Unit 2 · Cells
The Complete AP Biology Guide · AP BiologyUnit 2 · Cells
- The Complete AP Biology Guide
- AP Biology
- 8 sections
Unit 2: Cells accounts for 10% to 13% of the AP Biology exam score in Section I (multiple choice), and its mechanisms also appear in Section II free-response questions. Cell diagrams become useful only after compartments, boundaries, and movement are made explicit.
- How AP Biology assesses this 10–13% of the Section I score · mechanisms recur in Section II FRQs · 60 MCQs in 90 min
- Key skills cell structure and compartmentalization, cell size and surface area, membrane structure and transport, tonicity osmoregulation water potential
- How to study for Unit 2 This page turns cell images, transport graphs, water-potential data, and surface-area calculations into one route: identify the compartment, identify the species, determine the driving force, and predict the measured response.
- The organizing decision Across which boundary is the named substance moving, what determines its permeability, and what supplies the direction or energy for net movement?
Unit 2: Cells accounts for 10% to 13% of the AP Biology exam score in Section I (multiple choice), and its mechanisms also appear in Section II free-response questions.
Weighting is the College Board CED range for the current exam. Higher-weight units repay proportionally more review time; use this share to size your effort before drilling the mechanisms below. See the AP Biology exam guide & review for all eight units.
Cell diagrams become useful only after compartments, boundaries, and movement are made explicit. A membrane is not simply a wall, and an organelle is not merely a labeled shape. Each boundary creates a chemical environment, restricts some species, permits others, and makes a directional transport or processing step possible.
This page turns cell images, transport graphs, water-potential data, and surface-area calculations into one route: identify the compartment, identify the species, determine the driving force, and predict the measured response.
The decision that organizes this unit
Across which boundary is the named substance moving, what determines its permeability, and what supplies the direction or energy for net movement?
Mechanism route and repair branches
- Main route: Draw or name the two compartments and mark which side of the membrane contains each relevant concentration, charge, or pressure condition.
- Main route: Classify the transported species by size, polarity, and charge, then identify whether the bilayer, a protein, or a vesicle supplies the route.
- Diagnostic cue: The response says small molecules always cross or charged molecules never cross. Wrong branch: Size alone is treated as the permeability rule. Repair: Combine size with polarity, charge, membrane composition, and the presence of a channel, carrier, or vesicular route.
- Main route: Separate movement down an electrochemical gradient from transport coupled to ATP hydrolysis or another gradient.
- Diagnostic cue: A membrane protein is automatically called active transport. Wrong branch: Protein use is confused with energy use. Repair: Ask whether movement is down the electrochemical gradient and whether an energy source or coupled gradient is identified.
- Main route: For water, compare water potentials rather than solute concentration alone and distinguish initial net movement from the equilibrium state.
- Diagnostic cue: A dialysis-system mass change is explained as the impermeant solute moving through the membrane. Wrong branch: The observed mass change is assigned to solute motion without first deciding whether water and the named solute can cross. Repair: Judge water and solute permeability separately, then draw separate arrows for water and each solute and cross out any arrow blocked by the membrane.
- Main route: Connect flux, cell volume, compartment function, or surface-area-to-volume ratio to the measured cellular response without changing system boundaries.
