Unit 3 · Cellular Energetics

Enzyme ● (core concept) — A protein that acts as a biological catalyst, facilitating chemical reactions in cells by lowering the activation energy. Enzyme structure and function contribute to the regulation of biological processes.

Activation energy ● (core concept) — The minimum energy needed to start a chemical reaction; enzymes speed reactions by lowering it.

Substrate ● (core concept) — The molecule that an enzyme acts on; its shape and charge must be compatible with the enzyme's active site.

Active site ● (core concept) — The region of an enzyme where the substrate binds. The enzyme-substrate complex model illustrates that the substrate's shape and charge must be compatible with the active site for the reaction to occur.

Enzyme-substrate complex ● (core concept) — The bound state of enzyme and substrate that illustrates how substrate shape and charge must fit the active site for an enzyme-mediated reaction to occur.

Denaturation ● (core concept) — The disruption of a protein's structure (such as an enzyme's) by a change in temperature, pH, or chemical environment, eliminating its ability to catalyze reactions. In some cases denaturation is reversible, allowing the enzyme to regain activity.

Optimal enzyme conditions ● (core concept) — The temperature and pH at which an enzyme works most efficiently. Conditions outside the optimal range disrupt hydrogen bonds, changing enzyme structure and reducing catalytic efficiency. Higher temperatures increase molecular movement and enzyme-substrate collisions, raising the reaction rate until the optimal temperature is reached.

Competitive inhibitor ● (core concept) — An inhibitor molecule that binds reversibly to the active site of an enzyme, blocking the substrate.

Noncompetitive inhibitor ● (core concept) — An inhibitor molecule that binds to an allosteric site (not the active site), changing the activity of the enzyme.

Allosteric site ● (core concept) — A site on an enzyme, separate from the active site, where a noncompetitive inhibitor can bind and change enzyme activity.

Relative concentrations of substrates and products ● (core concept) — The relative concentrations of substrates and products determine how efficiently an enzymatic reaction proceeds.

First law of thermodynamics ● (core concept) — Energy cannot be created or destroyed, only transformed. Living systems maintain order because energy input exceeds energy loss.

Second law of thermodynamics ● (core concept) — Energy transformations increase disorder (entropy). Life maintains a highly ordered system despite this law; significant loss of order or energy flow results in death.

Energy coupling ● (core concept) — Cellular processes that release energy may be coupled with cellular processes that require energy.

Metabolic pathway ● (core concept) — A sequential series of reactions in which a product of one reaction is typically the reactant for the subsequent step, allowing a more controlled transfer of energy.

Conserved metabolic pathways ● (core concept) — Core metabolic pathways (e.g., glycolysis, oxidative phosphorylation) conserved across all currently recognized domains (Archaea, Bacteria, and Eukarya), supporting the concept of common ancestry for all organisms.

Photosynthesis ● (core concept) — The series of reactions that use carbon dioxide, water, and light energy to make carbohydrates and oxygen. Photosynthesis first evolved in prokaryotic organisms.

Stroma ● (core concept) — The fluid within the inner chloroplast membrane and outside the thylakoid; the site of the carbon fixation (Calvin cycle) reactions of photosynthesis.

Thylakoid ● (core concept) — The internal membrane system of the chloroplast containing chlorophyll pigments organized into two photosystems as well as electron transport proteins.

Granum ● (core concept) — Stacks of thylakoids; the light reactions of photosynthesis occur in the grana.

Chlorophyll ● (core concept) — Pigments in the thylakoid membranes that absorb energy from light, boosting electrons to a higher energy level in photosystems I and II.

Photosystems I and II ● (core concept) — Pigment-protein complexes embedded in the thylakoid membranes, connected by electron transfer through an electron transport chain. In photosystem I, electrons are ultimately transferred to NADP+, reducing it to NADPH; water splits to replace electrons lost from photosystem II.

Light reactions ● (core concept) — The light-powered reactions of photosynthesis in the thylakoid/grana that capture light energy to yield ATP and NADPH, which power the production of organic molecules in the Calvin cycle.

Electron transport chain ● (core concept) — A series of oxidation/reduction reactions that transfers electrons and establishes an electrochemical gradient of protons (hydrogen ions) across a membrane — across the thylakoid membrane in photosynthesis, the inner mitochondrial membrane in respiration, and prokaryotic plasma membranes. In photosynthesis protons are concentrated inside the thylakoid and low outside it; in respiration they are concentrated outside the inner mitochondrial membrane (intermembrane space) and low inside it (the matrix).

ATP synthase ● (core concept) — The membrane-bound enzyme through which protons flow back by chemiosmosis, driving the formation of ATP from ADP and inorganic phosphate.

Chemiosmosis ● (core concept) — The flow of protons back through membrane-bound ATP synthase down their gradient, driving ATP synthesis.

Photophosphorylation ● (core concept) — The formation of ATP from ADP and inorganic phosphate, driven by light-powered proton flow through ATP synthase.

NADPH ● (core concept) — An electron carrier produced in the light reactions when NADP+ is reduced in photosystem I; its energy powers carbohydrate production in the Calvin cycle.

Calvin cycle ● (core concept) — The carbon fixation reactions of photosynthesis that use the ATP and NADPH from the light reactions to produce carbohydrates from carbon dioxide in the stroma. (Memorizing the steps, structures, and enzyme names beyond ATP synthase is beyond AP scope.)

Cellular respiration ● (core concept) — The process that uses energy from biological macromolecules to synthesize ATP. Respiration and fermentation are characteristic of all forms of life.

Aerobic cellular respiration ● (core concept) — In eukaryotes, a series of coordinated enzyme-catalyzed reactions that capture energy from biological macromolecules, using oxygen as the terminal electron acceptor.

Glycolysis ● (core concept) — A biochemical pathway that releases the energy in glucose to form ATP (from ADP and inorganic phosphate), NADH, and pyruvate.

Pyruvate ● (core concept) — The product of glycolysis, transported from the cytosol to the mitochondrion where oxidation occurs.

Krebs cycle ● (core concept) — A cycle of reactions in the mitochondrial matrix that releases carbon dioxide from organic intermediates, synthesizes ATP, and transfers electrons via the coenzymes NAD+ and FAD.

Mitochondrial matrix ● (core concept) — The interior compartment of the mitochondrion where the Krebs (citric acid) cycle takes place.

NADH and FADH2 ● (core concept) — Electron-carrying coenzymes that deliver electrons extracted in glycolysis and the Krebs cycle to the electron transport chain in the inner mitochondrial membrane.

Terminal electron acceptor ● (core concept) — The final electron acceptor at the end of the electron transport chain — oxygen in aerobic respiration; anaerobic prokaryotes use other molecules.

Oxidative phosphorylation ● (core concept) — The formation of ATP from ADP and inorganic phosphate as protons flow back through ATP synthase by chemiosmosis during aerobic cellular respiration.

Fermentation ● (core concept) — A pathway that allows glycolysis to proceed in the absence of oxygen, producing organic molecules such as alcohol and lactic acid.

Decoupling oxidative phosphorylation ● (core concept) — In aerobic cellular respiration, separating oxidative phosphorylation from electron transport generates heat, which endothermic organisms can use to regulate body temperature.

Folding of the inner mitochondrial membrane ● (core concept) — The folding of the inner mitochondrial membrane increases its surface area, which allows for more ATP to be synthesized.