TLDR

• Enzymes lower activation energy but don’t change ΔG.

• ATP powers cellular work by coupling exergonic and endergonic reactions.

• Cellular respiration: glycolysis → Krebs cycle → ETC → ATP. Oxygen is the final electron acceptor.

• Photosynthesis: light reactions make ATP/NADPH, Calvin cycle builds sugar.

• ✅ Download your free Unit 3 Cheat Sheet PDF above

Why Unit 3 Matters

Unit 3 is where AP Biology moves from cell structure to cell processes. This unit explains how cells capture and use energy, which connects directly to every other system you’ll study. The exam often tests whether you can match location + input + output for respiration and photosynthesis — so keeping those straight is essential.

Enzymes: Catalysts of Life

• Speed up reactions by lowering activation energy.

• Active site binds substrate; induced fit = flexible model.

• Affected by temperature, pH, substrate concentration.

• Inhibition:

  • Competitive → blocks active site.

  • Noncompetitive → changes enzyme shape.

• Cofactors and coenzymes may be required.

👉 Tutor Tip: On the exam, when you see a graph of enzyme rate vs temperature or pH, always look for the “optimum.” Past that point, activity drops because of denaturation.

ATP and Energy Coupling

• ATP = adenosine triphosphate, the energy “currency” of the cell.

• Hydrolysis (ATP → ADP + Pi) releases energy.

• Coupling: energy from exergonic reactions drives endergonic reactions.

• Think of ATP as a charged battery, and ADP as the used battery.

Cellular Respiration

Equation:
C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ~36 ATP

Steps

1. Glycolysis (cytoplasm): glucose → 2 pyruvate, 2 ATP net, 2 NADH.

2. Link Reaction (matrix): pyruvate → Acetyl-CoA, NADH, CO₂.

3. Krebs Cycle (matrix): Acetyl-CoA → NADH, FADH₂, 2 ATP, CO₂.

4. ETC + Chemiosmosis (inner mitochondrial membrane):

   • Electrons passed along chain, O₂ is final acceptor → H₂O.

   • Proton gradient drives ATP synthase → ~34 ATP.

Total yield: ~36–38 ATP per glucose.

Fermentation (Anaerobic)

• Occurs when O₂ is absent.

• Lactic acid fermentation: pyruvate → lactate (animals).

• Alcohol fermentation: pyruvate → ethanol + CO₂ (yeast).

• Only 2 ATP from glycolysis — much less efficient.

Photosynthesis

Equation:
6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂

Light Reactions (thylakoid membranes)

• Photosystems II & I capture light.

• Water split → O₂ released.

• Produces ATP & NADPH.

Calvin Cycle (stroma)

• Uses ATP & NADPH to fix CO₂.

• Produces G3P → glucose.

👉 Mnemonic: “Light builds ATP/NADPH, Calvin builds sugar.”

Mini Formula Box

• ATP Hydrolysis: ATP → ADP + Pi + energy

• Respiration: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP

• Photosynthesis: 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂

Visual Mnemonics

• ATP: charged battery = ATP, used battery = ADP.

• Mitochondria: more folds = more ATP.

• Photosynthesis: sun builds ATP/NADPH, Calvin builds sugar.

Common Exam Pitfalls

• Thinking enzymes change ΔG (they don’t — they only lower activation energy).

• Confusing competitive vs noncompetitive inhibition.

• Assuming fermentation makes ATP (it only regenerates NAD⁺ for glycolysis).

• Forgetting oxygen is the final electron acceptor in the ETC.

• Mixing up locations: glycolysis in cytoplasm, Krebs & ETC in mitochondria, light reactions in thylakoids, Calvin cycle in stroma.

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