Energy in Living Systems – Biochemistry Module 3

Vocabulary flashcards summarising key terms and concepts from the lecture on thermodynamics, energy coupling, and ATP in biochemistry.

Exergonic reaction

A chemical reaction with a negative ΔG that releases energy and proceeds spontaneously toward products.

Endergonic reaction

A chemical reaction with a positive ΔG that requires an input of energy to proceed.

Gibbs free energy (G)

The energy available to do work in a closed system, defined by J. Willard Gibbs.

Change in free energy (ΔG)

Difference in free energy between products and reactants; determines reaction spontaneity.

Enthalpy (ΔH)

Heat content change of a system, reflecting bond energies released or absorbed during a reaction.

Entropy (ΔS)

Measure of disorder or randomness in a system; increased disorder contributes to negative ΔG.

Equilibrium constant (Keq)

Ratio of product to reactant concentrations at equilibrium; linked to ΔG by ΔG = –RT ln Keq.

Standard free energy change (ΔGo)

ΔG under standard conditions: 25 °C, 1 atm, 1 M reactants and products.

Standard biochemical free energy change (ΔGo’)

Standard ΔG at pH 7, used for biochemical reactions.

Coupled reaction

Process where an exergonic reaction drives an endergonic one via shared intermediates, lowering overall ΔG.

Catabolism

Metabolic pathways that break down complex molecules into simpler ones, releasing energy (exergonic).

Anabolism

Metabolic pathways that build complex molecules from simpler ones, requiring energy (endergonic).

Adenosine triphosphate (ATP)

Nucleotide composed of adenine, ribose, and three phosphates; primary energy carrier in cells.

ATP hydrolysis (ATP → ADP + Pi)

Exergonic reaction releasing ~30.5 kJ mol⁻¹ (–7.3 kcal mol⁻¹) used to drive cellular work.

Universal energy currency

Nickname for ATP because it transfers energy between diverse cellular reactions in all organisms.

Adenosine diphosphate (ADP)

Product of ATP hydrolysis that can be re-phosphorylated to regenerate ATP.

Ideal gas constant (R)

Physical constant 8.314 J mol⁻¹ K⁻¹ used in the equation ΔG = –RT ln Keq.

Metabolism

Sum of all catabolic and anabolic reactions sustaining life, coordinated through energy coupling.

Free energy of formation

Energy required to synthesize a molecule from its constituent atoms; influences molecular free energy content.

J. Willard Gibbs

Scientist who formulated concepts of free energy and thermodynamics of chemical reactions.

Reaction equilibrium

State where forward and reverse reaction rates are equal and ΔG equals zero.

Biological work

Processes such as mechanical movement, active transport, or biosynthesis powered by chemical energy.

Energy coupling via shared intermediates

Mechanism where a high-energy intermediate (e.g., ATP) links exergonic and endergonic reactions.

Phosphodiester bond

Energy-rich linkage between phosphate groups in ATP whose repulsion contributes to ATP’s stored energy.