You are alive, and so am I. The dog I can hear barking is alive, and so is the tree outside my window. However, snow falling from the clouds is not alive. The computer you’re using to read this article is not alive, and neither is a chair or table. The parts of a chair that are made of wood were once alive, but they aren’t any longer. If you were to burn the wood in a fire, the fire would not be alive either.

What is it that defines life? How can we tell that one thing is alive and another is not? Most people have an intuitive understanding of what it means for something to be alive. However, it’s surprisingly hard to come up with a precise definition of life. Because of this, many definitions of life are operational definitions—they allow us to separate living things from nonliving ones, but they don’t actually pin down what life is. To make this separation, we must come up with a list of properties that are, as a group, uniquely characteristic of living organisms.

## Properties of life

Biologists have identified various traits common to all the living organisms we know of. Although nonliving things may show some of these characteristic traits, only living things show all of them.

### 1. Organization

Living things are highly organized, meaning they contain specialized, coordinated parts. All living organisms are made up of one or more cells, which are considered the fundamental units of life.
Even unicellular organisms are complex! Inside each cell, atoms make up molecules, which make up cell organelles and structures. In multicellular organisms, similar cells form tissues. Tissues, in turn, collaborate to create organs (body structures with a distinct function). Organs work together to form organ systems.
Multicellular organisms—such as humans—are made up of many cells. The cells in multicellular organisms may be specialized to do different jobs and are organized into tissues, such as connective tissue, epithelial tissue, muscle, and nervous tissue. Tissues make up organs, such as the heart or lungs, which carry out specific functions needed by the organism as a whole.

### 2. Metabolism

Life depends on an enormous number of interlocking chemical reactions. These reactions make it possible for organisms to do work—such as moving around or catching prey—as well as growing, reproducing, and maintaining the structure of their bodies. Living things must use energy and consume nutrients to carry out the chemical reactions that sustain life. The sum total of the biochemical reactions occurring in an organism is called its metabolism.
Metabolism can be subdivided into anabolism and catabolism. In anabolism, organisms make complex molecules from simpler ones, while in catabolism, they do the reverse. Anabolic processes typically consume energy, whereas catabolic processes can make stored energy available.

### 3. Homeostasis

Living organisms regulate their internal environment to maintain the relatively narrow range of conditions needed for cell function. For instance, your body temperature needs to be kept relatively close to 98.6degreesF (37degreesC). This maintenance of a stable internal environment, even in the face of a changing external environment, is known as homeostasis

### 4. Growth

Living organisms undergo regulated growth. Individual cells become larger in size, and multicellular organisms accumulate many cells through cell division. You yourself started out as a single cell and now have tens of trillions of cells in your bodystart superscript, 1, end superscript! Growth depends on anabolic pathways that build large, complex molecules such as proteins and DNA, the genetic material.

### 5. Reproduction

Living organisms can reproduce themselves to create new organisms. Reproduction can be either asexual, involving a single parent organism, or sexual, requiring two parents. Single-celled organisms, like the dividing bacterium shown in the left panel of the image at right, can reproduce themselves simply by splitting in two!

### 6. Response

Living organisms show “irritability,” meaning that they respond to stimuli or changes in their environment. For instance, people pull their hand away—fast!—from a flame; many plants turn toward the sun; and unicellular organisms may migrate toward a source of nutrients or away from a noxious chemical.

### 7. Evolution

Populations of living organisms can undergo evolution, meaning that the genetic makeup of a population may change over time. In some cases, evolution involves natural selection, in which a heritable trait, such as darker fur color or narrower beak shape, lets organisms survive and reproduce better in a particular environment. Over generations, a heritable trait that provides a fitness advantage may become more and more common in a population, making the population better suited to its environment. This process is called adaptation.