Characteristics of Life
Sometimes it is difficult to tell the difference between living and nonliving things. At times, nonliving things have one or more of the characteristics of life, but it is necessary to have all of the characteristics of life to be considered living. Things that have all of the characteristics of life are known as organisms.
All living organisms share several key characteristics or functions: order, sensitivity or response to the environment, reproduction, adaptation, growth and development, regulation, homeostasis, energy processing, and evolution. Generally, when viewed together, these characteristics serve to define life.
Organisms are highly organized, coordinated structures that consist of one or more cells. Even very simple, single-celled organisms are remarkably complex. Inside each cell, atoms make up molecules. These in turn make up cell organelles and other cellular inclusions.
In multicellular organisms (see image above), similar cells form tissues. Tissues, in turn, collaborate to create organs (body structures with a distinct function). Organs work together to form organ systems.
Sensitivity or Response to Stimuli
Organisms respond to diverse stimuli. For example, plants can bend toward a source of light, climb on fences and walls, or respond to touch (see image above). Even tiny bacteria can move toward or away from chemicals (a process called chemotaxis) or light (phototaxis). Movement toward a stimulus is considered a positive response, while movement away from a stimulus is considered a negative response.
Another important characteristic of life is reproduction. Reproduction is the ability of an organism to make more of the same type of organism. The new organisms that are made are called offspring. Although reproduction is not needed for the survival of an individual organism, it must occur for the continuation of the organism’s species.
A species consists of a group of organisms that can mate with each other and produce offspring that are able to reproduce. For example, there are many species of crocodiles including the American crocodile (see image above), the Australian freshwater crocodile, and the saltwater crocodile. American crocodiles reproduce only American crocodiles. In fact, without reproduction, the species would die out.
When reproduction occurs, organisms pass along genes containing DNA to an their offspring. These genes ensure that the offspring will belong to the same species and will have similar characteristics, such as size and shape.
Growth and Development
Another characteristic of life is that growth and development must take place. An organism begins life as a single cell. As time passes, it grows and develops. As growth and development take place, the organism takes on the characteristics of its species. Growth results in the formation of new structures and an increase in the amount of living material. Development refers to the changes that occur in each organism’s life.