Glossary: The Chemical Level of Organization

acid: compound that releases hydrogen ions (H⁺) in solution

activation energy: amount of energy greater than the energy contained in the reactants, which must be overcome for a reaction to proceed

adenosine triphosphate (ATP): nucleotide containing ribose and an adenine base that is essential in energy transfer

amino acid: building block of proteins; characterized by an amino and carboxyl functional groups and a variable side-chain

anion: atom with a negative charge

atom: smallest unit of an element that retains the unique properties of that element

atomic number: number of protons in the nucleus of an atom

base: compound that accepts hydrogen ions (H⁺) in solution

bond: electrical force linking atoms

buffer: solution containing a weak acid or a weak base that opposes wide fluctuations in the pH of body fluids

carbohydrate: class of organic compounds built from sugars, molecules containing carbon, hydrogen, and oxygen in a 1-2-1 ratio

catalyst: substance that increases the rate of a chemical reaction without itself being changed in the process

cation: atom with a positive charge

chemical energy: form of energy that is absorbed as chemical bonds form, stored as they are maintained, and released as they are broken

colloid: liquid mixture in which the solute particles consist of clumps of molecules large enough to scatter light

compound: substance composed of two or more different elements joined by chemical bonds

concentration: number of particles within a given space

covalent bond: chemical bond in which two atoms share electrons, thereby completing their valence shells

decomposition reaction: type of catabolic reaction in which one or more bonds within a larger molecule are broken, resulting in the release of smaller molecules or atoms

denaturation: change in the structure of a molecule through physical or chemical means

deoxyribonucleic acid (DNA): deoxyribose-containing nucleotide that stores genetic information

disaccharide: pair of carbohydrate monomers bonded by dehydration synthesis via a glycosidic bond

disulfide bond: covalent bond formed within a polypeptide between sulfide groups of sulfur-containing amino acids, for example, cysteine

electron shell: area of space a given distance from an atom’s nucleus in which electrons are grouped

electron: subatomic particle having a negative charge and nearly no mass; found orbiting the atom’s nucleus

element: substance that cannot be created or broken down by ordinary chemical means

enzyme: protein or RNA that catalyzes chemical reactions

exchange reaction: type of chemical reaction in which bonds are both formed and broken, resulting in the transfer of components

functional group: group of atoms linked by strong covalent bonds that tends to behave as a distinct unit in chemical reactions with other atoms

hydrogen bond: dipole-dipole bond in which a hydrogen atom covalently bonded to an electronegative atom is weakly attracted to a second electronegative atom

inorganic compound: substance that does not contain both carbon and hydrogen

ionic bond: attraction between an anion and a cation

ion: atom with an overall positive or negative charge

isotope: one of the variations of an element in which the number of neutrons differ from each other

kinetic energy: energy that matter possesses because of its motion

lipid: class of nonpolar organic compounds built from hydrocarbons and distinguished by the fact that they are not soluble in water

macromolecule: large molecule formed by covalent bonding

mass number: sum of the number of protons and neutrons in the nucleus of an atom

matter: physical substance; that which occupies space and has mass

molecule: two or more atoms covalently bonded together

monosaccharide: monomer of carbohydrate; also known as a simple sugar

neutron: heavy subatomic particle having no electrical charge and found in the atom’s nucleus

nucleotide: class of organic compounds composed of one or more phosphate groups, a pentose sugar, and a base

organic compound: substance that contains both carbon and hydrogen

pH: negative logarithm of the hydrogen ion (H⁺) concentration of a solution

peptide bond: covalent bond formed by dehydration synthesis between two amino acids

periodic table of the elements: arrangement of the elements in a table according to their atomic number; elements having similar properties because of their electron arrangements compose columns in the table, while elements having the same number of valence shells compose rows in the table

phospholipid: a lipid compound in which a phosphate group is combined with a diglyceride

phosphorylation: addition of one or more phosphate groups to an organic compound

polar molecule: molecule with regions that have opposite charges resulting from uneven numbers of electrons in the nuclei of the atoms participating in the covalent bond

polysaccharide: compound consisting of more than two carbohydrate monomers bonded by dehydration synthesis via glycosidic bonds

potential energy: stored energy matter possesses because of the positioning or structure of its components

product: one or more substances produced by a chemical reaction

prostaglandin: lipid compound derived from fatty acid chains and important in regulating several body processes

protein: class of organic compounds that are composed of many amino acids linked together by peptide bonds

proton: heavy subatomic particle having a positive charge and found in the atom’s nucleus

purine: nitrogen-containing base with a double ring structure; adenine and guanine

pyrimidine: nitrogen-containing base with a single ring structure; cytosine, thiamine, and uracil

radioactive isotope: unstable, heavy isotope that gives off subatomic particles, or electromagnetic energy, as it decays; also called radioisotopes

reactant: one or more substances that enter into the reaction

ribonucleic acid (RNA): ribose-containing nucleotide that helps manifest the genetic code as protein

solution: homogeneous liquid mixture in which a solute is dissolved into molecules within a solvent

steroid: (also, sterol) lipid compound composed of four hydrocarbon rings bonded to a variety of other atoms and molecules

substrate: reactant in an enzymatic reaction

suspension: liquid mixture in which particles distributed in the liquid settle out over time

synthesis reaction: type of anabolic reaction in which two or more atoms or molecules bond, resulting in the formation of a larger molecule

triglyceride: lipid compound composed of a glycerol molecule bonded with three fatty acid chains

valence shell: outermost electron shell of an atom