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.
What are the characteristics of life?
One of the first things biologists look for when they are searching for characteristics of life is structure, or organization. Whether an organism is made of a single cell or billions of cells, all of its parts work together in an orderly living system.
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 (SPEE sheez) 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, the Australian freshwater crocodile, and the saltwater crocodile. American crocodiles reproduce only American crocodiles. Without reproduction, the species would die out.
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.
One more characteristic of life is the ability to adjust to surroundings, or the environment. Anything in the environment— air, water, temperature, weather, other organisms—that causes the organism to react is called a stimulus (plural, stimuli). The organism’s reaction to the stimulus is called a response.
An organism also has the ability to control its internal environment in order to maintain conditions suitable for survival. For example, an organism must make constant adjustments to maintain the right amount of water and minerals in its cells. This ability is called homeostasis (hoh mee oh STAY sus). Without the ability to adjust to internal changes, an organism would die.
How do organisms respond to change?
Organisms use energy to grow, develop, respond to stimuli, and maintain homeostasis. Energy is the ability to cause change. Organisms get their energy from food.
Any behavior, structure, or internal process that allows an organism to make changes in response to environmental factors and live long enough to reproduce is called an adaptation (a dap TAY shun).
For example, the leaves of many desert plants have a thick, waxy coating. This is an adaptation that helps these plants conserve water. Having large eyes is an adaptation that lets owls see well at night. The gradual change in a species over time due to adaptations is called evolution (e vuh LEW shun).