Life’s Levels of Organization
Nature is every substance and energy in the universe except what humans have manufactured. It includes flowers, water, animals, rocks, thunder, and so on. Biologists study the parts of nature that have to do with life, past and present. Through their work, we glimpse a great pattern of organization. The pattern starts with atoms, which are basic building blocks of all matter (1 – in the image above) . At the next level of organization, atoms join as molecules (2 – in the image above) . Only living cells make the molecules of life—complex carbohydrates and lipids, proteins, and nucleic acids—in nature. The pattern crosses the threshold to life when many molecules organize as a cell (3 – in the image above) . A cell is the smallest unit of life that can survive and reproduce on its own, given information in its DNA, energy, and raw materials. An organism is an individual that consists of one or more cells. In larger multicelled organisms, trillions of cells may be organized as tissues, organs, and organ systems that interact to keep the individual’s body working properly (4 – in the image above) .
At the next level of organization, a population is a group of individuals of the same type, or species, living in a given area (5 – in the image above) . An example would be all of the lake trout living in Lake Tahoe, California. At the next level, a community consists of all populations of all species in a given area (6 – in the image above) . An underwater ocean community, for example, may include many kinds of organisms that make their home in or on a particular reef. The next level of organization is the ecosystem: a community interacting with its environment (7 – in the image above) . The most inclusive level, the biosphere, encompasses all regions of Earth’s crust, waters, and atmosphere in which organisms live ( 8 – in the image above) .
Remember that life is more than the sum of its individual parts. In other words, some emergent property occurs at each successive level of life’s organization. An emergent property is a characteristic of a system that does not appear in any of a system’s component parts. For example, the molecules of life are themselves not alive. Considering them separately, no one would be able to predict that a particular arrangement of molecules would form a living cell. Life—an emergent property—appears first at the level of the cell.