Minimum amount of energy required to start a reaction; enzyme action lowers this energy barrier. Reactions differ in the amount required.
Chemically stable crevice in an enzyme where substrates bind and a reaction can be catalyzed repeatedly.
Adenosine diphosphate. A nucleotide with an adenine base and two phosphate groups.
Any enzyme or cofactor that helps neutralize free radicals before they damage tissues.
Adenosine triphosphate. A type of nucleotide that functions as the main energy carrier between reaction sites in cells. Consists of the base adenine, the five-carbon sugar ribose, and three phosphate groups.
How a cell regenerates its ATP supply. ADP forms when ATP gives up a phosphate group, then ATP forms as ADP binds to inorganic phosphate or a phosphate group split from a molecule.
Energy released as weak bonds form between a substrate, enzyme, and any cofactor.
Fluorescent light formed when certain organisms convert chemical bond energy to photon energy.
biosynthetic pathway (anabolic)
Any metabolic pathway by which one or more organic compounds are synthesized.
Potential energy in the bonds between atoms in molecules.
No net change in concentrations of reactants and products in a reversible chemical reaction.
An organic molecule that is a necessary participant in some enzymatic reactions; helps catalysis by donating or accepting electrons or functional groups; e.g., a vitamin, ATP, NAD+.
A metal ion or a coenzyme that assists an enzyme in catalysis by accepting or donating electrons or functional groups.
degradative pathway (catabolic)
Any of the stepwise series of metabolic reactions that break down organic compounds.
electron transfer chains
Array of enzymes and other molecules in a cell membrane that accept and give up electrons in sequence; operation of chain releases the energy of the electrons in small, usable increments.
A chemical reaction that requires a net energy input and converts more stable reactants into less stable products; not spontaneous.
A capacity to do work.
Measure of how much and how far a concentrated form of energy has been dispersed after an energy change.
A type of protein that catalyzes (speeds) a chemical reaction. Some RNAs also show catalytic activity.
Any chemical reaction with a net energy loss.
Flavin adenine dinucleotide. A type of nucleotide coenzyme; transfers electrons and H+ from one reaction site to another.
Mechanism by which a change that results from some cellular activity triggers responses that decrease or shut down the activity.
first law of thermodynamics
Energy cannot be created or destroyed.
Any unbound molecular fragment with an unpaired electron.
Explanation of how some enzymes work; their shape changes and fits a bound substrate more closely, and the tension destabilizes substrate bonds so that they can break.
A substance formed between the start and end of a metabolic pathway.
1,000 calories of heat energy; amount needed to raise the temperature of 1 kilogram of water by 1�C. Standard unit of measure for food�s caloric content.
Energy of motion.
A stepwise sequence of enzyme-mediated reactions.
All the controlled, enzyme-mediated chemical reactions by which cells acquire and use energy as they synthesize, store, degrade, and eliminate substances.
Nicotinamide adenine dinucleotide. A nucleotide coenzyme; after it accepts electrons and H+, abbreviated as NADH.
Nicotinamide adenine dinucleotide phosphate. A phosphorylated nucleotide coenzyme; after it accepts electrons and H+, abbreviated NADPH2.
oxidation-reduction reactions (redox)
Transfer of electrons between reactant molecules.
Enzyme-mediated transfer of a phosphate group to an organic compound.
A object�s capacity to do work owing to its position in space or the arrangement of its parts.
A substance remaining at the end of a reaction.
Substance that enters a reaction.
second law of thermodynamics
Energy tends to flow from concentrated to less concentrated forms.
A reactant molecule that is specifically acted upon by an enzyme.
Membrane protein that passively or actively assists specific ions or molecules into or out of a cell. The solutes move through the protein�s interior