AP Biology · Unit 3 · 12–16% of Exam ⚡ SPRINT MODE

Cellular Energetics

The highest-weighted unit on the entire exam. Every single topic here is ★★★. Master enzyme inhibition graphs, chemiosmosis, the 4-stage respiration table, and photosynthesis I/O cold — these are pure, predictable points.

Exam Weight12–16%

~MCQs7–10 questions

FRQ AppearanceAlmost Every Year

Sprint Time~2.5 hours

Activation EnergyInhibition GraphsOIL RIG ChemiosmosisLight ReactionsCalvin Cycle Glycolysis → ETCFermentation

⚡ Quick Glance — All Topics at a Glance

Topic	Priority	Exam Format	Key Trap / Must-Know
3.1 Enzymes	★★★	MCQData	Enzymes lower Ea only — ΔG and equilibrium UNCHANGED
3.2 Enzyme Regulation	★★★	MCQDataFRQ	Competitive: ↑Km, same Vmax. Noncompetitive: same Km, ↓Vmax
3.3 ATP & Redox	★★★	MCQFRQ	OIL RIG; NADH/FADH₂ carry electrons TO ETC — not ATP
3.4 Photosynthesis	★★★	MCQFRQData	O₂ from H₂O (NOT CO₂); Calvin cycle needs light products (not truly "dark")
3.5 Cellular Respiration	★★★	MCQFRQData	~90% ATP from ETC (not glycolysis); fermentation = NAD⁺ regeneration only

⚠ AP EXAM EXCLUSION — Do NOT memorize: specific glycolysis intermediate names, Krebs cycle intermediate names, exact ATP counts per step, or structural formulas. Know CONCEPTS, LOCATIONS, INPUTS/OUTPUTS, and relative ATP yield.

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Topic 3.1

Enzymes

★★★ HIGH PRIORITYMCQData Analysis

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Core Definition

⚗️ What Enzymes Do (and Don't)

Biological catalysts — speed up reactions; not consumed
Lower activation energy (Ea) — energy to reach transition state
Do NOT change ΔG (overall free energy difference reactants → products)
Do NOT change equilibrium — only reach it faster
Do NOT make unfavorable reactions favorable
Reusable; specific to one substrate/reaction type

Binding Models

🤝 Induced Fit (Current Model)

Lock-and-Key: rigid active site; overly simplistic; NOT the preferred model
Induced Fit: active site is flexible; changes shape when substrate binds → forms tighter enzyme–substrate complex → AP prefers this
Active site = specific region where substrate binds
Enzyme–substrate complex (ES complex) → product released → enzyme recycled

Special Cases

🔩 Cofactors, Coenzymes & Ribozymes

Cofactors: inorganic ions required by enzyme (Mg²⁺, Zn²⁺, Fe²⁺)
Coenzymes: organic helpers (NAD⁺, FAD, Coenzyme A)
Ribozymes: RNA molecules that catalyze reactions — ribosomal rRNA catalyzes peptide bond formation (evidence for RNA world hypothesis)
Most enzymes are proteins, but not all

🎯 Exam Sniper

Energy diagram MCQ (very common): Given a graph of reaction progress vs. free energy — with and without enzyme — identify: (1) Ea with/without enzyme, (2) ΔG of the reaction (same both curves), (3) which curve represents the enzyme-catalyzed reaction (lower peak)
MCQ trap: "The enzyme makes the reaction more thermodynamically favorable." → FALSE. Enzyme only lowers Ea peak, not ΔG endpoints
MCQ: "Which of the following is an example of a ribozyme?" → rRNA in the ribosome (catalyzes peptide bond formation) — connects to RNA world hypothesis and evolution
FRQ connection: Induced fit explains why inhibitors that bind the active site (competitive) or allosteric site (noncompetitive) affect enzyme function by altering the shape

💣 Trap Alert

❌ Enzymes do NOT change ΔG — the difference in free energy between reactants and products is unchanged. Enzymes only lower the hill (Ea), not the start or end points
❌ Enzymes do NOT shift equilibrium — reactions still reach the same equilibrium; enzymes just get there faster
❌ The induced-fit model is the AP-preferred model, not lock-and-key. Active sites are flexible and undergo conformational changes upon substrate binding

Topic 3.2

Enzyme Regulation & Environmental Impacts

★★★ HIGH PRIORITYMCQDataFRQ

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Competitive vs. Noncompetitive Inhibition — Graph Critical

🔵 Competitive Inhibition

Inhibitor binds active site (mimics substrate)
Competes directly with substrate for active site
Can be overcome by ↑ substrate concentration
Km ↑ (apparent — need more substrate for same rate)
Vmax unchanged (can still reach max with enough substrate)
Graph: same Vmax, curve shifted right
Example: drugs that mimic substrate (e.g., ACE inhibitors)

🔴 Noncompetitive Inhibition

Inhibitor binds allosteric site (not active site)
Changes enzyme shape → active site distorted
Adding more substrate does NOT overcome it
Km unchanged (affinity for substrate same)
Vmax ↓ (even saturating enzyme can't reach original Vmax)
Graph: lower Vmax, same Km (same curve shape, lower plateau)
Example: allosteric inhibitors; heavy metal poisoning (Pb²⁺, Hg²⁺)

Environmental Effects

🌡 Temperature & pH

Optimal temperature: fastest rate; above optimal → denaturation → loss of function (irreversible)
Optimal pH: each enzyme has a specific pH optimum (pepsin pH 2 in stomach; trypsin pH 8 in small intestine)
Above/below optimum: H-bonds, ionic bonds in enzyme broken → active site changes shape → substrate can't bind
Low temperature: slows reaction but does NOT denature enzyme (reversible when warmed)

Metabolic Control

🔄 Allosteric Regulation & Feedback Inhibition

Allosteric regulation: molecule binds site OTHER than active site → conformational change → activates or inhibits enzyme
Feedback inhibition: end product of a pathway inhibits an enzyme earlier in the same pathway → prevents overproduction; saves energy
Example: ATP inhibits phosphofructokinase (PFK) in glycolysis when energy is abundant
Allosteric activators: increase enzyme activity by stabilizing active conformation

🎯 Exam Sniper

Data Graph MCQ (top hit): Given a rate vs. [substrate] graph with and without inhibitor — if Vmax is same but curves shifts right → competitive inhibition; if Vmax is lower with parallel curve → noncompetitive inhibition
FRQ: "An inhibitor reduces enzyme activity even when substrate concentration is very high. Identify the type of inhibition and explain." → Noncompetitive — binds allosteric site → structural change → active site distorted → adding substrate can't help since inhibitor is at a different site
MCQ: "A metabolic pathway produces amino acid X. When X concentration is high, it inhibits the first enzyme in the pathway. This is an example of..." → Feedback (negative) inhibition / allosteric inhibition
MCQ: "An enzyme's activity drops sharply above 45°C. Which protein property explains this?" → Denaturation — hydrogen bonds and other non-covalent bonds maintaining 3° structure are disrupted → active site loses shape

Graph MCQTopic 3.2VERY HIGH FREQUENCY

An experiment measures the rate of an enzyme-catalyzed reaction at increasing substrate concentrations, with and without an inhibitor. With inhibitor present, the maximum reaction rate (Vmax) is unchanged, but the substrate concentration needed to reach half-maximal rate (Km) is doubled. What type of inhibition is this, and how does the inhibitor work?

(A) Noncompetitive inhibition — the inhibitor binds the active site and cannot be displaced by substrate
(B) Competitive inhibition — the inhibitor binds the active site and competes with substrate; adding more substrate eventually displaces it
(C) Allosteric inhibition — the inhibitor lowers Vmax by distorting the active site through a remote binding site
(D) Feedback inhibition — the inhibitor is the end product and reduces enzyme synthesis

Answer: (B) — The key clues are: Vmax unchanged + Km increased. This is the signature of competitive inhibition. The inhibitor mimics the substrate and occupies the active site, making it harder for substrate to bind (↑ apparent Km). However, because the inhibitor can be displaced by excess substrate (it's reversible), adding more substrate allows the enzyme to eventually reach the same Vmax. If Vmax had decreased, that would indicate noncompetitive inhibition.

Topic 3.3

Cellular Energy — ATP, Thermodynamics & Redox

★★★ HIGH PRIORITYMCQFRQ

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Energy Currency

⚡ ATP — The Universal Currency

ATP = adenosine triphosphate = adenine + ribose + 3 phosphate groups (a nucleotide!)
Hydrolysis: ATP → ADP + Pᵢ + energy (exergonic; ~7.3 kcal/mol released)
Synthesis: ADP + Pᵢ + energy → ATP (endergonic; phosphorylation)
ATP is not long-term storage — recharged continuously (muscle cell turns over ATP in seconds)
Three types of work: mechanical (muscle), transport (pumps), chemical (biosynthesis)

Thermodynamics

🔥 ΔG — Free Energy

Exergonic (ΔG < 0): releases free energy; spontaneous; products have less energy than reactants → cellular respiration, ATP hydrolysis
Endergonic (ΔG > 0): requires energy input; not spontaneous → photosynthesis, ATP synthesis, biosynthesis
Energy coupling: exergonic reaction (ATP hydrolysis) drives endergonic reactions
Enzymes lower Ea but do NOT change ΔG

Must Memorize

🔋 OIL RIG — Redox Reactions

OIL RIG: Oxidation Is electron Loss; Reduction Is electron Gain
Oxidation: lose electrons → lose H⁺ too (in biological systems) → NAD⁺ oxidizes glucose
Reduction: gain electrons → gain H⁺ → NAD⁺ is reduced to NADH
Oxidizing agent: causes oxidation (is itself reduced) → NAD⁺, FAD, O₂
Reducing agent: causes reduction (is itself oxidized) → glucose, NADH
Glucose is oxidized in respiration; CO₂ is reduced in photosynthesis

Electron Carriers

📦 NADH & FADH₂

NAD⁺: oxidized form (empty); accepts 2 electrons + H⁺ → becomes NADH (reduced, full)
FAD: oxidized form; accepts 2 electrons → becomes FADH₂
NADH and FADH₂ carry electrons to the electron transport chain (ETC)
ETC uses electron energy to pump H⁺ → drives ATP synthase
NADH yields ~2.5 ATP; FADH₂ yields ~1.5 ATP at the ETC
In photosynthesis: NADP⁺ → NADPH (similar role, different carrier)

🎯 Exam Sniper

MCQ: "In cellular respiration, glucose is _____ and oxygen is _____." → Glucose is oxidized (loses electrons); O₂ is reduced (gains electrons → becomes H₂O)
MCQ: "Which of the following is an endergonic reaction?" → Photosynthesis; ATP synthesis; protein synthesis. (NOT ATP hydrolysis or cellular respiration)
MCQ: "NADH functions in cellular respiration primarily by..." → Carrying electrons (and their energy) to the electron transport chain, where they are used to pump protons and generate ATP

Topic 3.4

Photosynthesis

★★★ HIGH PRIORITYMCQFRQData

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Overall Equation

6CO₂ + 6H₂O

+ Light →

C₆H₁₂O₆ + 6O₂

Stage-by-Stage Breakdown — Master This Table

Stage	Location	Inputs	Outputs	Key Point
Light Reactions (also: "Light-Dependent")	Thylakoid membrane (in grana)	Light energy, H₂O, ADP+Pᵢ, NADP⁺	ATP, NADPH, O₂	Water is split (photolysis) → releases O₂; PS II → PS I → NADPH; ATP via chemiosmosis
Calvin Cycle (also: "Carbon Fixation")	Stroma of chloroplast	CO₂, ATP, NADPH	G3P (→ used to build glucose), ADP+Pᵢ, NADP⁺	Fixes carbon (CO₂ → organic); uses ATP + NADPH from light reactions; enzyme = RuBisCO

Light Reactions — Critical Details

☀️ What Happens at the Thylakoid

Photosystem II (PS II): absorbs light → energizes electrons → water split (H₂O → O₂ + 2H⁺ + 2e⁻)
Electrons travel down ETC → H⁺ pumped into thylakoid lumen (high H⁺ inside)
Photosystem I (PS I): re-energizes electrons with more light → electrons used to reduce NADP⁺ → NADPH
H⁺ flows back through ATP synthase → ATP synthesized (chemiosmosis)
O₂ comes from H₂O, NOT CO₂ — proven by ¹⁸O isotope experiments

Calvin Cycle — Critical Details

🌿 What Happens in the Stroma

CO₂ is fixed (attached to organic molecule) by enzyme RuBisCO
3 CO₂ + 3 RuBP → 6 3-carbon molecules → reduced to G3P (glyceraldehyde-3-phosphate)
G3P used to build glucose, amino acids, fatty acids
Some G3P regenerates RuBP (must keep cycle running)
"Light-independent" is misleading — Calvin cycle needs ATP + NADPH from light reactions. It STOPS in darkness when these run out
3 turns of cycle fix 3 CO₂ → 1 net G3P (one "half" of glucose)

Chemiosmosis in Chloroplasts — The ATP Engine

Chemiosmosis — Same Mechanism in Both Photosynthesis and Respiration

☀️ In Chloroplasts (Thylakoid)

ETC pumps H⁺ INTO thylakoid lumen (from stroma)
High H⁺ inside thylakoid → concentration gradient
H⁺ flows OUT through ATP synthase (in thylakoid membrane) → synthesizes ATP in stroma
ATP + NADPH used by Calvin cycle

🔥 In Mitochondria (inner membrane)

ETC pumps H⁺ INTO intermembrane space (from matrix)
High H⁺ in IMS → concentration gradient
H⁺ flows BACK through ATP synthase (in inner membrane) → synthesizes ATP in matrix
O₂ is terminal electron acceptor → H₂O formed

🎯 Exam Sniper

MCQ (top hit): "What is the source of the oxygen released during photosynthesis?" → Water (H₂O) — split by PS II during the light reactions. NOT CO₂. CO₂ is fixed into carbohydrates
FRQ: "Explain why the Calvin cycle slows down if a plant is moved from light to dark." → Light reactions stop → ATP and NADPH not produced → Calvin cycle has no reactants → carbon fixation halts. The Calvin cycle is light-independent in mechanism but light-dependent for its substrates
Data Analysis: Experiments measuring O₂ production at different light intensities, CO₂ levels, or temperatures — know that increasing light, CO₂, temperature (up to optimum), and water all increase photosynthesis rate
MCQ: "Where does the Calvin cycle occur?" → Stroma of the chloroplast (NOT thylakoid membrane)
FRQ: "Describe the role of ATP synthase in photosynthesis" → Catalyzes ATP synthesis as H⁺ flows down electrochemical gradient from thylakoid lumen to stroma; same principle as in mitochondria

💣 Trap Alert

❌ O₂ comes from H₂O, NOT CO₂ — this is one of the most common wrong answers on the AP exam. The oxygen in CO₂ ends up in carbohydrate (G3P/glucose)
❌ Calvin cycle is NOT a "dark reaction" — it needs ATP and NADPH from light reactions. If there's no light, no ATP/NADPH, so the Calvin cycle stops within minutes
❌ Plants do cellular respiration too — in mitochondria, 24 hours a day. At night, only respiration. During bright daylight, photosynthesis rate > respiration rate
❌ PS II comes BEFORE PS I in the light reactions (numbered by discovery order, not pathway order)

Topic 3.5

Cellular Respiration

★★★ HIGHEST PRIORITYMCQFRQData

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Overall Equation — Aerobic Respiration

C₆H₁₂O₆ + 6O₂

→

6CO₂ + 6H₂O + ~30–32 ATP

The 4 Stages — Master This Table

Stage	Location	O₂ Needed?	Inputs	Direct Outputs	Net ATP
1. Glycolysis	Cytosol	❌ No	Glucose (1)	2 pyruvate, 2 NADH, 2 ATP net	2 ATP
2. Pyruvate Oxidation (pyruvate → acetyl-CoA)	Mitochondrial matrix	✅ Yes (O₂ indirectly)	2 pyruvate, CoA, NAD⁺	2 acetyl-CoA, 2 NADH, 2 CO₂	0 ATP
3. Krebs Cycle (Citric acid cycle)	Mitochondrial matrix	✅ Yes (indirectly)	2 acetyl-CoA, H₂O, NAD⁺, FAD	6 NADH, 2 FADH₂, 2 ATP, 4 CO₂	2 ATP
4. Oxidative Phosphorylation (ETC + ATP synthase)	Inner mitochondrial membrane	✅ Yes — O₂ is terminal e⁻ acceptor	10 NADH, 2 FADH₂, O₂	~26–28 ATP, H₂O	~26–28 ATP

Critical Concept

⚙️ Where Does ATP Actually Come From?

Glycolysis + Krebs: only 4 ATP total (substrate-level phosphorylation)
NADH + FADH₂ carry electrons to ETC → produce ~26–28 ATP (oxidative phosphorylation)
~90% of ATP from ETC/oxidative phosphorylation
H⁺ gradient (from ETC pumping) drives ATP synthase — chemiosmosis
NADH yields ~2.5 ATP; FADH₂ yields ~1.5 ATP (enters ETC lower down)
O₂ accepts electrons at the end → forms H₂O

Without Oxygen

🍺 Fermentation — The Backup Plan

Occurs when O₂ is absent (anaerobic conditions)
Glycolysis still runs → produces 2 ATP + 2 NADH + 2 pyruvate
Fermentation's ONLY purpose: regenerate NAD⁺ from NADH so glycolysis can continue
NO additional ATP produced in fermentation steps themselves
Two types: Lactic acid fermentation (animals, some bacteria: pyruvate → lactate + NAD⁺); Alcoholic fermentation (yeast: pyruvate → ethanol + CO₂ + NAD⁺)
Total ATP = only 2 ATP (from glycolysis)

Where CO₂ is Released

💨 Tracking Carbon

Glycolysis: NO CO₂ released (glucose → pyruvate, all C retained)
Pyruvate oxidation: 2 CO₂ released (one per pyruvate)
Krebs cycle: 4 CO₂ released (two per acetyl-CoA × 2 turns)
ETC/chemiosmosis: NO CO₂ released
Total: 6 CO₂ from 1 glucose (matches overall equation)
All carbon from glucose ends up as CO₂ by end of Krebs

🎯 Exam Sniper

FRQ (most common Unit 3 FRQ): Describe the role of the electron transport chain in ATP synthesis → Electrons from NADH and FADH₂ pass through protein complexes in inner mitochondrial membrane → energy used to pump H⁺ from matrix to intermembrane space → H⁺ gradient drives H⁺ back through ATP synthase → conformational change in synthase → ADP + Pᵢ → ATP. O₂ accepts electrons at end → H₂O
MCQ: "If a cell's mitochondria are destroyed, which ATP-producing process can still occur?" → Glycolysis (occurs in cytosol, no mitochondria needed)
MCQ: "Why does yeast produce ethanol during fermentation?" → To regenerate NAD⁺ from NADH — NAD⁺ is needed to keep glycolysis running (as an electron acceptor)
Data Analysis: Graph of CO₂ production in yeast — under anaerobic conditions: slow CO₂ (from fermentation); aerobic: more CO₂ (from respiration) — explains why winemaking uses anaerobic conditions
MCQ: "A drug blocks ATP synthase. Which stage of respiration is most directly affected?" → Oxidative phosphorylation — ETC still runs but H⁺ can't drive ATP synthesis. Result: H⁺ gradient builds up, ETC eventually stops too

💣 Trap Alert

❌ ~90% of ATP from oxidative phosphorylation (ETC), NOT glycolysis or Krebs. Glycolysis = 2 ATP, Krebs = 2 ATP, ETC = ~26–28 ATP
❌ Fermentation produces NO extra ATP — its only role is regenerating NAD⁺ so glycolysis can continue. Total ATP from fermentation = 2 (from glycolysis only)
❌ Glycolysis happens in the CYTOSOL, not the mitochondria. Prokaryotes can do glycolysis (no mitochondria)
❌ O₂ is the terminal electron acceptor, not the source of energy directly. O₂ pulls electrons down the ETC, allowing H⁺ to be pumped. It becomes H₂O at the end
❌ CO₂ is produced in pyruvate oxidation AND Krebs cycle — NOT in glycolysis or the ETC

FRQ-Style MCQTopic 3.5HIGHEST FREQUENCY

A researcher adds a chemical that makes the inner mitochondrial membrane permeable to H⁺ ions (a "proton uncoupler"). Which of the following best describes the effect on cellular respiration?

(A) Glycolysis would stop because glucose can no longer be phosphorylated
(B) The Krebs cycle would stop because acetyl-CoA cannot enter the matrix
(C) The ETC would continue but ATP synthesis would decrease because the H⁺ gradient collapses
(D) Oxygen consumption would decrease because the ETC no longer has electrons to transfer

Answer: (C) — ATP synthesis via chemiosmosis depends on the H⁺ concentration gradient across the inner mitochondrial membrane. A proton uncoupler dissipates this gradient by allowing H⁺ to leak back freely without going through ATP synthase. Result: the ETC still accepts electrons (NADH, FADH₂ still donate electrons; O₂ still accepts them → O₂ consumption may even INCREASE), but the H⁺ energy is released as heat rather than captured as ATP. This is why some uncouplers (like DNP) cause hyperthermia — the energy becomes heat. ATP production crashes.

Practice

Sprint Practice — Mixed Questions

Cross-Topic MCQPhotosynthesis + Respiration

A plant is kept in a sealed container in bright light. Initially, CO₂ decreases and O₂ increases. After several hours, gas concentrations stabilize with no further net change. Which explanation best accounts for the stabilization?

(A) The plant ran out of chlorophyll and can no longer photosynthesize
(B) Photosynthesis rate equals cellular respiration rate, so net O₂ and CO₂ exchange is zero
(C) The Calvin cycle stopped because there is no CO₂ left to fix
(D) The plant switched entirely to anaerobic fermentation

Answer: (B) — Plants carry out BOTH photosynthesis (consumes CO₂, produces O₂) and cellular respiration (consumes O₂, produces CO₂) simultaneously. As CO₂ is depleted and O₂ builds up, photosynthesis rate decreases (less substrate) while CO₂ from respiration is recycled into the system. Equilibrium is reached when the two rates are equal — this is called the compensation point. The plant is still metabolically very active; it's just that the inputs and outputs cancel each other out.

Data AnalysisEnzyme Inhibition

The following data were collected for an enzyme at varying substrate concentrations:

Without inhibitor: Vmax = 100 μmol/min, Km = 4 mM
With inhibitor X: Vmax = 100 μmol/min, Km = 12 mM
With inhibitor Y: Vmax = 50 μmol/min, Km = 4 mM

What are the types of inhibitors X and Y, and what do the results tell us about each mechanism?

(A) X = noncompetitive; Y = competitive
(B) X = competitive; Y = noncompetitive
(C) Both X and Y are competitive inhibitors
(D) X = allosteric activator; Y = feedback inhibitor

Answer: (B) — Inhibitor X: Vmax unchanged (100) but Km tripled (4 → 12 mM) → classic signature of competitive inhibition. The inhibitor competes for the active site; adding more substrate can displace it, so Vmax is still reachable. Inhibitor Y: Km unchanged (4 mM, same affinity for substrate) but Vmax halved (100 → 50) → classic noncompetitive inhibition. Binds allosteric site → distorts active site → even saturating substrate can't restore original Vmax. This data-reading skill is tested almost every year.

FRQ-StylePhotosynthesis Mechanism

A scientist treats isolated chloroplasts with a drug that blocks Photosystem II but leaves Photosystem I functional. Predict what happens to: (A) O₂ production, (B) NADPH production, and (C) ATP synthesis in the Calvin cycle.

(A) O₂ decreases; NADPH increases; Calvin cycle ATP synthesis increases
(B) O₂ production stops; NADPH production stops; Calvin cycle cannot proceed
(C) O₂ production stops; NADPH production continues normally; Calvin cycle is unaffected
(D) All three processes decrease proportionally but do not stop

Answer: (B) — PS II is blocked: (A) No water splitting → NO O₂ produced. (B) PS II feeds electrons into the ETC → PS I. If PS II is blocked, no electrons flow to PS I → PS I cannot reduce NADP⁺ → NO NADPH. (C) Without ATP (from ETC/chemiosmosis, which also requires electron flow from PS II) and NADPH, the Calvin cycle has no substrates → cannot fix CO₂. The entire photosynthetic system collapses from the blockage of PS II — it is the gateway of the light reactions.

⚠ Trap Alert

Unit 3 High-Frequency Exam Traps

🌿
Plants do NOT "only do photosynthesis" — they also respire 24/7Cellular respiration occurs in plant mitochondria continuously. At night: only respiration (O₂ consumed, CO₂ released). In bright light: photosynthesis rate > respiration rate, so net O₂ is released. The compensation point is when the two rates are equal. This distinction appears in almost every exam.
💧
O₂ in photosynthesis comes from H₂O — NOT CO₂PS II splits water (H₂O → O₂ + 2H⁺ + 2e⁻). The oxygen released to the atmosphere is from water. The carbon in CO₂ becomes incorporated into sugar (G3P). Proven by heavy oxygen (¹⁸O) labeling: label H₂O with ¹⁸O → ¹⁸O₂ is released.
🔋
Fermentation produces NO additional ATP beyond glycolysisFermentation (lactic acid or alcoholic) produces ZERO net ATP. It simply oxidizes NADH back to NAD⁺ so glycolysis can keep running. The 2 ATP from glycolysis is the TOTAL yield under anaerobic conditions. Fermentation is about NAD⁺ recycling, not energy production.
⚡
~90% of ATP from the ETC (oxidative phosphorylation) — NOT glycolysis or KrebsGlycolysis yields 2 ATP; Krebs yields 2 ATP. But NADH and FADH₂ from all stages feed the ETC, producing ~26–28 more ATP. Students who say "glycolysis is where most ATP comes from" lose points every time.
🧪
Competitive inhibitor: Vmax UNCHANGED; Km increased. Noncompetitive: Vmax DECREASED; Km unchangedThis is the most consistently tested graph-reading concept in Unit 3. On a rate vs. [substrate] graph: competitive inhibition = same plateau (Vmax) but shifted right curve; noncompetitive = lower plateau (Vmax) with same Km half-point.
🌑
The "Calvin cycle" (light-independent) DOES require light — indirectlyThe Calvin cycle doesn't use light directly, but it requires ATP and NADPH produced by the light reactions. If you cover a plant, the Calvin cycle stops within minutes as these substrates are depleted. "Dark reactions" is a misleading historical term that the AP exam has moved away from.
🔬
Glycolysis is in the CYTOSOL — NOT the mitochondriaGlycolysis is the only ATP-generating process that occurs outside the mitochondria. This means: (1) prokaryotes (no mitochondria) CAN do glycolysis; (2) even in hypoxic conditions, cells can still generate 2 ATP from glycolysis + fermentation; (3) if mitochondria are lost or inhibited, glycolysis still runs.
📊
Enzymes do NOT change ΔG or equilibrium — only EaOn an energy diagram, the enzyme lowers the activation energy peak but the starting and ending energy levels remain identical. The reaction's ΔG and the equilibrium position are determined by the molecules involved, not by the enzyme. This is a foundational concept that appears in thermodynamics questions.

✓ Last-Min Checklist

Pre-Exam 10-Minute Checklist

Click each item to check off before exam day.

Enzymes & Regulation (3.1–3.2)

Enzymes lower Ea ONLY — ΔG and equilibrium are UNCHANGED
Induced-fit model: active site changes shape upon substrate binding (preferred over lock-and-key)
Competitive inhibitor: binds active site; ↑ Km; Vmax UNCHANGED; can be overcome by ↑ substrate
Noncompetitive inhibitor: binds allosteric site; ↓ Vmax; Km unchanged; cannot be overcome
Feedback inhibition: end product inhibits first enzyme in pathway (allosteric, negative feedback)
Temperature: optimal = fastest; above optimal = denaturation (irreversible); cold = slows (reversible)

ATP & Redox (3.3)

OIL RIG: Oxidation = electron Loss; Reduction = electron Gain
Glucose is oxidized in respiration; CO₂ is reduced in photosynthesis
NADH and FADH₂ carry electrons to ETC; NADP⁺ → NADPH in photosynthesis
Exergonic (ΔG < 0) = spontaneous (respiration, ATP hydrolysis); Endergonic (ΔG > 0) = needs energy input (photosynthesis, ATP synthesis)

Photosynthesis (3.4)

Light reactions: thylakoid membrane → inputs: light, H₂O, ADP+Pᵢ, NADP⁺ → outputs: ATP, NADPH, O₂
Calvin cycle: stroma → inputs: CO₂, ATP, NADPH → outputs: G3P (→ glucose), ADP+Pᵢ, NADP⁺
O₂ released = from H₂O (PS II splits water) — NOT from CO₂
PS II → ETC → PS I → NADPH (order of light reactions); ATP synthase in thylakoid membrane
Calvin cycle stops in darkness (no ATP/NADPH from light reactions)

Cellular Respiration (3.5)

Glycolysis: cytosol, no O₂ needed, glucose → 2 pyruvate + 2 ATP + 2 NADH
Pyruvate oxidation: mitochondrial matrix, 2 pyruvate → 2 acetyl-CoA + 2 CO₂ + 2 NADH
Krebs cycle: mitochondrial matrix, 2 acetyl-CoA → 4 CO₂ + 6 NADH + 2 FADH₂ + 2 ATP
ETC + ATP synthase: inner mitochondrial membrane; O₂ = terminal acceptor → H₂O; ~26–28 ATP
~90% of ATP from ETC (oxidative phosphorylation) — NOT glycolysis or Krebs
Fermentation = 0 extra ATP; only purpose = regenerate NAD⁺ for glycolysis to continue
CO₂ released in: pyruvate oxidation (2) + Krebs (4) = 6 total; NOT in glycolysis or ETC

⚡ Final Sprint Strategy for Unit 3

Top 5 must-master concepts: (1) Inhibition graph reading (Km/Vmax changes), (2) Photosynthesis stage I/O table + O₂ from H₂O, (3) Respiration 4-stage location/I/O table, (4) Chemiosmosis mechanism (applies to BOTH organelles), (5) Fermentation = NAD⁺ only, no extra ATP
FRQ templates to know: "Trace electrons from glucose to ATP synthesis" (respiration ETC); "Explain why plants need both light and dark conditions for photosynthesis"; "Compare competitive and noncompetitive inhibition using a graph"
The AP CED explicitly excludes: intermediate names in glycolysis (glucose-6-phosphate, etc.), specific Krebs intermediates (citrate, oxaloacetate, etc.), precise ATP counts per step. Focus on relative amounts and conceptual understanding
Connections: Unit 1 (ATP = nucleotide derivative); Unit 2 (chemiosmosis uses membrane gradient like active transport); Unit 4 (ATP hydrolysis powers signal transduction); Unit 5 (enzymes used in DNA replication = same principles)