Biologists are fascinated by life and the different processes by which organisms use energy, interact, and reproduce. Scientists search for life on other planets. They try to create life in laboratories. How do they know when they find life? What is life?
Biologists have carefully considered the characteristics of living things. They agree that organisms must possess seven characteristics of life.
Cells and Organization
Cells are the basic unit of life. The smallest organisms consist of one cell. Every cell contains organelles that are responsible for carrying out various cell processes, such as energy production, RNA replication, and molecular transfer in and out of the cell. Organelles are made of molecules or macromolecules such as carbohydrates, lipids, proteins, and nucleic acids.
Similar cells combine to form tissues, such as the pericardium that surrounds 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.
Levels of organization go beyond the anatomy single organisms. A group of like organisms is a species. Organisms of the same species that live in the same area at the same time are a population. Different populations of different species that live in the same area are communities. An ecosystem consists of its communities and the surrounding environment.
Metabolism
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 acquire macronutrients. Respiration is a metabolic process by which oxygen is taken into the body and used by the cells for energy production.
Environmental Adaptation
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. Many animals respond to predators by running away or excreting toxins. An example of environmental adaptation in humans is skin pigmentation which is influenced by exposure to UV light.
Homeostasis
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.
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 into specialized cells that form different tissues and organs.
Reproduction
All living organisms have DNA, which is genetic material containing the information and instructions for forming an organism. All organisms 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 combining the genetic material of two organisms to produce a unique organism. Biologists cannot agree whether viruses are alive because they must use another organism's genetic material to replicate.
Evolution
In Biology, 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. Organisms must be able to evolve.
The characteristics of living things begin with 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 allow for the diverse number of species throughout the world. This diversity allows organisms to live within the various environments of the earth.
Reference
Brooker, Robert J., Widmaier, Eric P., Graham, Linda E., and Stiling, Peter D. Biology. New York: McGraw-Hill, 2008.
Diane Ursu - Diane Ursu joined Suite101 as a contributing writer in August 2009 and became a Feature Writer in January 2010. She is a freelance writer ...
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