nucleus 
(1) An atom's central core, containing protons and neutrons. (2) The chromosome-containing organelle of a eukaryotic cell. (3) A cluster of neurons.
element Any substance that cannot be broken down to any other substance.
atomic number The number of protons in the nucleus of an atom, unique for each element and designated by a subscript to the left of the elemental symbol.
atomic number The number of protons in the nucleus of an atom, unique for each element and designated by a subscript to the left of the elemental symbol.
ion (EYE-on) An atom that has gained or lost electrons, thus acquiring a charge.
isotope (EYE-so-tope) One of several atomic forms of an element, each containing a different number of neutrons and thus differing in atomic mass.
covalent bond (koh-VAY-lent) A type of strong chemical bond in which two atoms share one pair of electrons in a mutual valence shell.
molecule Two or more atoms held together by covalent bonds.
electronegativity The tendency for an atom to pull electrons toward itself.
polar covalent bond A type of 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.
nonpolar covalent bond A type of covalent bond in which electrons are shared equally between two atoms of similar electronegativity.
polar molecule A molecule (such as water) with opposite charges on opposite sides.
surface tension 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.
molecular formula A type of molecular notation indicating only the quantity of the constituent atoms.
aldehyde (AL-duh-hyde) An organic molecule with a carbonyl group located at the end of the carbon skeleton.
polymer (POL-eh-mur) A large molecule consisting of many identical or similar monomers linked together.
monosaccharide (MON-oh-SAK-ur-ide) The simplest carbohydrate, active alone or serving as a monomer for disaccharides and polysaccharides. Also known as simple sugars, the molecular formulas of monosaccharides are generally some multiple of CH2O.
disaccharide (dy-SAK-ur-ide) A double sugar, consisting of two monosaccharides joined by dehydration synthesis.
polysaccharide (POL-ee-SAK-ur-ide) A polymer of up to over a thousand monosaccharides, formed by condensation synthesis.
cellulose (SELL-yoo-lose) A structural polysaccharide of cell walls, consisting of beta glucose monomers joined by 1-4 glycosidic linkages.
glycogen (GLY-koh-jen) An extensively branched glucose storage polysaccharide found in the liver and muscle of animals; the animal equivalent of starch.
cellular respiration The most prevalent and efficient catabolic pathway for the production of ATP, in which oxygen is consumed as a reactant along with the organic fuel.
amino acid (uh-MEE-noh) An organic molecule possessing both carboxyl and amino groups. Amino acids serve as the monomers of proteins.
fatty acid A long carbon chain carboxylic acid. Fatty acids vary in length and in the number and location of double bonds; three fatty acids linked to a glycerol molecule form fat.
cellular respiration The most prevalent and efficient catabolic pathway for the production of ATP, in which oxygen is consumed as a reactant along with the organic fuel.
ribonucleic acid (ry-boh-noo-KLAY-ik) 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.
ATP (adenosine triphosphate) (uh-DEN-oh-sin try-FOS-fate) An adenine-containing nucleoside triphosphate that releases free energy when its phosphate bonds are hydrolyzed. This energy is used to drive endergonic reactions in cells.
deoxyribonucleic acid (DNA) (DEE-oks-ee-ry-boh-noo-KLAY-ik) A double-stranded, helical nucleic acid molecule capable of replicating and determining the inherited structure of a cell's proteins.
steroids A class of lipids characterized by a carbon skeleton consisting of four rings with various functional groups attached.
cholesterol (kol-ESS-teh-rol) A steroid that forms an essential component of animal cell membranes and acts as a precursor molecule for the synthesis of other biologically important steroids.
lipid (LIH-pid) One of a family of compounds, including fats, phospholipids, and steroids, that are insoluble in water.
phospholipids (FOS-foh-LIP-ids) Molecules that constitute the inner bilayer of biological membranes, having a polar, hydrophilic head and a nonpolar, hydrophobic tail.
potential energy The energy stored by matter as a result of its location or spatial arrangement.
hydrocarbon (HY-droh-kar-bon) An organic molecule consisting only of carbon and hydrogen.
saturated fatty acid A fatty acid in which all carbons in the hydrocarbon tail are connected by single bonds, thus maximizing the number of hydrogen atoms that can attach to the carbon skeleton.
unsaturated fatty acid A fatty acid possessing one or more double bonds between the carbons in the hydrocarbon tail. Such bonding reduces the number of hydrogen atoms attached to the carbon skeleton.
protein (PRO-teen) A three-dimensional biological polymer constructed from a set of 20 different monomers called amino acids.
enzyme A class of proteins serving as catalysts, chemical agents that change the rate of a reaction without being consumed by the reaction.
hemoglobin (HEE-moh-gloh-bin) An iron-containing protein in red blood cells that reversibly binds oxygen.
amino acid (uh-MEE-noh) An organic molecule possessing both carboxyl and amino groups. Amino acids serve as the monomers of proteins.
peptide bond The covalent bond between two amino acid units, formed by condensation synthesis.
polypeptide (POL-ee-PEP-tide) A polymer (chain) of many amino acids linked together by peptide bonds.
primary structure The level of protein structure referring to the specific sequence of amino acids.
secondary structure The localized, repetitive coiling or folding of the polypeptide backbone of a protein due to hydrogen bond formation between peptide linkages.
alpha helix A spiral shape constituting one form of the secondary structure of proteins, arising from a specific hydrogen-bonding structure.
quaternary structure (KWAT-ur-nair-ee) The particular shape of a complex, aggregate protein, defined by the characteristic three-dimensional arrangement of its constituent subunits, each a polypeptide.
tertiary structure (TUR-shee-air-ee) Irregular contortions of a protein molecule due to interactions of side chains involved in hydrophobic interactions, ionic bonds, hydrogen bonds, and disulfide bridges.
nucleic acid (polynucleotide) (PAHL-ee-NOO-klee-o-tide) A polymer consisting of many nucleotide monomers; serves as a blueprint for proteins and, through the actions of proteins, for all cellular activities. The two types are DNA and RNA.
gene A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses).
ribosome A cell organelle constructed in the nucleolus, functioning as the site of protein synthesis in the cytoplasm.
transfer RNA (tRNA) An RNA molecule that functions as an interpreter between nucleic acid and protein language by picking up specific amino acids and recognizing the appropriate codons in the mRNA.
polymer (POL-eh-mur) A large molecule consisting of many identical or similar monomers linked together.
nucleotide (NOO-klee-oh-tide) The building block of a nucleic acid, consisting of a five-carbon sugar covently bonded to a nitrogenous base and a phosphate group.
endergonic reaction (EN-dur-GON-ik) A nonspontaneous chemical reaction in which free energy is absorbed from the surroundings.
exergonic reaction (EKS-ur-GON-ik) A spontaneous chemical reaction in which there is a net release of free energy.
active site The specific portion of an enzyme that attaches to the substrate by means of weak chemical bonds.
prokaryotic cell (pro-KAR-ee-OT-ik) A type of cell lacking a membrane-enclosed nucleus and membrane-enclosed organelles; found only in the domains Bacteria and Archaea.
plasma membrane The membrane at the boundary of every cell that acts as a selective barrier, thereby regulating the cell's chemical composition.
hydrophobic (HY-droh-FOH-bik) Having an aversion to water; tending to coalesce and form droplets in water.
selective permeability A property of biological membranes that allows some substances to cross more easily than others.
facilitated diffusion The spontaneous passage of molecules and ions, bound to specific carrier proteins, across a biological membrane down their concentration gradients.
sodium-potassium pump A special transport protein in the plasma membrane of animal cells that transports sodium out of and potassium into the cell against their concentration gradients.
exocytosis (EKS-oh-sy-TOH-sis) The cellular secretion of macromolecules by the fusion of vesicles with the plasma membrane.
endocytosis (EN-doh-sy-TOH-sis) The cellular uptake of macromolecules and particulate substances by localized regions of the plasma membrane that surround the substance and pinch off to form an intracellular vesicle.
coenzyme (ko-EN-zyme) An organic molecule serving as a cofactor. Most vitamins function as coenzymes in important metabolic reactions.
glycolysis (gly-KOL-eh-sis) The splitting of glucose into pyruvate. Glycolysis is the one metabolic pathway that occurs in all living cells, serving as the starting point for fermentation or aerobic respiration.
Krebs cycle A chemical cycle involving eight steps that completes the metabolic breakdown of glucose molecules to carbon dioxide; occurs within the mitochondrion; the second major stage in cellular respiration.
oxidative phosphorylation (FOS-for-eh-LAY-shun) The production of ATP using energy derived from the redox reactions of an electron transport chain.
substrate-level phosphorylation The formation of ATP by directly transferring a phosphate group to ADP from an intermediate substrate in catabolism.
acetyl CoA The entry compound for the Krebs cycle in cellular respiration; formed from a fragment of pyruvate attached to a coenzyme.
electron transport chain A sequence of electron-carrier molecules (membrane proteins) that shuttle electrons during the redox reactions that release energy used to make ATP.
mitochondrial matrix The compartment of the mitochondrion enclosed by the inner membrane and containing enzymes and substrates for the Krebs cycle.
photosynthesis The conversion of light energy to chemical energy that is stored in glucose or other organic compounds; occurs in plants, algae, and certain prokaryotes.
chloroplast (KLOR-oh-plast) An organelle found only in plants and photosynthetic protists that absorbs sunlight and uses it to drive the synthesis of organic compounds from carbon dioxide and water.
light reactions The steps in photosynthesis that occur on the thylakoid membranes of the chloroplast and convert solar energy to the chemical energy of ATP and NADPH, evolving oxygen in the process.
thylakoid (THY-luh-koid) A flattened membrane sac inside the chloroplast, used to convert light energy to chemical energy.
Calvin cycle The second of two major stages in photosynthesis (following the light reactions), involving atmospheric CO2 fixation and reduction of the fixed carbon into carbohydrate.
stroma The fluid of the chloroplast surrounding the thylakoid membrane; involved in the synthesis of organic molecules from carbon dioxide and water.
photosystem The light-harvesting unit in photosynthesis, located on the thylakoid membrane of the chloroplast and consisting of the antenna complex, the reaction-center chlorophyll a, and the primary electron acceptor. There are two types of photosystems, I and II; they absorb light best at different wavelengths.
ATP synthase A cluster of several membrane proteins found in the mitochondrial cristae (and bacterial plasma membrane) that function in chemiosmosis with adjacent electron transport chains, using the energy of a hydrogen-ion concentration gradient to make ATP. ATP synthases provide a port through which hydrogen ions diffuse into the matrix of a mitrochondrion.
potential energy The energy stored by matter as a result of its location or spatial arrangement.
concentration gradient A regular increase of decrease in the intensity or density of a chemical substance. Cells often maintain concentration gradients of H+ions across their membranes. When a gradient exists, the ions or other chemical substances involved tend to move from where they are more concentrated to where they are less concentrated.
