atomic number
	The characteristic number of protons per atom of an element. Used as an identifying attribute.
atom
	The smallest unit of matter that has the characteristics of an element; consists of three main types of subatomic particles: protons, neutrons, and electrons.
biological community
	The populations of plants, animals, and microorganisms living and interacting in a certain area at a given time.
biomass
	The total mass or weight of all the living organisms in a given population or area.
carbon cycle
	The circulation and reutilization of carbon atoms, especially via the processes of photosynthesis and respiration.
carbon sinks
	Places of carbon accumulation, such as in large forests (organic compounds) or ocean sediments (calcium carbonate); carbon is thus removed from the carbon cycle for moderately long to very long periods of time.
carnivores
	Organisms that mainly prey upon animals.
cell
	Minute biological compartments within which the processes of life are carried out.
cellular respiration
	The process in which a cell breaks down sugar or other organic compounds to release energy used for cellular work; may be anaerobic or aerobic, depending on the availability of oxygen.
chemical energy
	Potential energy stored in chemical bonds of molecules.
compound
	A molecule made up of two or more kinds of atoms held together by chemical bonds.
conservation of matter
	In any chemical reaction, matter changes form; it is neither created nor destroyed.
consumer
	An organism that obtains energy and nutrients by feeding on other organisms or their remains. See also heterotroph.
decomposers
	Fungi and bacteria that break complex organic material into smaller molecules.
detritivores
	Organisms that consume organic litter, debris, and dung.
ecology
	The scientific study of relationships between organisms and their environment. It is concerned with the life histories, distribution, and behavior of individual species as well as the structure and function of natural systems at the level of populations, communities, and ecosystems.
ecosystem
	A specific biological community and its physical environment interacting in an exchange of matter and energy.
energy
	The capacity to do work (that is, to change the physical state or motion of an object).
enzymes
	Molecules, usually proteins or nucleic acids, that act as catalysts in biochemical reactions.
first law of thermodynamics
	States that energy is conserved; that is, it is neither created nor destroyed under normal conditions.
food chain
	A linked feeding series; in an ecosystem, the sequence of organisms through which energy and materials are transferred, in the form of food, from one trophic level to another.
food web
	A complex, interlocking series of individual food chains in an ecosystem.
half-life
	The time required for one-half of a sample to decay or change into some other form.
heat
	A form of energy transferred from one body to another because of a difference in temperatures.
herbivore
	An organism that eats only plants.
homeostasis
	Maintaining a dynamic, steady state in a living system through opposing, compensating adjustments.
ions
	Electrically charged atoms that have gained or lost electrons.
isotopes
	Forms of a single element that differ in atomic mass due to a different number of neutrons in the nucleus.
joule
	A unit of energy. One joule is the energy expended in 1 second by a current of 1 amp flowing through a resistance of 1 ohm.
kinetic energy
	Energy contained in moving objects such as a rock rolling down a hill, the wind blowing through the trees, or water flowing over a dam.
matter
	Anything that takes up space and has mass.
metabolism
	All the energy and matter exchanges that occur within a living cell or organism; collectively, the life processes.
molecule
	A combination of two or more atoms.
nitrogen cycle
	The circulation and reutilization of nitrogen in both inorganic and organic phases.
omnivore
	An organism that eats both plants and animals.
organic compounds
	Complex molecules organized around skeletons of carbon atoms arranged in rings or chains; includes biomolecules--molecules synthesized by living organisms.
pH
	A value that indicates the acidity or alkalinity of a solution on a scale of 0 to 14, based on the proportion of H+ ions present.
phosphorus cycle
	The movement of phosphorus atoms from rocks through the biosphere and hydrosphere and back to rocks.
photosynthesis
	The biochemical process by which green plants and some bacteria capture light energy and use it to produce chemical bonds. Carbon dioxide and water are consumed while oxygen and simple sugars are produced.
population
	A group of individuals of the same species occupying a given area.
potential energy
	Stored energy that is latent but available for use. A rock poised at the top of a hill and water stored behind a dam are examples of potential energy.
producer
	An organism that synthesizes food molecules from inorganic compounds by using an external energy source; most producers are photosynthetic.
productivity
	The amount of biomass (biological material) produced in a given area during a given period of time.
radioactive isotope
	An unstable form of an element that spontaneously emits either high-energy electromagnetic radiation or subatomic particles (or both). The decay rate describes the time necessary for a radioactive isotope to change to a stable element.
scavenger
	An organism that feeds on the dead bodies of other organisms.
second law of thermodynamics
	States that, with each successive energy transfer or transformation in a system, less energy is available to do work.
species
	A population of morphologically similar organisms that can reproduce sexually among themselves but that cannot produce fertile offspring when mated with other organisms.
sulfur cycle
	The chemical and physical reactions by which sulfur moves into or out of storage and through the environment.
temperature
	A measure of the speed of motion of a typical atom or molecule in a substance.
trophic level
	A step in the movement of energy through an ecosystem; an organism's feeding status in an ecosystem.

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