Unit 3 · Cellular Energetics
Unit 3 · Cellular Energetics
Unit 3: Cellular Energetics accounts for 12% to 16% 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.
Energy diagrams and organelle pathways become much easier when matter, electrons, protons, and ATP are tracked in separate columns. Calling every arrow energy hides the mechanism. Reactions transfer matter and electrons, membranes maintain gradients, and coupled processes use those gradients to change the probability or rate of another reaction.
You will learn to read enzyme curves and photosynthesis or respiration pathways without confusing reaction rate, stored gradient, ATP production, final equilibrium, and total product yield.
The decision that organizes this unit
What is being transferred at each step, which membrane or enzyme controls the transfer, and which measured output would change first after a perturbation?
Mechanism route and repair branches
- Main route: Name the substrate, product, electron carrier, proton compartment, or ATP-linked reaction represented by each arrow or axis.
- Main route: Locate the enzyme, membrane complex, or coupling step altered by the condition and distinguish direct targets from downstream consequences.
- Diagnostic cue: A lower product value is explained by saying the enzyme died. Wrong branch: Any reduction is treated as complete irreversible denaturation. Repair: Use the curve shape and condition range to distinguish competitive effects, saturation, reversible activity change, and loss of functional structure.
- Main route: Trace electrons and protons separately, preserving membrane sides and the direction of the electrochemical gradient.
- Diagnostic cue: Protons, electrons, ATP, and glucose are described as one energy stream. Wrong branch: Distinct carriers and forms are collapsed. Repair: Write a separate directional statement for electron transfer, proton pumping, gradient-driven flux, and ATP formation.
- Main route: Connect gradient formation or dissipation to ATP synthase operation, then connect ATP availability to the measured cellular process.
- Main route: Interpret a curve as rate, concentration, gas exchange, or yield and stop the prediction where the evidence no longer specifies a mechanism.
- Diagnostic cue: A faster initial rate is claimed to create more final product in every experiment. Wrong branch: Rate and endpoint are treated as identical. Repair: Check whether substrate, time, reversibility, inhibition, or equilibrium constrains the final amount independently of initial rate.
Load-bearing representation lab
Compare the curves in two ways: their approach to a maximum and the substrate concentration needed to reach a given rate. The competitive series is shifted to the right of control but approaches the same modeled maximum, which is the signature of a larger apparent Km with preserved Vmax. The noncompetitive series instead levels off below control while retaining the control Km in the displayed model, indicating reduced functional catalytic capacity. Calling every lower point denaturation loses those parameter distinctions and ignores the way extra substrate partly overcomes competitive inhibition. To audit the visual, select a rate below the control maximum and compare horizontal positions for control and competitive curves; then compare the high-substrate plateaus for control and noncompetitive curves. The graph supports those kinetic signatures, but it does not identify an inhibitor's molecular binding site or prove a perfectly pure inhibition mechanism in an unknown experiment. A response should name which parameter changes and cite the corresponding curve feature. Avoid comparing only one low-substrate point, where several mechanisms can produce a lower rate and the maximum-rate evidence has not yet become visible.
Two directional stories share the thylakoid membrane but must not be merged. The electron route runs from PSII through PQ, cytochrome b6f, PC, PSI, and Fd to the NADPH endpoint. Separately, the lumen is labeled as the high-proton side, and protons return to the stroma through ATP synthase. Electron transfer and proton translocation help establish the proton-motive force; ATP synthase uses the downhill proton flux to support ATP formation. The drawing also preserves sidedness: PC operates on the lumenal side of the chain, whereas Fd and NADPH formation are on the stromal side. A common response error calls electrons, H+, ATP, and NADPH one stream of energy and then reverses one of their arrows. Instead, trace the named carriers in order, pause at the membrane boundary, and then trace the proton arrow as a second path. The model does not quantify stoichiometry or photon number, so no ATP-to-NADPH ratio can be derived from it. Its supported claim is the coupled direction and compartment logic of the light reactions.
