Life on Earth is organized into different levels based on degree of complexity. These levels range from the very small and simple (like the atom) to the very large and complex (such as the biosphere). They are organized into a type of hierarchy, with the larger levels enveloping the smaller levels- sort of like Russian nesting dolls. In this tutorial, we will examine the complexity of life at the cellular, organismal and populational levels of biological organization. Cellular organization begins with the atom, which is the basic unit of matter. Atoms combine with other atoms to form molecules, which in turn can combine to form macromolecules, like the DNA shown here, which is used to store hereditary information. Atoms, molecules and macromolecules combine to form structures within cells, such as organelles, which are tiny compartments of eukaryotic cells. Each organelle has a specific cellular function. The nucleus, for example, is an organelle used for the storage of DNA. Atoms, molecules, macromolecules and organelles all combine to form a cell. A cell is the smallest level of organization that can be considered alive, and therefore is considered the smallest unit of life. At the organismal level, cells are organized into four levels of complexity. Cells combine to form tissues, which are groups of cells that act as a unit and perform a specific function. For example, cardiac muscle is a type of animal tissue that has the ability to contract. Tissues are grouped into organs that also perform specific functions. The heart, for example, is an organ composed of cardiac muscle tissue that contracts in order to circulate blood within the body of an animal. Organs, in turn, work together in organ systems. The circulatory system, for example, consists of the heart, blood and blood vessels, that all work together to move nutrients to the cells of the body, and remove waste materials from these same cells. And finally, various organ systems work together to form a complex organism, such as a human. The organization of life also extends beyond the level of individual organisms. The next level of organization is a population, which is a group of similar organisms living in the same place. A group of persons living in your hometown would be considered a population. All the populations of a particular kind of organism together form a species. Members of a species are similar in appearance and are able to interbreed. The modern species of humans, to which we all belong, is referred to as Homo sapiens. The next level of organization is a community, which consists of multiple different species occupying the same region. Humans, for example, typically share their neighborhoods with dogs, cats, birds, trees, grasses and numerous other species. When communities interact with parts of the non-living environment, like water, gases, soil and climate, they form what’s referred to as an ecosystem. Collectively, all of the ecosystems on the planet make up the biosphere. The biosphere contains all parts of the Earth, from the atmosphere, to underground and to the deepest trenches in the ocean where life is possible.