A words.

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Abiotic. A+biotic. Non living. For example, an abiotic factor is a non-living part of the environment.
Absorption spectrum. The amount of light taken up by a series of pigments across a particular range of wavelengths. This is important in photosynthesis because those wavelengths of light that are absorbed by the photosynthetic pigments are the wavelengths that can provide energy for photosynthesis.
Acetyl CoA. A two carbon energy rich residue attached to a coenzyme called Coenzyme A. Acetyl CoA feeds the two carbon residue into the Kreb's citric acid cycle.

Acid. Any substance that when added to water donates hydrogen ions to the solution. Acids maybe strong or weak. Strong acids require only a small amount of acid to donate a certain number of hydrogen ions compared to weak acids. Strong acids include hydrochloric, nitric and sulfuric acids. The principle weak acids are the organic acids such as formic, acetic and the fatty acids. See also carboxyl group, pH, base.

Activation energy. The amount of energy that must be added to the reactants(starting materials) in a chemical reaction in order for the chemical reaction to go to completion. This is an important concept in biology because enzymes operate by lowering the amount of activation energy required for biologically important reactions.

Active Transport. Refers to a series of energy requiring methods used by cells to get molecules, atoms or ions into or out of the cell. The energy used by these methods is generally ATP and the cell is typically acting against the natural tendency of diffusion.

Active Site. The active site is that region of the enzyme where the chemical reaction catalyzed by the enzyme takes place. The enzyme plus the chemicals involved in the reaction are jointly called the enzyme substrate complex.

Adaptation. 1.) Any physical, chemical or behavioral product of evolution that is important for the survival of an organism in it's usual environment. 2.) A physiological change in the state of an organism in response to an environmental change and that has the effect of restoring the organism's homeostasis.

Adaptive radiation. A common biogeographical pattern seen on remote islands in which a particular group of organisms is found in a wide variety of niches including many not normally inhabited by the group. Scientists infer that this pattern comes about by the an ancestral species evolving into all these niches giving rise to new species.

Adaptive zone . The way of life or functional role of an organism as seen from an evolutionary point of view. The idea is that once an evolutionary lineage has contact with a potential adaptive zone such as night flying insect eater then that lineage can evolve into that zone if its not completely occupied.
Adenosine tri-phosphate(ATP). A nucleotide like compound containing three phosphate groups. ATP is important because it serves as the principle means of energy transfer between energy producing and energy using reactions of the cell. When ATP gives up its energy it does so by a transfer of the last phosphate group to another molecule. After the energy transfer, the remainder from the ATP is called Adenosine di-phosphate or ADP.

Adhesion. The tendency of unlike substances to stick together. In biology water being polar is said to adhere to other substances such as cellulose that also have polar groups. Contrast with cohesion.

Aerobic. Literally means "with air." generally refers to cellular respiration that requires oxygen. Contrast Anaerobic.

Agonistic behavior. A class of behavior that includes both aggressive and submissive behavior.
Alcohol. A compound consisting of a hydrocarbon skeleton with one or more hydroxyl group. Ethyl alcohol is a familiar by product of yeast fermentation.
Alga(ae). A single celled or primitively multicellular aquatic eukaryotic organism that has cellulose cell walls and carries out photosynthesis.

Allele. An alternate form of a gene, in other words a gene that provides instructions for a slightly different version of the same polypeptide as the other member of a gene pair at the same location on a chromosome. See also gene, locus, chromosome.

Allopatric speciation. Evolution of new species involving the following steps: 1) isolation of an original species into two or more isolated groups due a climatic or geological event. 2) genetic divergence between the isolated groups through time due to natural selection and other mechanisms of evolution 3) Development of pre and post zygotic species isolating mechanisms. See also sympatric speciation.

Allosteric. The ability of an enzyme or other protein to exist in two or more forms. This is important because cells can exert control of an enzyme by switching it between a form with a functional active site and a form with a non functional active site. Coenzymes typically bind to a auxiliary site on the enzyme and the binding converts the enzyme from an inactive form to an active form.
Alternation of generations. The type of life cycle found in plants and some algae in which a diploid multicellular stage produces haploid spores by meiosis. The spores germinate in a suitable habitat and develop into a haploid multicellular organism which produces male and female gametes by mitosis. The gametes fuse and the diploid stage is produced from the resulting zygote by mitosis.

Altruism(Altruistic behavior). In biology, altruism is any behavior on the part of an individual organisms that benefits the reproductive success of relatives or unrelated individuals while reducing the individuals own reproductive success.

Alveolus(i). A small sac at the end of the bronchioles in the lungs. The walls of the sac consist of simple squamous epithelium which allows gas exchange between the sac and a network of capillaries which surrounds the sac. Each lung consists in part of many thousands of such sacs.

Amino. A functional group containing nitrogen and hydrogen.
Amino acid. An organic compound containing an amino group at one end, a carboxyl group at the other and one of any number of functional groups from the middle carbon. The amino acid shown is glycine, the simplest amino acid. Amino acids are the building blocks of proteins.

Amplification. In genetic engineering, the process by which a small amount of DNA is repeatedly duplicated to make a large amount of DNA. This can be done by inserting the DNA into a bacterial host and letting the bacteria multiply, thus making many copies of the DNA. More commonly today amplification involves an alternating sequence of mild heating and then incubation of the separated DNA halves with nucleotide bases and DNA polymerase This process is called the polymerase chain reaction or PCR.

Analogous structures. Analogous structures are structures with similar shape and function but differ in terms of their developmental origin. The inference that scientists make is that these structures do not have a common evolutionary origin.

Anabolism. The set of chemical reactions in metabolism that involve making more complex compounds from simpler compounds. For example, using amino acids to synthesize proteins is an anabolic process.
Anaerobic. Cellular Respiration without oxygen.
Androgen. This term refers to anyone of a number of steroid hormones that promote male secondary sex characteristics. Testosterone is the most familiar of these hormones, causing such characteristics as deepening voice, facial and chest hair.
Angiosperm. Flowering plant. The term refers to the fact that flowering plant seeds are hidden in a vessel rather than exposed.

Animalia. The Kingdom Animalia consists of multicellular eukaryotes. Animals do not have cell walls and are heterotrophic. Most animals ingest their food before digestion. In addition animals undergo an embryological development including a hollow ball stage called a blastula.

Antagonistic hormones. Hormones that have opposite effects and are typically part of a control system regulating the levels of a compound in the body. Insulin and glucagon are antagonistic hormones involved in regulating blood glucose level.
Antibody. A Y shaped protein produced by effector B cells(B lymphocytes)in the immune system of the body in response to the presence of certain foreign molecules or antigens produced by invading organisms. Antibodies are an important component of the body's specific defense system. Antigens are foreign molecules which in turn activate the immune system's production of antibodies that are specific to the foreign molecule.
Anticodon. A three nucleotide base triplet on transfer RNA that is complementary to a corresponding triplet on the messenger RNA. Anticodons are important because they allow the proper transfer RNA with its amino acid to attach to the ribosome, messenger RNA complex, so that the correct sequence of amino acids is produced during protein synthesis.
Antigen. A substance, generally foreign to the body that elicits an immune response. Antigens are often proteins or polysaccharides on the surface of pathogens.

Apoptosis. Cell death either as a pre-programmed part of development or in response to chemical signals released by certain cells of the immune system.
Arboreal. Living in trees.

Archea(Archaebacteria). A group of prokaryotes characterized by their ability to survive in extreme environments. Some live in hot springs while others live in environments with little air. Today the Archea are considered to be in a separate kingdom from the rest of the prokaryotes.

Asexual. Referring to any one of a number of common reproductive methods based on mitosis and thus producing offspring that genetically identical or clones of the parent..

Atom. The smallest unit of matter that can partake of chemical reactions and that has the chemical properties of an element.

Atomic number. The number of protons in an an atom. For instance, oxygen with an atomic number of 8 has eight protons in its nucleus.

Autocatylist. A chemical that enhances the production of copies of itself without getting destroyed in the process.

Autotroph. An organism that obtains its energy either from sunlight or from simple compounds. For instance trees carry out photosynthesis and thus are autotrophs.