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activation energy
Minimum amount of energy required to start a reaction; enzyme action lowers this energy barrier. Reactions differ in the amount required.
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active sites
Chemically stable crevice in an enzyme where substrates bind and a reaction can be catalyzed repeatedly.
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ADP
Adenosine diphosphate. A nucleotide with an adenine base and two phosphate groups.
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antioxidants
Any enzyme or cofactor that helps neutralize free radicals before they damage tissues.
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ATP
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.
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ATP/ADP cycle
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.
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binding energy
Energy released as weak bonds form between a substrate, enzyme, and any cofactor.
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bioluminescence
Fluorescent light formed when certain organisms convert chemical bond energy to photon energy.
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biosynthetic pathway (anabolic)
Any metabolic pathway by which one or more organic compounds are synthesized.
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chemical energy
Potential energy in the bonds between atoms in molecules.
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chemical equilibrium
No net change in concentrations of reactants and products in a reversible chemical reaction.
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coenzymes
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+.
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cofactors
A metal ion or a coenzyme that assists an enzyme in catalysis by accepting or donating electrons or functional groups.
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degradative pathway (catabolic)
Any of the stepwise series of metabolic reactions that break down organic compounds.
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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.
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endergonic reaction
A chemical reaction that requires a net energy input and converts more stable reactants into less stable products; not spontaneous.
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energy
A capacity to do work.
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entropy
Measure of how much and how far a concentrated form of energy has been dispersed after an energy change.
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enzymes
A type of protein that catalyzes (speeds) a chemical reaction. Some RNAs also show catalytic activity.
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exergonic reaction
Any chemical reaction with a net energy loss.
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FAD
Flavin adenine dinucleotide. A type of nucleotide coenzyme; transfers electrons and H+ from one reaction site to another.
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feedback inhibition
Mechanism by which a change that results from some cellular activity triggers responses that decrease or shut down the activity.
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first law of thermodynamics
Energy cannot be created or destroyed.
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free radicals
Any unbound molecular fragment with an unpaired electron.
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induced-fit model
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.
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intermediates
A substance formed between the start and end of a metabolic pathway.
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kilocalorie
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.
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kinetic energy
Energy of motion.
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metabolic pathways
A stepwise sequence of enzyme-mediated reactions.
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metabolism
All the controlled, enzyme-mediated chemical reactions by which cells acquire and use energy as they synthesize, store, degrade, and eliminate substances.
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NAD+
Nicotinamide adenine dinucleotide. A nucleotide coenzyme; after it accepts electrons and H+, abbreviated as NADH.
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NADP+
Nicotinamide adenine dinucleotide phosphate. A phosphorylated nucleotide coenzyme; after it accepts electrons and H+, abbreviated NADPH2.
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oxidation-reduction reactions (redox)
Transfer of electrons between reactant molecules.
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phosphorylation
Enzyme-mediated transfer of a phosphate group to an organic compound.
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potential energy
A object�s capacity to do work owing to its position in space or the arrangement of its parts.
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products
A substance remaining at the end of a reaction.
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reactants
Substance that enters a reaction.
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second law of thermodynamics
Energy tends to flow from concentrated to less concentrated forms.
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substrates
A reactant molecule that is specifically acted upon by an enzyme.
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transport proteins
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
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