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the part of the Earth, including air, land, surface rocks, and water, within which life occurs, and which biotic processes in turn alter or transform


a natural unit consisting of all plants, animals and micro-organisms in an area functioning together with all of the non-living physical factors of the environment


a group of interacting organisms sharing an environment


the collection of inter-breeding organisms of a particular species


an individual living system (such as animal, plant, fungus, or micro-organism)

Organ systems

A group of related organs


a group of tissues that perform a specific function or group of functions


a cellular organizational level intermediate between cells and a complete organism


the structural and functional unit of all known living organisms

Cell structures and viruses

An organelle, or the layout of organelles of the biological cell itself

Living and nonliving organic molecules

a sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds


the smallest unit of an element that retains the chemical properties of that element

Subatomic particles

an elementary or composite particle smaller than an atom

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