The approximate total mass of an atom; also called atomic weight. Given as a whole number, the atomic mass approximately equals the mass number.
A chemical substance that resists changes in pH by accepting hydrogen ions from or donating hydrogen ions to solutions.
An attraction between two atoms resulting from a sharing of outer-shell electrons or the presence of opposite charges on the atoms. The bonded atoms gain complete outer electron shells.
The making and breaking of chemical bonds, leading to changes in the composition of matter.
A substance containing two or more elements in a fixed ratio. For example, table salt (NaCl) consists of one atom of the element sodium (Na) for every atom of chlorine (Cl).
An attraction between atoms that share one or more pairs of outer-shell electrons; symbolized by a single line between the atoms.
A type of covalent bond in which two atoms share two pairs of electrons; symbolized by a pair of lines between the bonded atoms.
A subatomic particle with a single negative electrical charge. One or more electrons move around the nucleus of an atom.
An energy level representing the distance of an electron from the nucleus of an atom.
Thermal energy; the amount of energy associated with the movement of the atoms and molecules in a body of matter. Heat is energy in its most random form.
A type of weak chemical bond formed when the partially positive hydrogen atom participating in a polar covalent bond in one molecule is attracted to the partially negative atom participating in a polar covalent bond in another molecule (or in another part of the same macromolecule).
A variant form of an atom. Isotopes of an element have the same number of protons but different numbers of neutrons.
An electrically neutral particle (a particle having no electrical charge), found in the nucleus of an atom.
nonpolar covalent bond
A covalent bond in which electrons are shared equally between two atoms of similar electronegativity.
A measure of the relative acidity of a solution, ranging in value from 0 (most acidic) to 14 (most basic). The letters pH stand for potential hydrogen and refer to the concentration of hydrogen ions (H).
polar covalent bond
A covalent bond between atoms that differ in electronegativity. The shared electrons are pulled closer to the more electronegative atom, making it slightly negative and the other atom slightly positive.
A subatomic particle with a single positive electrical charge, found in the nucleus of an atom.
A liquid consisting of a homogeneous mixture of two or more substances, consisting of a dissolving agent, called the solvent, and a substance that is dissolved, called the solute.
A measure of how difficult it is to stretch or break the surface of a liquid. Water has a high surface tension because of the hydrogen bonding of surface molecules.
A measure of the intensity of heat in degrees, reflecting the average kinetic energy or speed of molecules.
alpha (α) helix
The spiral shape resulting from the coiling of a polypeptide in a protein's secondary structure.
An organic molecule containing a carboxyl group and an amino group; serves as the monomer of proteins.
In an organic molecule, a functional group consisting of a nitrogen atom bonded to two hydrogen atoms.
A synthetic variant of the male hormone testosterone that mimics some of its effects.
Member of the class of biological molecules consisting of simple single-monomer sugars (monosaccharides), two-monomer sugars (disaccharides), and other multiunit sugars (polysaccharides).
The chain of carbon atoms that forms the structural backbone of an organic molecule.
In an organic molecule, a functional group consisting of a carbon atom linked by a double bond to an oxygen atom.
In an organic molecule, a functional group consisting of an oxygen atom double-bonded to a carbon atom that is also bonded to a hydroxyl group.
A large polysaccharide composed of many glucose monomers linked into cable-like fibrils that provide structural support in plant cell walls.
A structural polysaccharide found in many fungal cell walls and in the exoskeletons of arthropods.
A steroid that is an important component of animal cell membranes and that acts as a precursor molecule for the synthesis of other steroids such as hormones.
A chemical process in which two molecules become covalently bonded to each other with the removal of a water molecule. Also called condensation.
A process in which a protein unravels, losing its specific structure and hence function; can be caused by changes in pH or salt concentration or by high temperature. Also refers to the separation of the two strands of the DNA double helix, caused by similar factors.
deoxyribonucleic acid (DNA)
A double-stranded helical nucleic acid molecule consisting of nucleotide monomers with deoxyribose sugar and the nitrogenous bases adenine (A), cytosine (C), guanine (G), and thymine (T). Capable of replicating, is an organism's genetic material.
The form of native DNA, referring to its two adjacent polynucleotide strands wound into a spiral shape.
A protein that serves as a biological catalyst, changing the rate of a chemical reaction without itself being changed into a different molecule in the process.
A large lipid molecule made from an alcohol called glycerol and three fatty acids; a triglyceride. Most function as energy-storage molecules.
An assemblage of atoms commonly attached to the carbon skeletons of organic molecules and usually involved in chemical reactions.
A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses).
An extensively branched polysaccharide of many glucose monomers; serves as an energy-storage molecule in liver and muscle cells; the animal equivalent of starch.
A chemical process in which polymers are broken down by the chemical addition of water molecules to the bonds linking their monomers; an essential part of digestion.
"Water-loving"; pertaining to polar, or charged, molecules (or parts of molecules) that are soluble in water.
"Water-fearing"; pertaining to nonpolar molecules (or parts of molecules) that do not dissolve in water.
In an organic molecule, a functional group consisting of a hydrogen atom bonded to an oxygen atom.
Organic compounds with the same molecular formula but different structures and, therefore, different properties.
An organic compound consisting mainly of carbon and hydrogen atoms linked by nonpolar convalent bonds, making the compound mostly hydrophobic; includes fats, phospholipids, and steroids; insoluble in water.
A giant molecule in a living organism formed by the joining of smaller molecules: a protein, carbohydrate, or nucleic acid.
The simplest carbohydrate; a simple sugar with a molecular formula that is generally some multiple of CH2O; building blocks of disaccharides and polysaccharides.
A polymer consisting of many nucleotide monomers; serves as a blueprint for proteins and, through the actions of proteins, for all cellular structures and activities; two types are DNA and RNA.
An organic monomer consisting of a five-carbon sugar covalently bonded to a nitrogenous base and a phosphate group; building blocks of nucleic acids.
A chemical compound containing the element carbon and usually synthesized by cells.
The covalent linkage between two amino acid units in a polypeptide; formed by a dehydration reaction.
A functional group consisting of a phosphorus atom covalently bonded to four oxygen atoms.
A lipid made up of glycerol joined to two fatty acids and a phosphate group, giving the molecule a nonpolar hydrophobic tail and a polar hydrophilic head; form bilayers that function as biological membranes.
A large molecule consisting of many identical or similar molecular units, called monomers, covalently joined together in a chain.
A carbohydrate polymer consisting of hundreds to thousands of monosaccharides (sugars) linked by dehydration synthesis.
The first level of protein structure; the specific sequence of amino acids making up a polypeptide chain.
A functional biological molecule consisting of one or more polypeptides folded into a specific three-dimensional structure.
The fourth level of protein structure; the shape resulting from the association of two or more polypeptide subunits.
ribonucleic acid (RNA)
A type of nucleic acid consisting of nucleotide monomers with a ribose sugar and the nitrogenous bases adenine (A), cytosine (C), guanine (G), and uracil (U); usually single-stranded; functions in protein synthesis and as the genome of some viruses.
Pertaining to fats and fatty acids whose hydrocarbon chains contain the maximum number of hydrogens and therefore have no double covalent bonds; solidify at room temperature.
The second level of protein structure; the regular local patterns of coils or folds of a polypeptide chain.
A storage polysaccharide found in the roots of plants and certain other cells; a polymer of glucose.
A type of lipid whose carbon skeleton is in the form of four fused rings with various chemical groups attached; examples are cholesterol, testosterone, and estrogen.
The third level of protein structure; the overall, three-dimensional shape of a polypeptide due to interactions of the R groups of the amino acids making up the chain.