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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 2 · Topic 2.3

Plasma Membrane

Cell organization and membrane properties control exchange, signaling, and the efficiency of cellular work.

1. Topic Lens

Plasma Membrane is studied through cells. Connect the system boundary, interacting parts, and measurable evidence before applying a formula.

\[SA:V=\frac{6s^2}{s^3}\]

2. Why the Formula Works

The relationship is built from definitions and conservation reasoning:

  1. Model a cell as a cube of side length s.
  2. Add the six faces to obtain surface area 6s^2 and use s^3 for volume.
  3. Divide and simplify; increasing size lowers surface area per unit volume.
\[SA:V=\frac{6}{s}\]

3. Detailed Visual Model

Pixel diagram for Plasma MembraneOriginal schematic connecting Plasma Membrane to Cells.
Plasma Membrane: an original pixel-style model. Use it as a schematic, not a literal scale drawing.

4. Worked Example and Lab Link

Compare cubic cells with side lengths 1 mm and 2 mm. Which has the larger surface-area-to-volume ratio?

Answer: The 1 mm cell has ratio 6:1; the 2 mm cell has 3:1, so the smaller cell exchanges material more efficiently.

Investigation idea: Use agar cubes and a pH indicator to measure diffusion depth while changing cube size.

Common trap: Tonicity describes a solution relative to a cell; it is not the same as solute concentration alone.

Checkpoint · Topic 2.3

Explain how plasma membrane supports or limits this conclusion: The 1 mm cell has ratio 6:1; the 2 mm cell has 3:1, so the smaller cell exchanges material more efficiently.

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