What Is Life?
Biologists have identified seven characteristics that all living things share.
There are characteristics that all of life on earth share. While it may seem obvious that a rock is not alive and a person is, there are questions regarding other things, such as viruses. The following are seven characteristics that are used in the science of biology to define what life is.
Cells and Organization
Cells are the basic unit of life. The smallest organism consists of one cell. Within that cell is the cellular equivalent of organs, called organelles. Organelles are made of molecules or macromolecules such as carbohydrates, lipids, proteins, and nucleic acids. These organelles serve different functions including transferring ribonucleic acid (RNA), making adenosine triphosphate (ATP) for energy, or transferring molecules in and out of the cell.
Similar cells combine to form tissues, such as the pericardium, the membrane around the heart. Tissues give rise to organs, like the liver, heart, or skin. Unlike the single-celled organism, complex organisms consist of a combination of cells, tissues, and organs.
Organisms must acquire and use energy in order to maintain their complex, living systems. Cells metabolize molecules, such as nutrients, to produce energy and use it for other processes, like replicating deoxyribonucleic acid (DNA).
Some organisms, including plants and some bacteria, can convert light energy to produce nutrients. This is called photosynthesis. Other organisms, such as animals, must consume other organisms to meet their energy needs.
Organisms must be able to adapt to their changing environments. For example, during the season when the sun is directly overhead, plants will grow upward, but they will grow more toward the side during those seasons when the sun is lower in the sky. Horses shed their coats in the spring and summer to allow for easier cooling during the warmer weather. They acquire a thick coat for the winter months when it is necessary to keep warm air close to the skin.
Organisms must maintain an internal environment that is conducive to cell metabolism. For example, humans must maintain a specific body temperature. When they are cold, they will shiver, a response that causes the muscles to produce more heat. When they are too warm, the body will produce sweat which carries heat away from the body when it evaporates. The degree to which this must be done varies among organisms.
Growth and Development
All organisms undergo a process of growth and development. Single-celled organisms start as smaller cells that grow. The contents of the cell may become more diverse and complex. More complex organisms, such as humans, start out as a single cell that divides to create more cells. These new cells differentiate to develop into specialized cells that form different tissues and organs.
All living organisms have DNA, which is genetic material containing the information and instructions for forming an organism. All organisms die and must be able to pass this genetic material on to create more organisms. This is done through reproduction. Single-celled organisms may divide in half and grow. Other organisms reproduce sexually, a process that requires another organism with which to share its DNA.
The book, Biology, by Robert J. Brooker, et al., defines biological evolution as “the phenomenon that populations of organisms change over the course of many generations.” This can include the beak length and shape of a particular species of bird that is better suited to acquiring certain kinds of food in a particular environment.
The characteristics of living things begin with the organization from the molecular level to the organismal level. Organisms are able to acquire energy and break it down for cellular use. This energy is used to maintain an internal environment that is conducive to cellular processes. Organisms can adapt to environmental changes, and a species as a whole can evolve over the period of many generations. Living organisms can reproduce to create more organisms that will grow and develop. Essentially, these properties of life, together, are the definition of life.
Living organisms share these seven characteristics, but that does not make them the same. The ability of the individual to adapt and the population to evolve allows for a diverse number of species throughout the world. This diversity allows organisms to live within the various environments of the earth.
Brooker, Robert J., Widmaier, Eric P., Graham, Linda E., and Stiling, Peter D. Biology. New York: McGraw-Hill, 2008.