Unit 3 Cellular Energetics: How Cells Capture, Store, and Use Energy

First Law of Thermodynamics

Energy is conserved; cells cannot create energy from nothing, only transform it from one form to another.

Second Law of Thermodynamics

Energy transformations increase the entropy (disorder) of the universe overall; cells maintain local order by increasing disorder elsewhere (often as heat).

Gibbs Free Energy (G)

A measure of usable energy to do work; used to predict whether a process is energetically favorable (ΔG = ΔH − TΔS).

Exergonic Reaction

A reaction with ΔG < 0 that releases free energy and is thermodynamically spontaneous (but not necessarily fast).

Endergonic Reaction

A reaction with ΔG > 0 that requires an input of free energy and is not thermodynamically spontaneous.

Activation Energy

The energy barrier reactants must overcome to reach the transition state; a high activation energy can make a spontaneous reaction proceed very slowly.

Enzyme

A catalyst that speeds reactions by lowering activation energy (often by stabilizing the transition state) without changing ΔG or the equilibrium position.

ATP (Adenosine Triphosphate)

A primary cellular energy coupler; its hydrolysis is exergonic and can drive endergonic cellular work.

ATP Hydrolysis

ATP + H2O → ADP + Pi + energy; an exergonic reaction that cells harness to power processes (often via phosphorylation).

Phosphorylation (Energy Coupling)

Transfer of a phosphate group (often from ATP) to a reactant or protein to create a higher-energy intermediate or induce a conformational change that drives an otherwise unfavorable step.

Redox Reaction

A reaction involving electron transfer; central to cellular energetics because electrons carry potential energy.

Oxidation

Loss of electrons from a substance.

Reduction

Gain of electrons by a substance.

NADP+ / NADPH

Electron carrier pair in photosynthesis: NADP+ is the oxidized form; NADPH is the reduced, electron-rich form that provides reducing power (electrons) to other reactions (e.g., the Calvin cycle).

Chemiosmosis

ATP-producing mechanism that uses a proton (H+) gradient across a membrane to drive ATP synthase.

Electron Transport Chain (ETC)

A series of membrane proteins that pass electrons “downhill,” using released energy to pump protons and build an electrochemical gradient.

Proton Motive Force

The electrochemical gradient of protons across a membrane (combining concentration and charge differences) that can power ATP synthesis.

ATP Synthase

A membrane protein complex that makes ATP as protons flow down their gradient through it, causing conformational changes that catalyze ATP formation.

Chloroplast

Organelle in plants/algae where photosynthesis occurs; contains thylakoid membranes (light reactions) and stroma (Calvin cycle).

Thylakoid Membrane

Internal chloroplast membrane where the light reactions occur; houses photosystems, ETC components, and ATP synthase.

Stroma

Fluid region of the chloroplast outside thylakoids where the Calvin cycle takes place.

Photosystem

A pigment-protein complex (antenna + reaction center) that captures light energy and transfers excited electrons to a primary electron acceptor.

Linear Electron Flow

Main light-reaction pathway: electrons move from water → PSII → ETC → PSI → NADP+, producing ATP and NADPH and releasing O2.

Cyclic Electron Flow

Alternative pathway around PSI that produces extra ATP without producing NADPH or O2 (electrons cycle back through an electron transport pathway).

Calvin Cycle

Stroma-based pathway that uses ATP and NADPH to reduce CO2 into carbohydrate; outputs G3P and regenerates RuBP to continue the cycle.