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.
- Organization
Living things are highly organized, and all living organisms are made up of one or more cells, which are considered the fundamental units of life. Individual cells perform complex biochemical processes needed to maintain their structure and function, and each cell is highly organized.
Unicellular organisms consist of only a single cell, while 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.
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.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 98.6∘^\circ∘degreeF (37∘^\circ∘degreeC). This maintenance of a stable internal environment, even in the face of a changing external environment, is known as homeostasis.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 body1^11start superscript, 1, end superscript! Growth depends on anabolic pathways that build large, complex molecules such as proteins and DNA, the genetic material.