Unit 3: Cellular Energetics

AP Biology: 139 practice questions with detailed explanations.

Unit Study Guide

Executive Summary

Unit 3 follows energy through enzymes, ATP, photosynthesis, and respiration. Structure gives enzymes specificity; gradients power ATP synthesis.

Enzymes

Enzymes lower activation energy by stabilizing the transition state. Substrates fit the active site (lock-and-key or induced fit). They are reusable and specific; they never change ΔG. Denaturation (heat, pH, salt) destroys the active-site shape and function. Cofactors and coenzymes assist.

Environmental effects

Each enzyme has temperature and pH optima. Below the optimum, molecules move slowly; above it, the enzyme denatures. Substrate concentration saturates the enzyme — Vmax marks full occupancy; Km measures affinity (lower Km = tighter binding). Inhibitors: competitive (bind the active site, overcome by more substrate) and noncompetitive (bind elsewhere, lower Vmax).

Energy and ATP

Catabolic pathways release energy (exergonic); anabolic pathways consume it (endergonic). ATP couples the two: hydrolysis of its terminal phosphate releases energy that drives unfavorable reactions.

Photosynthesis

Light reactions (thylakoid membranes): photosystem II splits water, releasing O₂; electron flow through the chain pumps H⁺ into the thylakoid lumen; chemiosmosis through ATP synthase makes ATP; NADP⁺ is reduced to NADPH. Calvin cycle (stroma): RuBisCO fixes CO₂ onto RuBP; ATP and NADPH reduce 3-PGA to G3P; three CO₂ yield one net G3P while RuBP regenerates.

Cellular respiration

Glycolysis (cytosol): glucose → 2 pyruvate + 2 ATP (net) + 2 NADH. Pyruvate oxidation and the Krebs cycle (mitochondrial matrix): per glucose, 2 ATP (GTP) + 6 NADH + 2 FADH₂ + CO₂. Oxidative phosphorylation (inner membrane): NADH/FADH₂ feed the electron transport chain, pumping H⁺; ATP synthase harvests the gradient. Oxygen is the final electron acceptor → water. Modern estimates: ~30–32 ATP per glucose. Without O₂, fermentation regenerates NAD⁺: lactate in muscle, ethanol + CO₂ in yeast — only 2 ATP per glucose.

Top 5 Concepts to Master

  1. 1Explain catalysis as lowering activation energy without changing ΔG.
  2. 2Predict enzyme activity from temperature, pH, and inhibitor type.
  3. 3Trace electron flow: water → PSII → ETC → NADPH, with chemiosmotic ATP.
  4. 4Balance the Calvin cycle: 3 CO₂ → 1 net G3P, RuBP regenerated.
  5. 5Compare ATP yield: respiration ~30–32 vs fermentation 2 per glucose.

Key Terms & Definitions

Practice with Flashcards
Activation energy

Energy barrier reactants must cross; enzymes lower it.

Active site

Pocket where substrate binds and reacts.

Denaturation

Loss of enzyme shape and function from heat/pH/salt.

Competitive inhibitor

Binds the active site; outcompeted by excess substrate.

Noncompetitive inhibitor

Binds elsewhere and lowers Vmax regardless of substrate.

ATP

Adenosine triphosphate; the cell's energy currency.

Chemiosmosis

H⁺ gradient drives ATP synthase.

Photolysis

Light-driven splitting of water at photosystem II.

RuBisCO

Enzyme fixing CO₂ onto RuBP in the Calvin cycle.

Calvin cycle

Light-independent CO₂ fixation in the stroma.

Glycolysis

Glucose → 2 pyruvate + 2 ATP + 2 NADH in the cytosol.

Krebs cycle

Acetyl-CoA oxidation yielding NADH, FADH₂, ATP, CO₂.

Oxidative phosphorylation

ETC + chemiosmosis; most ATP of respiration.

Fermentation

NAD⁺ regeneration without O₂; lactate or ethanol produced.

Common Misconceptions: Exam Traps

Enzymes add energy to reactions.

Correct: Enzymes only lower activation energy; ΔG is unchanged.

Enzymes are consumed by reactions.

Correct: Enzymes emerge unchanged and catalyze many cycles.

The Calvin cycle runs only in the dark.

Correct: It runs whenever ATP and NADPH are available; light just refreshes the supply.

Oxygen released in photosynthesis comes from CO₂.

Correct: The O₂ comes from splitting water at photosystem II.

Fermentation makes extra ATP beyond glycolysis.

Correct: Fermentation only regenerates NAD⁺; total yield stays 2 ATP per glucose.

Question Bank Breakdown

By difficulty

easy 48medium 63hard 28

By topic

Oxidative Phosphorylation 26The Light-Dependent Reactions 25Cellular Energy 24Cellular Respiration Overview 21Glycolysis 18Photosynthesis Overview 17Enzyme Catalysis 16The Krebs Cycle 16Photosynthesis: The Calvin Cycle 16Fermentation 12Enzyme Structure 7Environmental Impacts on Enzyme Function 7

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