Unit 3 Activation energy (Ea) Tap / Space to flip
Unit 3 Energy needed to start a reaction. Enzymes lower Ea by stabilizing the transition state.
Unit 3 · 12–16% of exam
Cellular Energetics
Enzymes, photosynthesis, cellular respiration, fermentation, ΔG. One of the highest-weight units. See the cellular respiration deep dive for stage-by-stage detail.
Must-know content
- Enzymes lower activation energy and are specific to substrate. They're unaffected by reaction overall but reusable. Activity depends on temperature, pH, [substrate], [enzyme], and inhibitors. Cofactors (inorganic) and coenzymes (organic) assist.
- Inhibition:
- Competitive — binds active site; outcompeted by more substrate.
- Noncompetitive (allosteric) — binds elsewhere; changes active-site shape; cannot be reversed by substrate.
- Photosynthesis (chloroplast):
- Light reactions (thylakoid membrane): H₂O is split (O₂ released); PSII → ETC → PSI; produces ATP via chemiosmosis and NADPH; H⁺ pumped into thylakoid lumen.
- Calvin cycle (stroma): CO₂ fixed by RuBisCO onto RuBP → 3-PGA → G3P; uses ATP and NADPH. Three turns produce one G3P.
- Net: 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂.
- Cellular respiration (full breakdown on the priority page): glycolysis → pyruvate oxidation → Krebs → ETC + chemiosmosis. ~30–32 ATP/glucose.
- Fermentation regenerates NAD⁺ anaerobically. Lactic acid (animals) or ethanol + CO₂ (yeast). Net 2 ATP from glycolysis only.
- Coupled reactions: exergonic (−ΔG) drives endergonic (+ΔG) via ATP hydrolysis.
- Feedback regulation: end-product inhibition (ATP inhibits phosphofructokinase in glycolysis).
Example questions
MCQ In the light reactions, the proton gradient across the thylakoid membrane is established primarily by: (A) Reduction of NADP⁺ (B) Splitting of water and electron transport (C) The Calvin cycle (D) Diffusion of CO₂
Answer: B. Water-splitting deposits H⁺ in the thylakoid lumen; the ETC pumps additional H⁺ into the lumen as electrons pass between PSII and PSI.
Long FRQ A student measures O₂ produced by spinach chloroplasts under varying light intensities. Predict the shape of the curve and justify your prediction.
Answer: As light intensity increases from zero, O₂ production rises approximately linearly because more photons drive water-splitting at PSII. At higher intensities the curve plateaus, because another factor (RuBisCO availability, CO₂ concentration, NADP⁺ pool, or photosystem saturation) becomes limiting. Adding more light beyond that point can no longer increase the rate.
MCQ Cyanide blocks complex IV. Immediate effect on the cell? (A) Glycolysis stops (B) Krebs cycle accelerates (C) Proton gradient collapses and ATP synthesis halts (D) NADH levels drop
Answer: C. With ETC blocked, no H⁺ pumping → no proton gradient → no oxidative phosphorylation. NADH actually accumulates because it can't deposit electrons; this then halts upstream pathways that need NAD⁺.
Drill flashcards
Unit 3 Competitive inhibitor Tap / Space to flip
Unit 3 Binds the active site and competes with substrate. Effect can be reversed by adding more substrate.
Unit 3 Noncompetitive (allosteric) inhibitor Tap / Space to flip
Unit 3 Binds an allosteric site, changing the active site's shape. Cannot be reversed by adding more substrate.
Unit 3 ATP Tap / Space to flip
Unit 3 Adenosine triphosphate — the cell's energy currency. Hydrolysis of the terminal phosphate releases usable energy.
Unit 3 Chemiosmosis Tap / Space to flip
Unit 3 ATP synthesis powered by H⁺ flowing down its gradient through ATP synthase.
Unit 3 Photosystem II Tap / Space to flip
Unit 3 Splits water (releasing O₂), captures light energy, and feeds excited electrons into the thylakoid ETC.
Unit 3 Calvin cycle Tap / Space to flip
Unit 3 Light-independent reactions in the stroma; uses ATP & NADPH to fix CO₂ onto RuBP, producing G3P.
Unit 3 RuBisCO Tap / Space to flip
Unit 3 Enzyme that fixes CO₂ to RuBP in the Calvin cycle — the most abundant protein on Earth.