(1) An evolutionary modification that improves an organism's chances of survival and reproductive success. (2) A decline in the response of a receptor subjected to repeated or prolonged stimulation.
Prokaryotic organisms with a number of features, such as the absence of peptidoglycan in their cell walls, that set them apart from the bacteria. Archaea is the name of one of the two prokaryotic domains. Compare with bacteria.
Reproduction in which there is no fusion of gametes and in which the genetic makeup of parent and of offspring is usually identical. Compare with sexual reproduction.
The smallest quantity of an element that retains the chemical properties of that element.
An organism that synthesizes complex organic compounds from simple inorganic raw materials; also called producer or primary producer. Compare with heterotroph. See chemoautotroph and photoautotroph.
Prokaryotic organisms that have peptidoglycan in their cell walls; most are decomposers, but some are parasites and others are autotrophs. Bacteria is the name of one of the two prokaryotic domains. Compare with archaea.
binomial system of nomenclature
System of naming a species by the combination of the genus name and a specific epithet.
All of Earth's living organisms, collectively.
The basic structural and functional unit of life, which consists of living material enclosed by a membrane.
Mechanisms of communication between cells. Cells signal one another with secreted signaling molecules, or a signaling molecule on one cell combines with a receptor on another cell. See signal transduction.
The theory that the cell is the basic unit of life, of which all living things are composed, and that all cells are derived from pre-existing cells.
cilium (pl., cilia)
One of many short, hairlike structures that project from the surface of some eukaryotic cells and are used for locomotion or movement of materials across the cell surface.
A taxonomic category made up of related orders.
An association of populations of different species living together in a defined habitat with some degree of interdependence. Compare with ecosystem.
In a scientific experiment, a group in which the experimental variable is kept constant. The control provides a standard of comparison used to verify the results of the experiment.
Microbial heterotrophs that break down dead organic material and use the decomposition products as a source of energy. Also called saprotrophs or saprobes.
The reasoning that operates from generalities to specifics and can make relationships among data more apparent. Compare with inductive reasoning. See hypothetico-deductive approach.
deoxyribonucleic acid (DNA)
Double-stranded nucleic acid; contains genetic information coded in specific sequences of its constituent nucleotides.
All the progressive changes that take place throughout the life of an organism.
See deoxyribonucleic acid.
(1) A structural and functional region of a protein. (2) The broadest taxonomic category; each domain includes one or more kingdoms.
A discipline of biology that studies the interrelations among living things and their environments.
The interacting system that encompasses a community and its nonliving, physical environment. Compare with community.
Characteristics of an object, process, or behavior that could not be predicted from its component parts; emergent properties can be identified at each level as we move up the hierarchy of biological organization. energy The capacity to do work; expressed in kilojoules or kilocalories.
An organism whose cells have nuclei and other membrane-enclosed organelles. Includes protists, fungi, plants, and animals. Compare with prokaryote.
Any cumulative genetic changes in a population from generation to generation. Evolution leads to differences in populations and explains the origin of all the organisms that exist today or have ever existed.
A taxonomic category made up of related genera.
flagellum (pl., flagella)
A long, whiplike structure extending from certain cells and used in locomotion. (1) Eukaryote flagella consist of two central, single microtubules surrounded by nine double microtubules (9 + 2 structure), all covered by a plasma membrane. (2) Prokaryote flagella are filaments rotated by special structures located in the plasma membrane and cell wall.
fungus (pl., fungi)
A heterotrophic eukaryote with chitinous cell walls and a body usually in the form of a mycelium of branched, threadlike hyphae. Most fungi are decomposers; some are parasitic.
A segment of DNA that serves as a unit of hereditary information; includes a transcribable DNA sequence (plus associated sequences regulating its transcription) that yields a protein or RNA product with a specific function.
All the alleles of all the genes present in a freely interbreeding population.
A taxonomic category made up of related species.
An organism that cannot synthesize its own food from inorganic raw materials and therefore must obtain energy and body-building materials from other organisms. Also called consumer. Compare with autotroph. See chemoheterotroph and photoheterotroph.
The balanced internal environment of the body; the automatic tendency of an organism to maintain such a steady state
An organic chemical messenger in multicellular organisms that is produced in one part of the body and often transported to another part where it signals cells to alter some aspect of metabolism.
A testable statement about the nature of an observation or relationship. Compare with theory.
The reasoning that uses specific examples to draw a general conclusion or discover a general principle. Compare with deductive reasoning. See hypothetico-inductive approach.
A broad taxonomic category made up of related phyla; many biologists currently recognize six kingdoms of living organisms.
The sum of all the chemical processes that occur within a cell or organism; the transformations by which energy and matter are made available for use by the organism. See anabolism and catabolism.
The smallest particle of a covalently bonded element or compound; two or more atoms joined by covalent bonds.
Any change in DNA; may include a change in the nucleotide base pairs of a gene, a rearrangement of genes within the chromosomes so that their interactions produce different effects, or a change in the chromosomes themselves.
The mechanism of evolution proposed by Charles Darwin; the tendency of organisms that have favorable adaptations to their environment to survive and become the parents of the next generation. Evolution occurs when natural selection results in changes in allele frequencies in a population.
A chemical signal used by neurons to transmit impulses across a synapse.
A molecule consisting of one or more phosphate groups, a five-carbon sugar (ribose or deoxyribose), and a nitrogenous base (purine or pyrimidine).
nucleus (pl., nuclei)
(1) The central region of an atom that contains the protons and neutrons. (2) A cell organelle in eukaryotes that contains the DNA and serves as the control center of the cell. (3) A mass of nerve cell bodies in the central nervous system. Compare with ganglion.
A taxonomic category made up of related families.
A specialized structure, such as the heart or liver, made up of tissues and adapted to perform a specific function or group of functions.
An organized group of tissues and organs that work together to perform a specialized set of functions, e.g., the digestive system or circulatory system.
One of the specialized structures within the cell, such as the mitochondria, Golgi complex, ribosomes, or contractile vacuole; many organelles are membrane-enclosed.
Any living system consisting of one or more cells.
The biological process that captures light energy and transforms it into the chemical energy of organic molecules (e.g., carbohydrates), which are manufactured from carbon dioxide and water.
The selectively permeable surface membrane that encloses the cell contents and through which all materials entering or leaving the cell must pass.
A group of organisms of the same species that live in a defined geographic area at the same time. producer See autotroph.
A cell that lacks a nucleus and other membrane-enclosed organelles; includes the bacteria and archaea (kingdoms Eubacteria and Archaea). Compare with eukaryote.
A large, complex organic compound composed of covalently linked amino acid subunits; contains carbon, hydrogen, oxygen, nitrogen, and sulfur.
One of a vast kingdom of eukaryotic organisms, primarily unicellular or simple multicellular; mostly aquatic.
The gain of one or more electrons (or hydrogen atoms) by an atom, ion, or molecule. Compare with oxidation.
Permanently attached to one location, e.g., coral animals.
A type of reproduction in which two gametes (usually, but not necessarily, contributed by two different parents) fuse to form a zygote. Compare with asexual reproduction.
According to the biological species concept, one or more populations whose members are capable of interbreeding in nature to produce fertile offspring and do not interbreed with members of other species. Compare with evolutionary species concept.
The second part of the name of a species; designates a specific species belonging to that genus.
The scientific study of the diversity of organisms and their evolutionary relationships. Taxonomy is an aspect of systematics. See taxonomy.
A field of biology that synthesizes knowledge of many small parts to understand the whole. Also referred to as integrative biology or integrative systems biology.
A formal taxonomic group at any level, e.g., phylum or genus.
The science of naming, describing, and classifying organisms; see systematics.
A widely accepted explanation supported by a large body of observations and experiments. A good theory relates facts that appear unrelated; it predicts new facts and suggests new relationships. Compare with hypothesis.
A group of closely associated, similar cells that work together to carry out specific functions.