Biological organization is the classification of biological structures, ranging from atoms at the bottom of the hierarchy to the Earth's biosphere. Each level of the hierarchy represents an increase in complexity that builds upon the previous level.
Molecules Are Composed of Atoms, and Biomolecules Are Assembled from Molecules:
The most basic levels include atoms, molecules, and biomolecules. Atoms, the smallest unit of ordinary matter, are composed of a nucleus and electrons. Molecules comprise two or more atoms held together by chemical bonds, most commonly covalent, ionic, or metallic bonds. Biomolecules are molecules found in living organisms, including proteins, nucleic acids, lipids, and carbohydrates. Biomolecules are often polymers—large molecules created from smaller, repeating units. For instance, proteins are composed of amino acids, and nucleic acids are composed of nucleotides. Biomolecules can be endogenous or exogenous. Endogenous means that the biomolecule is produced inside a living organism. Biomolecules can also be consumed; for example, a cow gets carbohydrates from digesting grass (exogenous), but the grass must produce the carbohydrates through photosynthesis (endogenous).
Organelles Are Cellular Compartments Built from Biomolecules:
The next hierarchical level is comprised of subcellular structures called organelles. Organelles are made up of biomolecules and compartmentalize eukaryotic cells. The word organelle means "little organ" as it has specialized functions within a cell. For example, the lysosome allows the degradation of molecules without detrimental effects on other structures within the cell, and chloroplasts enable plants to perform photosynthesis.
Organisms Can Have One, Multiple, or Several Trillions of Cells:
The next level of biological organization is the cell. Organisms can be unicellular (i.e., consist of a single cell) or multicellular. Scientists estimated that the human body consists of 37 trillion cells.
Tissues, Organs, and Organ Systems Are Formed by Specialized Cells:
In complex organisms such as plants and animals, cells form tissues, organs, and organ systems. Tissues are groups of cells that have similar structures and functions. For example, connective tissues, one of the four types of animal tissue, are found between organs. Organs are groups of tissues that carry out a specific function or set of functions and may be comprised of either similar or different tissue types. The heart, for example, is found in most animals and pumps blood throughout the body to provide other organs with oxygen and nutrients as well as remove waste products. Finally, groups of organs make up organ systems that work together to carry out vital processes. For example, the animal's cardiovascular system is comprised of the heart, vessels, and blood. In plants, a diverse set of tissues create the root and shoot system that provides different functions.
Individual Organisms Create Populations and Communities:
Several organisms of the same species that exist at the same place and time form populations. Communities are composed of individuals of different species at the same location and time. Communities can include multiple species. Ecosystems are comprised of not just the living (biotic) community but also the non-living (abiotic) environmental factors that influence the community. For example, an oasis is an ecosystem that may contain populations of date palms, fig trees, and small lizards found at the same location and time. This community lives and interacts with an environment of fertile soil and fresh water in an otherwise arid environment. The biosphere comprises all areas (air, soil, and water) on Earth that harbors living organisms. As such, the biosphere is composed of many ecosystems.
