Unit 3: Cellular Energetics: Quick Review

Enzymes

• Biological catalysts (mostly proteins) that lower activation energy ($E_a$) — do not change $\Delta G$.
• Induced fit: substrate binding reshapes active site.
• Affected by temperature, pH, [substrate], [enzyme], inhibitors.
• Competitive inhibitor: binds active site; overcome by more substrate.
• Noncompetitive/allosteric inhibitor: binds elsewhere; can't be overcome.
• Feedback inhibition: end product shuts down upstream enzyme.

Thermodynamics

• 1st Law: energy conserved. 2nd Law: entropy increases.
• $\Delta G = \Delta H - T\Delta S$
• Exergonic ($-\Delta G$): spontaneous, releases energy.
• Endergonic ($+\Delta G$): requires energy input.
• Cells couple endergonic reactions to ATP hydrolysis (~–7.3 kcal/mol).

Cellular Respiration

$C_6H_{12}O_6 + 6\,O_2 \rightarrow 6\,CO_2 + 6\,H_2O + \sim 30\text{–}32\,\text{ATP}$

• Glycolysis (cytoplasm): glucose → 2 pyruvate. Net 2 ATP, 2 NADH.
• Pyruvate oxidation (matrix): pyruvate → acetyl-CoA + $CO_2$. 2 NADH.
• Krebs cycle (matrix, ×2 turns): 2 ATP, 6 NADH, 2 FADH₂, 4 CO₂.
• Oxidative phosphorylation (inner membrane):
  • NADH/FADH₂ → ETC → pumps $H^+$ into intermembrane space.
  • O₂ is the final electron acceptor → forms H₂O.
  • Proton gradient drives ATP synthase (chemiosmosis).
  • Produces ~26 ATP.

Fermentation (no O₂)

• Regenerates NAD⁺ so glycolysis continues.
• Lactic acid (muscles, bacteria) or ethanol + CO₂ (yeast).
• Only 2 ATP per glucose.

💡 Exam Tip: If O₂ runs out, the ETC backs up → NADH can't unload → glycolysis stalls without fermentation. This cause-and-effect chain is a common FRQ.

Photosynthesis

$6\,CO_2 + 6\,H_2O + \text{light} \rightarrow C_6H_{12}O_6 + 6\,O_2$

Light Reactions (thylakoid membrane)

• PSII absorbs light → excites electrons → splits H₂O (releases O₂).
• Electrons travel ETC → pump H⁺ into thylakoid lumen.
• PSI re-excites electrons → reduce NADP⁺ → NADPH.
• Proton gradient → ATP synthase → ATP (photophosphorylation).
• Products: ATP, NADPH, O₂.

Calvin Cycle (stroma, light-independent)

1. Fixation: RuBisCO attaches CO₂ to RuBP.
2. Reduction: ATP + NADPH → G3P.
3. Regeneration: most G3P recycles to RuBP.

• 3 turns = 1 net G3P; 6 turns = 1 glucose.

C3 vs C4 vs CAM

• C3: direct fixation, suffers photorespiration in heat.
• C4: spatial separation (mesophyll → bundle sheath). Hot, sunny climates. Corn, sugarcane.
• CAM: temporal separation (stomata open at night). Deserts. Cacti, pineapple.

💡 Exam Tip: When a question asks why C4 beats C3 in hot weather, mention photorespiration and RuBisCO's affinity for O₂ at high temperatures.

Key Terms

• Activation energy ($E_a$) — energy barrier to reach transition state.
• Chemiosmosis — ATP synthesis powered by a proton gradient.
• Proton-motive force — potential energy stored in H⁺ gradient.
• Substrate-level phosphorylation — direct ATP formation from phosphorylated intermediate.
• Oxidative phosphorylation — ATP formation via ETC + chemiosmosis.
• RuBisCO — enzyme that fixes CO₂ in the Calvin cycle.
• Photolysis — splitting water in PSII.
• Photophosphorylation — ATP production in light reactions.

Must-Know for the Exam

• Enzymes lower $E_a$, not $\Delta G$.
• Distinguish competitive vs noncompetitive inhibition on a graph.
• Know ATP/NADH/FADH₂ tallies for each respiration stage.
• O₂ is the final electron acceptor, not the fuel.
• Chemiosmosis uses a proton gradient across the inner mitochondrial membrane (respiration) or thylakoid membrane (photosynthesis).
• Light reactions → ATP + NADPH + O₂; Calvin cycle → uses ATP + NADPH to fix CO₂ into G3P.
• RuBisCO's oxygenase activity causes photorespiration; C4 and CAM are adaptations.
• Mitochondria and chloroplasts both use ETCs, proton gradients, and ATP synthase — a major evolutionary parallel.
• Fermentation regenerates NAD⁺ so glycolysis can keep producing 2 ATP without O₂.