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Credit Sprint's Guide to Biology Unit 3: Cellular Energetics

AP® weight 12–16%CLEP® weight 10–14%

Enzymes speed up reactions by lowering activation energy, and cells capture and release energy through photosynthesis and cellular respiration. Both depend on electron transport chains that pump protons and power ATP synthase.

Activation energy with and without an enzyme Progress of reactionEnergy with enzyme without enzyme lower Ea reactantsproductsΔG same

What you need to know

  • Enzymes are catalysts: they lower activation energy without being used up. The substrate binds the active site, which fits by shape and chemistry.
  • Temperature and pH change enzyme shape. Too far from the optimum and the enzyme denatures, so the rate drops.
  • A competitive inhibitor binds the active site (overcome by more substrate); a noncompetitive inhibitor binds elsewhere and changes the enzyme's shape.
  • Raising substrate concentration speeds the reaction until every active site is busy (saturation).
  • Photosynthesis: light reactions (thylakoid) split water, release O₂ and make ATP and NADPH; the Calvin cycle (stroma) uses them to fix CO₂ into sugar.
  • Cellular respiration: glycolysis (cytoplasm) → pyruvate oxidation and the Krebs cycle (mitochondrial matrix) → oxidative phosphorylation (inner membrane).
  • The electron transport chain pumps H⁺ across a membrane; H⁺ flows back through ATP synthase, making ATP. This is chemiosmosis.
  • O₂ is the final electron acceptor in aerobic respiration, forming water. Without O₂, fermentation regenerates NAD⁺ so glycolysis can keep going.
  • Fermentation makes only the 2 ATP from glycolysis, plus lactate (animals) or ethanol and CO₂ (yeast).
  • Energy flows; matter cycles. Respiration's products (CO₂, H₂O) are photosynthesis's inputs.

Equations and rules

Photosynthesis: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂
Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP
Rate of reaction = change in amount ÷ change in time

Exam traps

Watch forEnzymes don't change ΔG (whether a reaction releases energy); they only lower activation energy.
Watch forThe Calvin cycle doesn't need light directly, but it stops in the dark because it runs out of ATP and NADPH.
Watch forMost ATP in respiration comes from oxidative phosphorylation, not glycolysis or the Krebs cycle.

Key terms

  • Activation energy: The energy needed to start a reaction. Enzymes lower it.
  • Active site: The pocket on an enzyme where the substrate binds.
  • Competitive inhibitor: A molecule that blocks the active site. More substrate can outcompete it.
  • Noncompetitive inhibitor: Binds an allosteric site and changes the enzyme's shape, lowering activity even with lots of substrate.
  • Denaturation: Loss of a protein's shape (and function) from heat, pH or salt.
  • ATP: The cell's energy currency. Releasing its last phosphate releases usable energy.
  • Light reactions: Thylakoid reactions that split water, release O₂ and make ATP and NADPH.
  • Calvin cycle: Stroma reactions that use ATP and NADPH to fix CO₂ into sugar (G3P). Rubisco fixes the carbon.
  • Glycolysis: Splits glucose into two pyruvate in the cytoplasm. Net 2 ATP and 2 NADH. No oxygen needed.
  • Krebs cycle: Mitochondrial matrix cycle that releases CO₂ and loads NADH and FADH₂ with electrons.
  • Chemiosmosis: H⁺ flows down its gradient through ATP synthase, powering ATP production.
  • Fermentation: Regenerates NAD⁺ without oxygen so glycolysis can continue. Makes lactate or ethanol.

Practice questions

Five of the 24 Unit 3 questions in Credit Sprint. Every choice is explained in the app.

1. Enzymes speed up reactions by
  1. being permanently consumed
  2. raising the temperature of the cell
  3. making the reaction release more energy
  4. lowering the activation energy
Show answer

lowering the activation energy. They provide an easier pathway.

2. Where do the light reactions of photosynthesis take place?
  1. Stroma
  2. Thylakoid membranes
  3. Mitochondrial matrix
  4. Cytoplasm
Show answer

Thylakoid membranes. Photosystems and the electron transport chain are there.

3. Where does glycolysis occur, and does it need oxygen?
  1. In the chloroplast; light is required
  2. In the cytoplasm; no oxygen is required
  3. In the mitochondrial matrix; oxygen is required
  4. In the inner mitochondrial membrane; oxygen is required
Show answer

In the cytoplasm; no oxygen is required. It's anaerobic.

4. Muscle cells sprinting without enough oxygen make lactate. The purpose of this fermentation is to
  1. regenerate NAD⁺ so glycolysis can continue making ATP
  2. make more ATP than aerobic respiration
  3. produce oxygen
  4. store glucose as glycogen
Show answer

regenerate NAD⁺ so glycolysis can continue making ATP. Without NAD⁺, glycolysis stops.

5. During the Krebs cycle, the carbon atoms from glucose leave the cell mainly as
  1. CO₂
  2. water
  3. lactate
  4. O₂
Show answer

CO₂. Decarboxylation releases CO₂.