Microscopic organism that is a member of one of the two divisions of prokaryotes; often found in hostile environments such as hot springs or concentrated brine.

Microscopic organism that is a member of one of the two divisions of prokaryotes; some species cause disease. The term is sometimes used to refer to any prokaryotic microorganisms, although the world of prokaryotes also includes archaea, which are only distantly related to each other.

The basic unit from which a living organism is made; an aqueous solution of chemicals, enclosed by a membrane, that has an ability to self-replicate.

Specialized organelle in algae and plants that contains chlorophyll and serves as the site for photosynthesis.

Long, threadlike structure composed of DNA and proteins that carries the genetic information of an organism; becomes visible as a distinct entity when a plant or animal cell prepares to divide.

Contents of a cell that are contained within its plasma membrane but, in the case of eukaryotic cells, outside the nucleus.

System of protein filaments in the cytoplasm of a eukaryotic cell that gives the cell shape and the capacity for directed movement. Its most abundant components are actin filaments, microtubules, and intermediate filaments.

Contents of the main compartment of the cytoplasm, excluding membrane-enclosed organelles such as endoplasmic reticulum and mitochondria. The cell fraction remaining after membranes, cytoskeletal components, and other organelles have been removed.

Double-stranded polynucleotide formed from two separate chains of covalently linked deoxyribonucleotide units. It serves as the cell’s store of genetic information that is transmitted from generation to generation.

Instrument that illuminates a specimen using beams of electrons to reveal and magnify the structure of very small objects, such as organelles and large molecules.

Labyrinthine membrane-enclosed compartment in the cytoplasm of eukaryotic cells where lipids and proteins are made.

An organism whose cells have a distinct nucleus and cytoplasm.

Process of gradual modification and adaptation that occurs in living organisms over generations.

Instrument used to visualize a specimen that has been labled with a fluorescent dye; samples are illuminated with a wavelength of light that excites the dye, causing it to fluoresce.

The total genetic information carried by all the chromosomes of a cell or organism; in humans, the total number of nucleotide pairs in the 22 autosomes plus the X and Y chromosomes.

Membrane-enclosed organelle in eukaryotic cells that modifies the proteins and lipids made in the endoplasmic reticulum and sorts them for transport to other sites.

Describes genes, chromosomes, or any structures that are similar because of their common evolutionary origin. Can also refer to similarities between protein sequences or nucleic acid sequences.

Unit of length equal to one millionth (10–6) of a meter or 10–4 centimeter.

Instrument for viewing extremely small objects. Some use a focused beam of visible light and are used to examine cells and organelles. Others use a beam of electrons and can be used to examine objects as small as individual molecules.

Membrane-enclosed organelle, about the size of a bacterium, that carries out oxidative phosphorylation and produces most of the ATP in eukaryotic cells.

In biology, refers to the prominent, rounded structure that contains the DNA of a eukaryotic cell. In chemistry, refers to the dense, positively charged center of an atom.

A living thing selected for intensive study as a representative of a large group of species. Examples include the mouse(representing mammals), the yeast Saccharomyces cerevisiae.

A discrete structure of subcompartment of a eukaryotic cell that is specialized to carry out a particular function. Examples include mitochondria and the Golgi apparatus.

The process by which plants, algae, and some bacteria use the energy of sunlight to drive the synthesis of organic molecules from carbon dioxide and water.

The protein-containing lipid bilayer that surrounds a living cell.

Major category of living cells distinguished by the absence of a nucleus; includes the archaea and the eubacteria(commonly called bacteria).

Macromolecule built from amino acids that provides cells with their shape and structure and performs most of their activities.

A free-living, nonphotosynthetic, single-celled, motile eukaryote.

Large macromolecular complex, composed of rnas and proteins, that translates a messenger RNA into a polypeptide chain.

Molecule produced by the transcription of DNA; usually single-stranded, it is a polynucleotide composed of covalently linked ribonucleotide subunits. Serves as variety of informational, structural, catalytic, and regulatory functions in cells.

A molecule that releases a proton when dissolved in water; this dissociation generates hydronium (H3O+) ions, thereby lowering the ph.

Small organic molecule containing both an amino group and a carboxyl group; it serves as the building block of proteins.

The smallest particle of an element that still retains its distinctive chemical properties; consist of a positively charged nucleus surrounded by a cloud of negatively charged electrons.

The mass of an atom relative to the mass of a hydrogen atom; equal to the number of protons plus the number of neutrons that the atom contains.

Activated carrier that serves as the principal carrier of energy in cells; a nucleoside triphosphate composed of adenine, ribose, and three phosphate groups.

The number of molecules in a mole, the quantity of a substance equal to its molecular weight in grams; approximately 6 x 10^23.

Molecule that accepts a proton when dissolved in water; also used to refer to the nitrogen containing purines or pyrimidines in DNA and RNA.

Mixture of weak acids and bases that maintains the pH of a solution by releasing anf taking up protons.

A sharing or transfer of electrons that holds two atoms together.

A combination of atoms, such as a hydroxyl group (-OH) or an amino group (-NH2), with distinct chemical and physical properties that influence the behavior of the molecule in which it resides.

Chemical reaction in which a covalent bond is formed between two molecules as water is expelled; used to build polymers, such as proteins, polysaccharides, and nucleic acids.

Precise, three-dimensional shape of a protein or other macromolecule, based on spatial location of its atoms in relation to one another.

Stable chemical link between two atoms produced by sharing one or more pairs of electrons.

Negatively charged subatomic particle that occupies space around an atomic nucleus (e–).

The tendency of an atom to attract electrons.

Force that draws together oppositely charged atoms. Examples include ionic bonds and the attractions between molecules containing polar covalent bonds.

Molecule that consists of a carboxylic acid attached to a long hydrocarbon chain. Used as a major source of energy during metabolism and as a starting point for the synthesis of phospholipids.

A weak noncovalent interaction between a positively charged hydrogen atom in one molecule and a negatively charged atom, such as nitrogen or oxygen, in another.

Chemical reaction that involves cleavage of a covalent bond with the accompanying consumption of water (its –H being added to one product of the cleavage and its –OH to the other); the reverse of a condensation reaction.

The form taken by a proton (H+) in aqueous solution.

Molecule or part of a molecule that readily forms hydrogen bonds with water, allowing it to readily dissolve; literally, “water loving.”

Nonpolar, uncharged molecule or part of a molecule that forms no hydrogen bonds with water molecules and therefore does not dissolve; literally, “water fearing.”

A noncovalent interaction that forces together the hydrophobic portions of dissolved molecules to minimize their disruption of the hydrogen-bonded network of water; causes membrane phospholipids to self-assemble into a bilayer and helps to fold proteins into a compact, globular shape.

Not composed of carbon atoms.

An atom carrying an electrical charge, either positive or negative.

Interaction formed when one atom donates electrons to another; this transfer of electrons causes both atoms to become electrically charged.

An organic molecule that is insoluble in water but dissolves readily in nonpolar organic solvents; typically contains long hydrocarbon chains or multiple rings. One class, the phospholipids, forms the structural basis for biological membranes.

Thin pair of closely juxtaposed sheets, composed mainly of phospholipid molecules, that forms the structural basis for all cell membranes.

Polymer built form covalently linked subunits; includes proteins, nucleic acids, and polysaccharides, with a molecular mass greater than a few thousand daltons.

Sum of the atomic weights of the atoms in a molecule; as a ration of molecular masses, it is a number without units.

Group of atoms joined together by covalent bonds.

Small molecule that can be linked to others of a similar type to form a larger molecule (polymer).

Chemical association that does not involve the sharing of electrons; singly they are relatively weak, but they can sum together to produce strong, highly specific interactions between molecules. Examples are hydrogen bonds and van der Waals attractions.

Basic building block of the nucleic acids, DNA and RNA; a nucleoside linked to a phosphate.

Chemical compound that contains carbon and hydrogen.

Concentration of hydrogen ions in a solution, expressed as a logarithm.

In chemistry, describes a molecule or bond in which electrons are distributed unevenly.

Long molecule made by covalently linking multiple identical or similar subunits (monomers).

Positively charged particle found in the nucleus of every atom; also, another name for a hydrogen ion (H+).

The linear order of monomers in a large molecule—for example, amino acids in a protein or nucleotides in DNA; encodes information that specifies a macromolecule’s precise biological function.

A monomer that forms part of a larger molecule, such as an amino acid residue in a protein or a nucleotide residue in a nucleic acid. Can also refer to a complete molecule that forms part of a larger molecule.

A substance made of carbon, hydrogen, and oxygen with the general formula (CH2O)n. A carbohydrate or saccharide.

Weak noncovalent interaction, due to fluctuating electrical charges, that comes into play between two atoms within a short distance of each other.

Region on the surface of an enzyme that binds to a substrate molecule and catalyzes its chemical transformation.

Describes a protein that can exist in multiple conformations depending on the binding of a molecule (ligand) at a site other than the catalytic site; such changes from one conformation to another often alter the protein’s activity or ligand affinity.

Folding pattern, common in many proteins, in which a single polypeptide chain twists around itself to form a rigid cylinder stabilized by hydrogen bonds between every fourth amino acid.

The order of the amino acid subunits in a protein chain. Sometimes called the primary structure of a protein.

Protein produced by B lymphocytes in response to a foreign molecule or invading organism. Binds to the foreign molecule or cell extremely tightly, thereby inactivating it or marking it for destruction.

Molecule or fragment of a molecule that is recognized by an antibody.

Folding pattern found in many proteins in which neighboring regions of the polypeptide chain associate side-by-side with each other through hydrogen bonds to give a rigid, flattened structure.

Region on the surface of a protein, typically a cavity or groove, that interacts with another molecule (a ligand) through the formation of multiple noncovalent bonds.

The end of a polypeptide chain that carries a free carboxyl group (–COOH).

Technique used to separate the individual molecules in a complex mixture on the basis of their size, charge, or their ability to bind to a particular chemical group. In a common form of the technique, the mixture is run through a column filled with a material that binds the desired molecule, and it is then eluted from the column with a solvent gradient.

Small molecule that binds tightly to an enzyme and helps it to catalyze a reaction.

Technique for observing the detailed structure of a macromolecule at very low temperatures after freezing native structures in ice.

Covalent cross-link formed between the sulfhydryl groups on two cysteine side chains; often used to reinforce a secreted protein’s structure or to join two different proteins together.

Technique for separating a mixture of proteins or DNA fragments by placing them on a polymer gel and subjecting them to an electric field. The molecules migrate through the gel at different speeds depending on their size and net charge.

A protein that catalyzes a specific chemical reaction.

A form of metabolic control in which the end product of a chain of enzymatic reactions reduces the activity of an enzyme early in the pathway.

A protein with an elongated, rodlike shape, such as collagen or a keratin filament.

Any protein in which the polypeptide chain folds into a compact, rounded shape. Includes most enzymes.

Intracellular signaling protein whose activity is determined by its association with either GTP or GDP. Includes both trimeric G proteins and monomeric GTPases, such as Ras.

An elongated structure whose subunits twist in a regular fashion around a central axis, like a spiral staircase.

A large aggregate of phase-separated macromolecules that creates a region with a special biochemistry without the use of an encapsulating membrane.

Region in a polypeptide chain that lacks a definite structure.

General term for a small molecule that binds to a specific site on a macromolecule.

Enzyme that severs the polysaccharide chains that form the cell walls of bacteria; found in many secretions including saliva and tears, where it serves as an antibiotic.

Sensitive technique that enables the determination of the exact mass of all of the molecules in a complex mixture.

The concentration of substrate at which an enzyme works at half its maximum velocity; serves as a measure of how tightly the substrate is bound.