Cellular Energetics

What this unit covers

The topics below follow the published Biology course framework for Unit 3. This unit is worth 12–16% of the exam, so budget your time against that rather than against how long the unit takes to teach.

Lessons in this unit

• Enzymes & Catalysis14 min · 3 objectivesExplain how enzymes lower activation energy without being consumed · Relate active-site shape and the induced-fit model to substrate specificity · Predict how temperature, pH, and inhibitors change reaction rate
• Photosynthesis15 min · 3 objectivesWrite the overall equation of photosynthesis and identify its inputs and outputs · Describe how the light reactions convert light energy into ATP and NADPH · Explain how the Calvin cycle uses ATP and NADPH to fix CO₂ into sugar
• Cellular Respiration15 min · 3 objectivesSequence glycolysis, pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation · Trace how electron carriers deliver electrons to the ETC to drive ATP synthesis · Account for the approximate ATP yield of aerobic respiration from one glucose
• Energy Flow & Molecular Fitness12 min · 3 objectivesExplain how ATP powers cellular work through its cycle of hydrolysis and regeneration · Distinguish exergonic (catabolic) from endergonic (anabolic) reactions and energy coupling · Connect photosynthesis and respiration as a coupled cycle of matter and energy
• Photosynthesis Deep Dive15 min · 3 objectivesTrace electrons through Photosystem II, the electron transport chain, and Photosystem I to NADPH, and link the proton gradient to photophosphorylation · Account quantitatively for the Calvin cycle: how many turns, CO₂, ATP, and NADPH yield one G3P and one glucose · Compare C3, C4, and CAM strategies for avoiding photorespiration
• Cellular Respiration Deep Dive14 min · 3 objectivesTally the exact per-glucose outputs of glycolysis, pyruvate oxidation, and the Krebs cycle in ATP and electron carriers · Explain how the proton-motive force and ATP synthase convert NADH/FADH₂ into ~26–28 ATP by chemiosmosis · Justify the ~30–32 ATP total and contrast it with the 2-ATP yield of anaerobic fermentation

Formulas in Unit 3

Every term in Unit 3

All 40 terms we publish for Cellular Energetics, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.

Chemiosmosis

H⁺ flows back through ATP synthase down its electrochemical gradient, and that flow drives ATP synthesis. The gradient, not the electrons, makes the ATP.

Enzyme mechanism

Lowers activation energy by binding substrate in the active site and stabilizing the transition state. It is not consumed and does not change the reaction's ΔG.

Induced fit

The active site changes shape slightly on substrate binding, tightening the fit — a refinement of the older lock-and-key picture.

Competitive inhibition

Inhibitor binds the active site itself. Raising substrate concentration can outcompete it, so Vmax is unchanged.

Noncompetitive inhibition

Inhibitor binds an allosteric site and changes active-site shape. More substrate does not help, so Vmax falls.

Feedback inhibition

The end product of a pathway inhibits an early enzyme, shutting the pathway down when supply is sufficient — a control loop, not a defect.

Effect of temperature on enzymes

Rate rises with temperature until the protein denatures, after which it falls sharply. The curve peaks rather than plateauing.

Coupled reactions

An exergonic reaction drives an endergonic one; ATP hydrolysis is the usual donor, which is why ATP is the cell's energy currency.

ATP structure

Adenine, ribose and three phosphates. The negative charges repel, so hydrolyzing the terminal phosphate releases free energy.

Glycolysis

Splits glucose into two pyruvate in the cytosol. Net 2 ATP and 2 NADH, and no oxygen required — evidence of its ancient origin.

Pyruvate oxidation

Pyruvate enters the mitochondrion, loses CO₂ and joins coenzyme A to form acetyl-CoA, producing NADH.

Krebs cycle

Oxidizes acetyl-CoA in the matrix, yielding 2 CO₂, 3 NADH, 1 FADH₂ and 1 ATP per turn — two turns per glucose.

Electron transport chain

Electrons from NADH and FADH₂ pass down carriers in the inner membrane, and the energy released pumps H⁺ into the intermembrane space.

Why oxygen matters

Oxygen is the final electron acceptor. Without it the chain backs up, NAD⁺ is not regenerated, and the Krebs cycle stops.

Fermentation

Regenerates NAD⁺ without oxygen so glycolysis can continue. Yields lactic acid in animals, ethanol and CO₂ in yeast — 2 ATP per glucose, not 32.

Photosystems II and I

Light-driven complexes in the thylakoid membrane. PSII splits water and starts the electron flow; PSI re-energises electrons to reduce NADP⁺.

Light-dependent reactions

In the thylakoid membrane: water is split, O₂ released, and the proton gradient produces ATP and NADPH.

Calvin cycle

In the stroma: rubisco fixes CO₂ onto RuBP, ATP and NADPH reduce it to G3P, and RuBP is regenerated. Three turns per G3P.

Rubisco and photorespiration

Rubisco can bind O₂ instead of CO₂. On hot dry days with stomata closed, O₂ builds up and the wasteful photorespiration pathway runs.

C4 and CAM plants

Adaptations that concentrate CO₂ around rubisco — C4 separates fixation spatially into bundle-sheath cells, CAM separates it in time by fixing at night.

Free energy and ΔG

Negative ΔG means the reaction releases free energy and is spontaneous (exergonic); positive ΔG means it requires input (endergonic).

Activation energy

The energy barrier that must be crossed for a reaction to proceed. Enzymes lower it; they do not change ΔG or the equilibrium position.

Enzyme specificity

Only substrates whose shape and charge complement the active site bind, which is why one enzyme catalyses one reaction.

Cofactors and coenzymes

Non-protein helpers required for activity — metal ions are cofactors, organic molecules such as vitamins are coenzymes.

Allosteric regulation

A molecule binds away from the active site and shifts the enzyme between active and inactive shapes — the basis of both activation and noncompetitive inhibition.

Substrate concentration and rate

Rate rises with substrate then plateaus at saturation, when every active site is continuously occupied.

Anabolic vs catabolic pathways

Anabolic pathways build complex molecules and consume energy; catabolic ones break molecules down and release it.

Redox in metabolism

Oxidation is loss of electrons, reduction is gain. Glucose is oxidized and oxygen reduced, and the energy released comes from that electron transfer.

NAD⁺ and FAD

Electron carriers that are reduced to NADH and FADH₂ during glucose oxidation and deliver electrons to the transport chain.

ATP yield per glucose

About 30–32 ATP under aerobic conditions: roughly 2 from glycolysis, 2 from the Krebs cycle, and the rest from chemiosmosis.

Substrate-level vs oxidative phosphorylation

Substrate-level transfers a phosphate directly from a substrate to ADP; oxidative phosphorylation uses the proton gradient and ATP synthase.

Proton gradient

H⁺ accumulated on one side of a membrane stores potential energy in both a concentration and a charge difference — the actual currency between the chain and ATP synthase.

Uncoupling agents

Molecules that let H⁺ leak back across the membrane, so the gradient dissipates as heat and ATP output collapses. Brown fat does this deliberately.

Absorption spectrum vs action spectrum

The absorption spectrum shows which wavelengths a pigment absorbs; the action spectrum shows which drive photosynthesis. Their close match identifies the working pigments.

Chlorophyll a and accessory pigments

Chlorophyll a is the reaction-center pigment; chlorophyll b and carotenoids broaden the range of wavelengths captured and pass energy to it.

Photolysis of water

PSII splits water to replace lost electrons, releasing O₂ as a by-product and contributing H⁺ to the thylakoid space.

Products of the light reactions

ATP, NADPH and O₂. The first two power the Calvin cycle; the oxygen is waste from the plant's perspective.

Lab: Enzyme Activity

Catalase or peroxidase assayed while varying pH, temperature or substrate concentration. Rate is measured as product formed per unit time, not total product.

Lab: Photosynthesis (floating disk)

Vacuum-infiltrated spinach disks sink, then rise as photosynthesis produces oxygen. ET50 — the time for half to float — is the standard measure.

Lab: Cellular Respiration

A respirometer with KOH absorbing CO₂ measures oxygen consumption by germinating seeds, comparing germinating with dormant and warm with cold.

What examiners penalize here

• The AP throughline for enzymes is "**shape → function.**" Any factor that changes the enzyme’s shape — temperature, pH, or a noncompetitive inhibitor — changes the active site and thus the rate. When you explain a rate change, name the effect on shape and the active site explicitly.
• Master the hand-off: the **light reactions** (thylakoid) turn light + water into **ATP, NADPH, and O₂**; the **Calvin cycle** (stroma) spends that ATP and NADPH to fix **CO₂** into **sugar**, returning ADP and NADP⁺. Tracing which product feeds the next stage earns free-response points.
• Keep the tally straight: **glycolysis** (cytoplasm) → 2 ATP + 2 NADH; **pyruvate oxidation + Krebs cycle** (matrix) → 2 ATP + more NADH/FADH₂ + CO₂; **oxidative phosphorylation** (inner membrane) → the bulk of ATP, with O₂ as final electron acceptor forming H₂O. Anaerobically, only glycolysis’s 2 ATP remain.
• Zoom out for the big picture: **photosynthesis and respiration form one coupled cycle.** Photosynthesis captures energy and stores it in glucose (exergonic light harvest driving endergonic sugar synthesis); respiration releases that energy to make ATP. Energy flows one way (sunlight → heat), but matter (C, H, O) cycles.
• Free-response scoring hinges on the hand-off and the numbers: the **light reactions** (thylakoid membrane) turn light + H₂O into **ATP, NADPH, and O₂**; the **Calvin cycle** (stroma) spends **18 ATP + 12 NADPH** per glucose across **6 turns / 6 CO₂**. If asked why the ATP:NADPH demand is unequal, cite **RuBP regeneration** consuming the extra ATP.
• Lock in the ledger: **glycolysis** → 2 ATP + 2 NADH (cytosol); **pyruvate oxidation** → 2 NADH; **Krebs** → 2 ATP + 6 NADH + 2 FADH₂ (matrix); **oxidative phosphorylation** → ~26–28 ATP from 10 NADH + 2 FADH₂ via the proton-motive force, with **O₂ as final electron acceptor → H₂O**. Total **~30–32**; strip away O₂ and you are left with glycolysis’s **2**.

Practice Biology

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

Questions about this unit

All 8 units of AP Biology

1. Unit 1 · Chemistry of Life
2. Unit 2 · Cell Structure & Function
3. Unit 3 · Cellular Energetics
4. Unit 4 · Cell Communication & Cycle
5. Unit 5 · Heredity
6. Unit 6 · Gene Expression & Regulation
7. Unit 7 · Natural Selection
8. Unit 8 · Ecology