Unit 2 · Cells
Unit 2 · Cells
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.
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.
Load-bearing representation lab
The comparison holds equal external concentration and ranks passive crossing of a bare phospholipid bilayer. O2 is small and nonpolar, the steroid is larger but nonpolar, glucose is large and polar, and Na+ is small but fully charged. The displayed order O2, steroid, glucose, Na+ therefore cannot be reconstructed from size alone: the small ion ranks below the larger nonpolar steroid because charge is strongly unfavorable in the hydrophobic core. That pattern exposes the error of ranking permeability by size alone, not the unrelated placement of protein segments in membrane regions. Classify full charge, polarity, and size before comparing the rank. The protein-assisted routes are shown separately: glucose can use a carrier and Na+ can use a channel, but those pathways are not entries in the bare-bilayer ranking. A common error mixes the two comparisons and claims that a channel makes the ion intrinsically permeable through lipid. The visual supports a relative permeability order under the stated equal-concentration condition; it does not provide absolute fluxes, channel open probabilities, or a claim that any solute has literally zero passive crossing.
Cell mass increases at the low external-solute condition, reaches zero change at the marked 0.3 mol/L crossing, and decreases at the higher condition. The sign of mass change reports net water movement under the stated permeability conditions; it does not show the impermeant solute entering or leaving the cell. At the zero crossing, the experiment has located an isotonic condition for this system, where there is no net water-driven mass change over the measured interval. The canonical misconception assigns the changing mass to movement of the named impermeant solute. Repair it by tracking water and solute separately: ask which species can cross, keep their movements distinct, and connect water movement to the relevant water-potential difference. Do not call the entire curve osmosis without identifying the selectively permeable boundary, and do not assume isotonic means equal concentrations of every solute. A credit-bearing interpretation names external solute concentration as the manipulated quantity, cell-mass change as the response, and the zero crossing as the evidence for the equilibrium point. The graph alone does not reveal the molecular identity of the water channel or the time required to reach that state.
