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AP Course

AP Biology

Updated for the AP Biology framework effective fall 2025, with concept lessons, data reasoning, quizzes, and practice problems.

Explore life from molecules and cells to evolution and ecosystems through the official AP Biology unit sequence.

Lessons
Structure of Water and Hydrogen BondingElements of LifeIntroduction to MacromoleculesCarbohydratesLipidsNucleic AcidsProteinsCell Structure and FunctionCell SizePlasma MembraneMembrane PermeabilityMembrane TransportFacilitated DiffusionTonicity and OsmoregulationMechanisms of TransportCell CompartmentalizationOrigins of Cell CompartmentalizationEnzymesEnvironmental Impacts on Enzyme FunctionCellular EnergyPhotosynthesisCellular RespirationCell CommunicationIntroduction to Signal TransductionSignal Transduction PathwaysFeedbackCell CycleRegulation of Cell CycleMeiosisMeiosis and Genetic DiversityMendelian GeneticsNon-Mendelian GeneticsEnvironmental Effects on PhenotypeDNA and RNA StructureDNA ReplicationTranscription and RNA ProcessingTranslationRegulation of Gene ExpressionGene Expression and Cell SpecializationMutationsBiotechnologyIntroduction to Natural SelectionNatural SelectionArtificial SelectionPopulation GeneticsHardy–Weinberg EquilibriumEvidence of EvolutionCommon AncestryContinuing EvolutionPhylogenySpeciationVariations in PopulationsOrigins of Life on EarthResponses to the EnvironmentEnergy Flow Through EcosystemsPopulation EcologyEffect of Density on PopulationsCommunity EcologyBiodiversityDisruptions in Ecosystems
Quizzes
Practice Problems Lab design, data analysis, and FRQ practice resources can be added here.

AP Biology · Unit 3 · Topic 3.5

Cellular Respiration

Energy transformations depend on enzymes, redox carriers, membranes, and the coupling of favorable and unfavorable processes.

1. Topic Lens

Cellular Respiration is studied through cellular energetics. Connect the system boundary, interacting parts, and measurable evidence before applying a formula.

\[\Delta G_{\text{net}}=\sum_i \Delta G_i\]

2. Why the Formula Works

The relationship is built from definitions and conservation reasoning:

  1. Define the coupled reactions as one thermodynamic system.
  2. Gibbs free-energy changes are state functions, so pathway steps add.
  3. A negative total can drive a positive step when energy transfer is physically coupled.
\[\Delta G_{\text{coupled}}=\Delta G_{\text{endergonic}}+\Delta G_{\text{exergonic}}\]

3. Detailed Visual Model

Pixel diagram for Cellular RespirationOriginal schematic connecting Cellular Respiration to Cellular Energetics.
Cellular Respiration: an original pixel-style model. Use it as a schematic, not a literal scale drawing.

4. Worked Example and Lab Link

A synthesis step has ΔG=+12 kJ/mol and is coupled to a reaction with ΔG=-30 kJ/mol. Is the pair favorable?

Answer: The net change is -18 kJ/mol, so the coupled process is thermodynamically favorable.

Investigation idea: Measure catalase reaction rate across pH or temperature while holding enzyme and substrate amounts constant.

Common trap: A negative ΔG predicts favorability, not reaction speed; enzymes change activation energy, not ΔG.

Checkpoint · Topic 3.5

Explain how cellular respiration supports or limits this conclusion: The net change is -18 kJ/mol, so the coupled process is thermodynamically favorable.

Official curriculum reference: College Board AP Biology course page. The explanation and worked example are independently written for this study site.