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Change in the free energy during a reaction: the free energy of the product molecules minus the free energy of the starting molecules. A large negative value of ΔGindicates that the reaction has a strong tendency to occur. (Panel 2–7 - pp. 102–103)



Small water-soluble activated carrier molecule. Consists of an acetyl group linked to coenzyme A (CoA) by an easily hydrolyzable thioester bond. (Figure 2–38)

acetyl CoA


A proton donor. Substance that releases protons (H+) when dissolved in water - forming hydronium ions (H3O+) and lowering the pH. (Panel 2–2 - pp. 92–93)



Small diffusible molecule that stores easily exchangeable energy in the form of one or more energy-rich covalent bonds. Examples are ATP - acetyl CoA - FADH2 - NADH - and NADPH. (Figure 2–31)

activated carrier


The extra energy that must be acquired by atoms or molecules in addition to their ground-state energy in order to reach the transition state required for them to undergo a particular chemical reaction. (Figure 2–21)

activation energy


Nucleotide produced by hydrolysis of the terminal phosphate of ATP. Regenerates ATP when phosphorylated by an energy-generating process such as oxidative phosphorylation. (Figure 2–33)

ADP (adenosine 5′-diphosphate)


Process by which a cell obtains energy from sugars or other organic molecules by allowing their carbon and hydrogen atoms to combine with the oxygen in air to produce CO2 and H2O - respectively.

aerobic respiration


Nucleoside triphosphate composed of adenine - ribose - and three phosphate groups. The principal carrier of chemical energy in cells. The terminal phosphate groups are highly reactive in the sense that their hydrolysis - or transfer to another molecule - takes place with the release of a large amount of free energy. (Figure 2–33)

ATP (adenosine 5′-triphosphate)


(1) A substance that can reduce the number of protons in solution - either by accepting H+ ions directly - or by releasing OH– ions - which then combine with H+ to form H2O. (2) The purines and pyrimidines in DNA and RNA are organic nitrogenous bases and are often referred to simply as bases. (Panel 2–2 - pp. 92–93)



Solution of weak acid or weak base that resists the pH change that would otherwise occur when small quantities of acid or base are added.



Substance that can lower the activation energy of a reaction (thus increasing its rate) - without itself being consumed by the reaction.



Certain combinations of atoms—such as methyl (–CH3) - hydroxyl (–OH) - carboxyl (–COOH) - carbonyl (–C=O) - phosphate (–PO32–) - sulfhydryl (–SH) - and amino (–NH2) groups—that have distinct chemical and physical properties and influence the behavior of the molecule in which the group occurs.

chemical group


Central metabolic pathway found in aerobic organisms. Oxidizes acetyl groups derived from food molecules - generating the activated carriers NADH and FADH2 - some GTP - and waste CO2. In eukaryotic cells - it occurs in the mitochondria. (Panel 2–9 - pp. 106–107)

citric acid cycle [tricarboxylic acid (TCA) cycle - Krebs cycle]


Linked pair of chemical reactions in which the free energy released by one serves to drive the other. (Figure 2–29)

coupled reaction


Stable chemical link between two atoms produced by sharing one or more pairs of electrons. (Panel 2–1 - pp. 90–91)

covalent bond


The net drift of molecules through space due to random thermal movements.



Series of reactions in which electron carrier molecules pass electrons “down the chain” from higher to successively lower energy levels. The energy released during such electron movement can be used to power various processes. Electron-transport chains present in the inner mitochondrial membrane (called the respiratory chain) and in the thylakoid membrane of chloroplasts generate a proton gradient across the membrane that is used to drive ATP synthesis. See especially Figures 14–18 and 14–52.

electron-transport chain


A noncovalent - ionic bond between two molecules carrying groups of opposite charge. (Panel 2–3 - pp. 94–95)

electrostatic attraction


Thermodynamic quantity that measures the degree of disorder or randomness in a system; the higher the entropy - the greater the disorder. (Panel 2–7 - pp. 102–103)

entropy (S)


State in a chemical reaction where there is no net change in free energy to drive the reaction in either direction. The ratio of product to substrate reaches a constant value at chemical equilibrium. (Figure 2–30)



Electron carrier system that functions in the citric acid cycle and fatty acid oxidation. One molecule of FAD gains two electrons plus two protons in becoming the activated carrier FADH2. (Figure 2–39)

FAD/FADH2 (flavin adenine dinucleotide/reduced flavin adenine dinucleotide)


Energy-storage lipid in cells. Composed of triglycerides—fatty acids esterified with glycerol.



Anaerobic energy-yielding metabolic pathway involving the oxidation of organic molecules. Anaerobic glycolysis refers to the process whereby pyruvate is converted into lactate or ethanol - with the conversion of NADH to NAD+. (Figure 2–47)



The energy that can be extracted from a system to drive reactions. Takes into account changes in both energy and entropy. (Panel 2–7 - pp. 102–103)

free energy (G) (Gibbs free energy)


See ΔG.

free-energy change (Δ ΔG.)


Polysaccharide composed exclusively of glucose units. Used to store energy in animal cells. Large granules of glycogen are especially abundant in liver and muscle cells. (Figure 2–51 and Panel 2–4 - pp. 96–97)



Ubiquitous metabolic pathway in the cytosol in which sugars are incompletely degraded with production of ATP. Literally - “sugar splitting.” (Figure 2–46 and Panel 2–8 - pp. 104–105)



Nucleoside triphosphate produced by the phosphorylation of GDP (guanosine diphosphate). Like ATP - it releases a large amount of free energy on hydrolysis of its terminal phosphate group. Has a special role in microtubule assembly - protein synthesis - and cell signaling. (Figure 2–58)

GTP (guanosine 5′-triphosphate)


Noncovalent bond in which an electropositive hydrogen atom is partially shared by two electronegative atoms. (Panel 2–3 - pp. 94–95)

hydrogen bond


Water molecule associated with an additional proton. The form generally taken by protons in aqueous solution.

hydronium ion (H3O+)