The basics
What it covers: Enzymes and how they work, the role of ATP in cells, photosynthesis (light reactions + Calvin cycle), cellular respiration (glycolysis + Krebs + ETC), and fermentation.
Exam weight: 12–16% of the AP Biology exam — one of the heaviest units.
The big question: How do cells transform energy from one form to another to do the work of being alive?
Big Ideas covered: Evolution (BI 1), Energetics (BI 2), Systems Interactions (BI 4).
Key topics at a glance
Enzymes & the Active Site
Enzymes (proteins) speed up reactions by lowering activation energy. Each enzyme has a specific active site shaped to fit only its substrate. The induced fit model: substrate binding causes a slight conformational change.
Enzyme Regulation
Competitive inhibitors mimic substrate and block the active site. Noncompetitive (allosteric) inhibitors bind elsewhere, changing the enzyme's shape. Temperature and pH outside the optimum cause denaturation.
ATP — Energy Currency
Adenosine + 3 phosphates. Hydrolysis of the last phosphate (ATP → ADP + P) releases energy. Used in coupled reactions to drive endergonic processes like biosynthesis and active transport.
Photosynthesis Overview
6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂. Two stages: light reactions (in thylakoid membranes) make ATP + NADPH + O₂; the Calvin cycle (in stroma) uses them to fix CO₂ into glucose.
Light Reactions
Chlorophyll absorbs light → splits H₂O (releasing O₂) → electrons travel through thylakoid ETC → H⁺ pumped into thylakoid → ATP synthase makes ATP (chemiosmosis). Outputs: ATP, NADPH, O₂.
Calvin Cycle
Light-INDEPENDENT (uses ATP + NADPH from light reactions). Rubisco attaches CO₂ to RuBP, then ATP + NADPH reduce it to G3P, which becomes glucose. Occurs in the stroma.
Cellular Respiration Overview
C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ~30–32 ATP. Four stages: glycolysis (cytoplasm), pyruvate oxidation (matrix), Krebs cycle (matrix), oxidative phosphorylation (inner membrane).
Glycolysis & Krebs
Glycolysis: glucose → 2 pyruvate + 2 ATP + 2 NADH (cytoplasm, no O₂ needed). Krebs: acetyl-CoA → CO₂ + 3 NADH + 1 FADH₂ + 1 ATP per cycle (×2 per glucose; in mitochondrial matrix).
Electron Transport & Chemiosmosis
NADH/FADH₂ drop electrons into the inner-membrane ETC. Electrons cascade to O₂ (final acceptor → H₂O). Energy released pumps H⁺ into intermembrane space. H⁺ flows back through ATP synthase, making most of the ATP.
Fermentation
Anaerobic — no oxygen, no Krebs, no ETC. Glycolysis runs alone (2 ATP/glucose). Pyruvate becomes lactate (animal muscle) or ethanol + CO₂ (yeast) — just to regenerate NAD⁺ so glycolysis can keep running.
The key terms you must know
- Activation energy — the energy barrier that must be overcome for a reaction to proceed. Enzymes lower it.
- Active site / induced fit — the substrate-binding region of an enzyme; the enzyme molds around the substrate to grip it tightly.
- Competitive vs. noncompetitive inhibition — blockers at the active site (can be overcome with more substrate) vs. blockers elsewhere (cannot be overcome).
- Denaturation — loss of an enzyme's 3D shape (and function) from extreme heat, pH, or chemicals.
- ATP hydrolysis — breaking ATP into ADP + Pᵢ to release usable energy; the cell's main energy reaction.
- Endergonic vs. exergonic — energy-requiring reactions (+ΔG) vs. energy-releasing reactions (–ΔG).
- Coupled reactions — pairing an exergonic and an endergonic reaction so the released energy drives the energy-requiring one.
- Chlorophyll — pigment in the thylakoid membrane that absorbs light energy, mostly in the red and blue wavelengths.
- Light reactions — capture light energy → ATP, NADPH, O₂ (in thylakoid membrane).
- Calvin cycle — uses ATP and NADPH to fix CO₂ into glucose (in stroma).
- Glycolysis — glucose → 2 pyruvate + 2 ATP + 2 NADH (cytoplasm, anaerobic).
- Krebs (citric acid) cycle — fully oxidizes acetyl-CoA, producing 3 NADH + 1 FADH₂ + 1 ATP per cycle (matrix).
- Electron transport chain — proteins in the inner mitochondrial membrane that pass electrons; powers H⁺ pumping.
- Chemiosmosis — H⁺ flowing back through ATP synthase down its gradient, producing ATP.
- Fermentation — anaerobic ATP production via glycolysis; regenerates NAD⁺ by converting pyruvate to lactate or ethanol.
Key themes to remember
- Enzymes are catalysts, not consumed. One enzyme can run a reaction thousands of times. Their shape is everything.
- Photosynthesis and cellular respiration are reverses of each other — at least at the overall equation level. Together they form the most important energy cycle on Earth.
- Most ATP comes from chemiosmosis. Both in the chloroplast (light reactions) and in the mitochondrion (electron transport chain), the H⁺ gradient powering ATP synthase makes the most ATP.
- The electron transport chain is universal. Same logic in the thylakoid and in the mitochondrion — electrons cascade down energy levels, pumping H⁺.
- Compartmentalization makes metabolism work. Light reactions only work in the thylakoid; Krebs only in the matrix. Membranes separate steps that would otherwise interfere.
- Anaerobic pathways came first. Glycolysis works without oxygen and is found in every kind of organism — evidence it evolved before O₂ was abundant.
Common exam traps
- Enzymes lower activation energy — they don't change ΔG. An enzyme makes a reaction go faster, but it doesn't change whether the reaction is endergonic or exergonic.
- Don't confuse competitive and noncompetitive inhibition. Competitive blocks the active site (more substrate solves it). Noncompetitive binds elsewhere and changes shape (more substrate doesn't help).
- O₂ is the FINAL electron acceptor in respiration — it doesn't power the ETC. Glucose's electrons flow through the chain; O₂ just catches them at the end and forms water.
- The Calvin cycle is light-INDEPENDENT, but it still depends on light reactions. Calvin needs the ATP and NADPH that light reactions produce. In the dark, those run out and Calvin stops.
- Fermentation isn't "the next step after glycolysis when there's no oxygen." It's a recycling mechanism — pyruvate is converted to lactate/ethanol just to regenerate NAD⁺ so glycolysis can continue.
- Most ATP comes from oxidative phosphorylation, not from substrate-level phosphorylation. Glycolysis + Krebs only make ~4 ATP. Chemiosmosis makes ~26.
- Rubisco is the enzyme of the Calvin cycle, not the light reactions. It fixes CO₂ to RuBP. Don't put it in the wrong stage.
- Photosynthesis is NOT the opposite of respiration in detail. Equation-wise yes; but mechanistically, photosynthesis stores energy in glucose by using light to push electrons uphill, while respiration releases that stored energy step by step.