemble a living thing. It fails step 1, because it has no cells. It also has no genetic material, so it cannot pass on inherited information. It is a chemical reaction called combustion, not an organism.

What are the basic needs of organisms?

All organisms depend on their environment. The needs below are shared by almost every living thing, although the details differ widely between species.

• Energy: Animals get it from food, plants and many other organisms capture it from sunlight.
• Water: Cell chemistry takes place in water, so every organism needs it, even desert species that store it carefully.
• Nutrients: Building materials such as carbon, nitrogen and phosphorus are needed to make new cells.
• Gases: Most organisms use oxygen for respiration. Plants also need carbon dioxide for photosynthesis.
• Suitable temperature: Each species functions within a range of temperatures.
• Space: Organisms need room to live, find resources and avoid being crowded out.

Some organisms do not need oxygen. Certain bacteria live deep in mud or in the guts of animals and are harmed by it. This shows why a need should be described for a group of organisms, not stated as a universal rule.

Living, nonliving and borderline examples

The next table applies the characteristics to a small set of familiar items. A check mark means the item shows that characteristic as an organism, and a dash means it does not.

Viruses are the best-known borderline case. They contain genetic material and evolve, but they have no cells and cannot carry out metabolism or reproduce without a host cell. Most biologists treat them as lying at the boundary of the definition, and the question is still discussed.

How did scientists come to define life?

For centuries, many people thought that living things could arise directly from nonliving matter, for example maggots from rotting meat. This idea is called spontaneous generation. In 1668 the Italian physician Francesco Redi showed that maggots did not appear on meat covered with gauze, which weakened the idea for larger animals.

The question for microorganisms was settled later. In the 1860s Louis Pasteur used flasks with long curved necks that let air in but kept dust out. Broth in such flasks stayed clear, which supported the principle that life comes from existing life.

In the nineteenth century, cell theory added the idea that cells are the basic units of life. Modern definitions continue to change. Some researchers, for example, describe life as a self-sustaining chemical system capable of evolving, which is a shorter definition but harder to apply in a school laboratory.

Why does the definition of life matter today?

The characteristics of life are used in real research. Astrobiologists who search for life on other planets must decide what signs to look for. They often look for evidence of metabolism, such as gases produced by chemical reactions, because cells are hard to find from a distance.

Medicine and ethics also depend on such definitions. Questions about viruses, about the start of an individual human life and about artificial cells built in laboratories all require clear criteria. In agriculture and conservation, the same characteristics help biologists decide whether a seed or spore is dormant or truly dead.

Which mistakes are common when classifying life?

Mistake 1: Treating movement as proof of life

Clouds, rivers and wind-blown sand all move. Many organisms, such as trees and corals, move little or not at all. Movement alone is not a reliable sign, and response to stimuli is a better test.

Mistake 2: Calling dead things nonliving

A dead mouse and a piece of wood were once alive, so they differ from a stone that never was. Using the three groups, living, once living and never living, avoids this confusion.

Mistake 3: Requiring every characteristic from every individual

A sterile mule or a person who chooses not to have children is alive. Reproduction is a characteristic of the species, not a test that each individual must pass.

Mistake 4: Assuming growth means life

Crystals grow by adding identical layers from outside, while organisms grow by taking in nutrients and building new cells from them. The method of growth is what counts, not only the increase in size.