Cellular Energetics
Unit 3 is the highest-weighted unit in AP Biology. It covers how enzymes catalyze life's reactions and how cells harvest and store energy through two complementary processes: photosynthesis and cellular respiration. Understanding the flow of energy and electrons through these pathways is essential for every subsequent unit.
Enzymes
Enzymes are biological catalysts — almost all are proteins — that speed up chemical reactions without being consumed in the process. They lower the activation energy (Ea) of a reaction: the minimum energy needed to start the reaction. By lowering Ea, enzymes make reactions occur orders of magnitude faster than they would spontaneously.
Enzymes do not change the overall free energy difference (ΔG) between reactants and products — they do not make unfavorable reactions favorable; they only accelerate reactions that are already thermodynamically possible. They do not alter equilibrium — they just help the system reach equilibrium faster.
The Active Site and Enzyme–Substrate Complex
The active site is the specific region of the enzyme where the substrate binds. The shape and chemical properties of the active site are precisely complementary to the substrate — this is the basis of enzyme specificity. Two models describe substrate binding:
The active site is a rigid, fixed shape that is exactly complementary to the substrate — like a key fitting a specific lock. Simple but overly rigid; does not fully explain how enzymes interact with multiple substrates or how inhibitors work.
The active site is flexible. When the substrate approaches, the enzyme undergoes a conformational change — the active site reshapes slightly to fit the substrate more tightly. This creates the enzyme–substrate (ES) complex. The induced fit generates the optimal orientation for catalysis.
How Enzymes Lower Activation Energy
- Orientation: Enzymes hold substrate molecules in the optimal position for the reaction to occur.
- Microenvironment: The active site may have a different pH or polarity than the surrounding cytoplasm, creating ideal reaction conditions locally.
- Chemical participation: Amino acid R groups in the active site may directly donate or accept protons (acid-base catalysis) or transiently bond to the substrate (covalent catalysis).
- Stress and strain: Binding distorts the substrate's bonds, bringing it closer to the transition state.
Enzyme Characteristics
- Enzymes are reusable — they are not consumed in the reaction and can catalyze thousands of reactions per second.
- Enzymes are specific — each enzyme typically catalyzes only one type of reaction (or a narrow class).
- Most enzymes are proteins; some are RNA molecules called ribozymes (e.g., the catalytic core of the ribosome, rRNA, is a ribozyme).
- Many enzymes require cofactors (inorganic metal ions: Mg²⁺, Zn²⁺, Fe²⁺) or coenzymes (organic molecules: NAD⁺, FAD, CoA) to function.
Activation energy diagrams: The AP exam frequently shows energy diagrams and asks you to identify the effect of an enzyme. On the diagram, Ea is the energy difference between reactants and the transition state peak. With enzyme, the peak is lower. The overall ΔG (reactants → products) remains the same — enzymes only lower the peak, not the endpoints.
Ribozymes: The fact that rRNA (not a protein) catalyzes peptide bond formation in the ribosome is evidence for the RNA world hypothesis and commonly appears in evolution-related questions.
Induced fit vs. lock-and-key: The AP exam prefers the induced-fit model as the accurate description. Know that conformational changes occur upon substrate binding.
An enzyme lowers the activation energy of a reaction from 80 kJ/mol to 30 kJ/mol. Which of the following correctly describes the effect of this enzyme on the reaction?
- (A) The enzyme changes the overall free energy change (ΔG) of the reaction, making it more favorable.
- (B) The enzyme is consumed during the reaction and must be replenished.
- (C) The enzyme increases the rate of the reaction without changing the equilibrium position or ΔG.
- (D) The enzyme shifts the equilibrium toward product formation, increasing product concentration at equilibrium.
Environmental Impacts on Enzyme Function
Because enzyme function depends on its precise 3D shape, any factor that alters protein structure will affect enzyme activity. The key environmental factors are temperature, pH, substrate concentration, and inhibitors.
Temperature
As temperature increases, molecules move faster → more frequent enzyme–substrate collisions → reaction rate increases. This continues up to the enzyme's optimal temperature (for most human enzymes, ~37°C). Above the optimal temperature, excessive heat disrupts hydrogen bonds and hydrophobic interactions → denaturation → enzyme loses its shape → active site is distorted → activity rapidly drops to zero.
Denaturation at high temperatures is often irreversible for most proteins (cooked egg white cannot be uncooked). However, some denaturation (e.g., from slightly elevated temperature) may be reversible when conditions normalize.
pH
Each enzyme has an optimal pH at which its activity is maximum. Changes in pH alter the ionization state of amino acid R groups in the active site → disrupts ionic bonds and hydrogen bonds → distorts active site shape → reduced activity or denaturation. Examples:
| Enzyme | Optimal pH | Location / Reason |
|---|---|---|
| Pepsin | ~2 | Stomach — acidic environment from HCl; breaks down proteins |
| Salivary Amylase | ~6.8–7.0 | Mouth — neutral pH; breaks down starch |
| Trypsin | ~8 | Small intestine — slightly alkaline (neutralized by bile); breaks down proteins |
| Most intracellular enzymes | ~7.2–7.4 | Cytoplasm — near-neutral pH |
Substrate Concentration and Saturation Kinetics
As substrate concentration increases (with enzyme amount fixed), reaction rate initially increases proportionally. However, as all enzyme active sites become occupied, the rate reaches a maximum (Vmax) — enzyme saturation. Increasing substrate further has no effect. The Michaelis-Menten constant (Km) is the substrate concentration at which rate = ½Vmax; a low Km indicates high affinity (enzyme binds substrate tightly).
Enzyme Inhibition — A Critical Exam Topic
| Type | Where It Binds | Effect on Active Site | Effect on Vmax | Effect on Km | Can Be Overcome By? |
|---|---|---|---|---|---|
| Competitive Inhibitor | Active site (competes with substrate) | Blocks substrate from binding; shape mimics substrate | Unchanged | ↑ (apparent — enzyme seems lower affinity) | Adding more substrate (outcompetes the inhibitor) |
| Noncompetitive Inhibitor | Allosteric site (different from active site) | Changes enzyme shape → active site distorted; substrate can still bind but catalysis reduced | ↓ (fewer effective catalytic events) | Unchanged | Cannot be overcome by adding more substrate |
Allosteric Regulation
Many enzymes have allosteric sites — binding sites separate from the active site. When a molecule binds the allosteric site, it causes a conformational change that either activates or inhibits the enzyme. This is the basis of feedback inhibition: the end product of a metabolic pathway inhibits an early enzyme in that pathway, preventing overproduction. Example: excess ATP inhibits an early enzyme in cellular respiration, slowing ATP production when energy is already plentiful.
Graph interpretation: Rate vs. substrate concentration curves are very common. A plateau → enzyme saturation. A curve that doesn't plateau at normal Vmax but reaches a lower plateau → noncompetitive inhibition. A curve shifted right (higher substrate needed to reach same rate) → competitive inhibition.
Distinguish inhibitor types using the "can more substrate help?" test: If adding more substrate restores activity → competitive inhibitor (substrate wins). If more substrate cannot help → noncompetitive inhibitor (active site is still distorted regardless).
Feedback inhibition = allosteric noncompetitive inhibition by the pathway's end product. This is a fundamental regulatory mechanism in metabolism (seen in amino acid biosynthesis, nucleotide synthesis, etc.).
A researcher adds a molecule that binds to the allosteric site of an enzyme, reducing its activity. When the researcher then doubles the substrate concentration, the rate of the reaction does NOT return to the uninhibited level. This inhibitor is best classified as
- (A) a competitive inhibitor, because it reduced enzyme activity
- (B) a competitive inhibitor, because it binds to the active site
- (C) a noncompetitive inhibitor, because it binds at the allosteric site and cannot be overcome by excess substrate
- (D) a denaturation agent, because it alters enzyme structure
❌ Noncompetitive inhibitor does NOT bind the active site. If it did, it would be competitive. "Non-competitive" = doesn't compete for the active site = binds elsewhere.
❌ Temperature and pH effects are about shape, not just speed. Above optimal temperature, the enzyme DENATURES (loses its 3D structure) — it's not just "moving too fast." Below optimal, it's just slow. At wrong pH, the active site's charge distribution is disrupted.
Cellular Energy — ATP and Thermodynamics
All living systems require a constant input of energy to maintain order — to counteract the natural tendency toward disorder (increased entropy) described by the Second Law of Thermodynamics. The First Law states that energy is neither created nor destroyed, only converted. Cells are not exempt from these laws — they harness energy from the environment (sunlight or chemical bonds in food) and use it to do biological work.
ATP — The Universal Energy Currency
ATP (adenosine triphosphate) is the cell's primary energy carrier. It consists of adenosine (adenine + ribose) + three phosphate groups. ATP hydrolysis releases usable free energy because the products (ADP + Pᵢ) are more stable than ATP under cellular conditions — the phosphate groups repel each other electrostatically, and the products have greater resonance stabilization. When the terminal phosphate is hydrolyzed:
This energy release is used to power endergonic (energy-requiring) cellular processes: muscle contraction, active transport, biosynthesis, and signal transduction. ATP is regenerated from ADP + Pᵢ by cellular respiration (and photosynthesis in plants) — forming a continuous ATP cycle.
Energy Coupling
Cells use energy coupling to drive unfavorable (endergonic) reactions by linking them to favorable (exergonic) ones. ATP hydrolysis is the most common exergonic reaction used to power endergonic processes. The two reactions are physically coupled — the phosphate released from ATP is transferred directly to the molecule that needs to be activated (phosphorylation), changing its shape/energy state.
Redox Reactions and Electron Carriers
Energy is transferred in metabolism primarily through oxidation-reduction (redox) reactions. Oxidation = loss of electrons; Reduction = gain of electrons (mnemonic: OIL RIG — Oxidation Is Loss, Reduction Is Gain). The molecule that loses electrons is oxidized (and acts as a reducing agent); the one that gains electrons is reduced (and acts as an oxidizing agent).
Key electron carriers in cellular energetics:
NAD⁺ (oxidized form) accepts 2 electrons + 1 H⁺ → becomes NADH (reduced, high-energy electron carrier). NADH delivers electrons to the electron transport chain (ETC), where they are used to generate ATP. Each NADH yields ~2.5 ATP via oxidative phosphorylation.
FAD (oxidized) accepts 2 electrons + 2 H⁺ → becomes FADH₂ (reduced). Also delivers electrons to the ETC. Each FADH₂ yields ~1.5 ATP (enters ETC at a lower point than NADH, pumping fewer protons).
NADP⁺ (oxidized) → NADPH (reduced). The photosynthesis equivalent of NADH. NADPH carries electrons from the light reactions to the Calvin cycle, where they are used to reduce CO₂ to sugar.
Conservation of Core Metabolic Pathways
The fact that glycolysis and the core reactions of cellular respiration are conserved across all three domains of life (Bacteria, Archaea, Eukarya) is strong evidence for common ancestry. These pathways evolved very early in life's history and have been maintained because they are fundamental to survival.
OIL RIG: Know oxidation vs. reduction definitively. When glucose is oxidized in cellular respiration, its carbons lose electrons → CO₂ (fully oxidized carbon). When CO₂ is reduced in photosynthesis, electrons are added → glucose (reduced carbon). This is the core of the carbon cycle at a molecular level.
ATP is NOT a long-term energy storage molecule. Fats and carbohydrates store energy long-term. ATP is the immediate energy currency — rapidly produced and consumed. A cell may recycle its entire ATP supply every 1–2 minutes under high demand.
Photosynthesis
Photosynthesis is the process by which photoautotrophs (plants, algae, cyanobacteria) convert light energy into chemical energy stored in glucose. The overall equation:
Photosynthesis first evolved in prokaryotes (cyanobacteria ~2.7 billion years ago) and was responsible for oxygenating Earth's early atmosphere. The eukaryotic chloroplast evolved from endosymbiosis of a cyanobacterium (Unit 2.10).
Chloroplast Structure and the Two Stages of Photosynthesis
| Stage | Location | Inputs | Outputs | Key Process |
|---|---|---|---|---|
| Light Reactions | Thylakoid membranes (grana) | Light energy, H₂O, ADP, Pᵢ, NADP⁺ | ATP, NADPH, O₂ (byproduct of water splitting) | Photosystems I & II, ETC, chemiosmosis/photophosphorylation |
| Calvin Cycle (Light-independent reactions) | Stroma | CO₂, ATP, NADPH | G3P (glyceraldehyde-3-phosphate) → glucose; ADP, Pᵢ, NADP⁺ recycled | Carbon fixation; RuBisCO fixes CO₂ into organic molecules |
The Light Reactions — Step by Step
The light reactions convert light energy into chemical energy (ATP and NADPH) and produce O₂ as a byproduct. They occur in the thylakoid membranes and involve two photosystems connected by an electron transport chain.
Chlorophyll absorbs light → electrons are excited to a higher energy level. Water is split (photolysis): 2H₂O → 4H⁺ + 4e⁻ + O₂. The O₂ is released as a byproduct. The electrons replace those lost from PSII.
High-energy electrons from PSII flow through the ETC (series of protein complexes in the thylakoid membrane). As electrons cascade downward in energy, H⁺ ions are pumped from the stroma into the thylakoid lumen, building an electrochemical (proton) gradient.
Electrons from the ETC arrive at PSI, where they are re-energized by another photon. These re-energized electrons are then used to reduce NADP⁺ → NADPH. NADPH carries this energy to the Calvin cycle.
The proton gradient established across the thylakoid membrane drives H⁺ ions back through ATP synthase (chemiosmosis) → ATP is synthesized from ADP + Pᵢ. This is photophosphorylation — identical mechanism to oxidative phosphorylation in mitochondria.
Both photosynthesis and cellular respiration use the same fundamental ATP synthesis mechanism: chemiosmosis. Electron transport through a membrane-bound ETC pumps H⁺ ions across a membrane, creating an electrochemical gradient. H⁺ flows back through ATP synthase (a molecular turbine), driving ATP synthesis. In chloroplasts, H⁺ is pumped INTO the thylakoid (stroma → lumen → back through ATP synthase into stroma). In mitochondria, H⁺ is pumped into the intermembrane space (matrix → IMS → back through ATP synthase into matrix).
The Calvin Cycle — Using ATP and NADPH to Fix Carbon
The Calvin cycle (occurring in the stroma) uses the ATP and NADPH from the light reactions to convert CO₂ into organic molecules (G3P), which can be used to make glucose and other carbohydrates. The enzyme RuBisCO catalyzes carbon fixation (the incorporation of CO₂ into an organic molecule). For every 3 CO₂ molecules fixed, one net G3P is produced.
Factors Affecting the Rate of Photosynthesis
↑ Light → ↑ photosynthesis rate until light-saturation point. At low light, light reactions are limiting. At very high light, light may damage photosystems (photoinhibition).
Affects enzyme activity (Calvin cycle enzymes). Too cold → enzymes slow. Too hot → denaturation. Optimal ~25–30°C for most C3 plants.
CO₂ is the substrate for RuBisCO. ↑ CO₂ → ↑ Calvin cycle rate. At current atmospheric CO₂ (~420 ppm), CO₂ is often a limiting factor for C3 plants.
Water is split in PSII to provide electrons. Drought → stomata close → CO₂ entry limited → Calvin cycle limited → photorespiration increases (O₂ binds RuBisCO instead of CO₂).
O₂ from photosynthesis comes from WATER, not CO₂. Isotope labeling experiments (¹⁸O) confirmed this. Water is split in PSII; the oxygen from CO₂ ends up in carbohydrates.
Light reactions supply ATP and NADPH to the Calvin cycle. If light reactions are blocked, the Calvin cycle stops (runs out of ATP and NADPH). If CO₂ is removed, the Calvin cycle stops but the light reactions continue briefly (until they run out of ADP and NADP⁺ to recycle).
Photosystem II comes BEFORE Photosystem I in the electron pathway — despite the name. This is because PSII was discovered second, but it is first in the electron flow. Remember: "II before I" in the electron path.
A plant is placed in the dark for 24 hours. Predict and explain what would happen to the concentration of ATP, NADPH, and G3P in the chloroplast stroma during this time.
NADPH: Concentration would decrease. NADPH is produced in Photosystem I (PS I) by the reduction of NADP⁺ using energy from light-excited electrons. Without light, PS I cannot function, so NADPH production stops. Existing NADPH is consumed by the Calvin cycle (used to reduce G3P intermediates) until depleted.
G3P: Concentration would decrease (and eventually the Calvin cycle stops). G3P is continuously produced by CO₂ fixation and consumed to regenerate RuBP. In darkness, ATP and NADPH are no longer replenished by the light reactions → the Calvin cycle cannot regenerate RuBP or fix CO₂ → G3P production ceases → G3P levels fall as remaining G3P is used up or converted to other products.
Cellular Respiration
Cellular respiration is the process by which cells extract energy from organic molecules (primarily glucose) and store it as ATP. All living organisms perform cellular respiration — both plants and animals. The overall aerobic equation:
Glucose is oxidized (loses electrons → CO₂). Oxygen is reduced (gains electrons → H₂O) as the terminal electron acceptor. The energy released from breaking C–H bonds is captured in ATP, NADH, and FADH₂.
The Four Stages of Aerobic Cellular Respiration
| Stage | Location | Inputs | Outputs | Net ATP |
|---|---|---|---|---|
| 1. Glycolysis | Cytosol (cytoplasm) | 1 glucose (6C) | 2 pyruvate (3C each), 2 ATP net, 2 NADH | 2 ATP |
| 2. Pyruvate Oxidation | Mitochondrial matrix | 2 pyruvate, 2 NAD⁺, 2 CoA | 2 acetyl-CoA (2C), 2 CO₂, 2 NADH | 0 ATP (direct) |
| 3. Krebs Cycle (Citric Acid Cycle) | Mitochondrial matrix | 2 acetyl-CoA, 6 NAD⁺, 2 FAD, 2 ADP | 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP | 2 ATP |
| 4. Oxidative Phosphorylation (ETC + Chemiosmosis) | Inner mitochondrial membrane | 10 NADH, 2 FADH₂, O₂ | H₂O, 26–28 ATP | ~26–28 ATP |
Stage 1: Glycolysis — In Detail
Glycolysis splits one glucose (6C) into two molecules of pyruvate (3C each) through a series of enzymatic reactions. This occurs in the cytosol — no mitochondria needed, so all cells (prokaryotes and eukaryotes) can perform glycolysis. Net yield per glucose: 2 ATP + 2 NADH + 2 pyruvate. Both aerobic and anaerobic respiration begin with glycolysis.
Stage 3: The Krebs Cycle — In Detail
Each acetyl-CoA (2C) enters the cycle by combining with oxaloacetate (4C) → citrate (6C). Through a series of reactions, 2 carbons are released as CO₂, and the 4C oxaloacetate is regenerated. Per acetyl-CoA turn: 3 NADH, 1 FADH₂, 1 ATP, 2 CO₂. The cycle turns twice per glucose (two acetyl-CoA).
Key fact: The Krebs cycle does not directly produce much ATP — its main function is to transfer electrons from organic molecules to NADH and FADH₂, which then carry those electrons to the ETC for the bulk of ATP production.
Stage 4: Oxidative Phosphorylation — The ATP Powerhouse
This is where ~90% of aerobic respiration's ATP is produced. Two coupled processes:
NADH and FADH₂ donate electrons to protein complexes (I, II, III, IV) in the inner mitochondrial membrane. As electrons cascade downward in energy toward O₂, the energy released pumps H⁺ from the matrix → intermembrane space. O₂ is the terminal electron acceptor → reduced to H₂O. Without O₂, the ETC stops.
The proton gradient across the inner mitochondrial membrane is potential energy. H⁺ flows back into the matrix through ATP synthase, driving its rotation and synthesizing ATP from ADP + Pᵢ. This is oxidative phosphorylation. The cristae (inner membrane folds) massively increase surface area → more ATP synthase → more ATP.
Fermentation — Anaerobic Alternatives
When oxygen is absent, the ETC cannot function (no terminal electron acceptor). NADH cannot be oxidized back to NAD⁺ by the ETC. Without NAD⁺, glycolysis halts (cannot accept electrons from glycolysis). Fermentation regenerates NAD⁺ from NADH by transferring electrons to an organic molecule — allowing glycolysis to continue. No additional ATP is produced beyond glycolysis's 2 ATP.
Pyruvate → acetaldehyde + CO₂ → ethanol + NAD⁺ (regenerated). Performed by yeast and some bacteria. Used in brewing, bread making. CO₂ production causes bread to rise and beer/wine to be carbonated.
Pyruvate + NADH → lactate + NAD⁺ (regenerated). Performed by animal muscle cells during intense exercise (when O₂ delivery is insufficient), some bacteria (yogurt, cheese). The primary function is NAD⁺ regeneration — this allows glycolysis to keep producing ATP when oxygen is limited. Note: lactate itself is not the primary cause of delayed muscle soreness; current evidence attributes soreness to muscle microtrauma and inflammation.
Oxygen is the terminal electron acceptor in the ETC — it accepts electrons at the end of the chain and is reduced to water. Without O₂, the ETC stops, the proton gradient collapses, ATP synthase stops, and oxidative phosphorylation ceases. The cell must rely on fermentation to regenerate NAD⁺ for continued glycolysis. Net ATP production drops from ~30–32 ATP to just 2 ATP per glucose.
Note: Oxygen is NOT a direct reactant in glycolysis or the Krebs cycle — it is only consumed at the ETC. However, aerobic respiration depends on oxygen because the ETC requires it to regenerate NAD⁺ and FAD. Without oxygen, the ETC halts, NAD⁺ and FAD cannot be regenerated, and the Krebs cycle stops. Glycolysis alone can continue briefly without oxygen (anaerobic), but the Krebs cycle cannot sustain itself.
Comparing Photosynthesis and Cellular Respiration
| Feature | Photosynthesis | Cellular Respiration |
|---|---|---|
| Organism | Autotrophs (plants, algae, cyanobacteria) | ALL organisms (including plants) |
| Energy direction | Light energy → chemical energy (glucose) | Chemical energy (glucose) → ATP |
| CO₂ | Consumed (fixed into glucose) | Produced (released from Krebs cycle) |
| O₂ | Produced (from water splitting in PSII) | Consumed (terminal electron acceptor) |
| H₂O | Consumed (split in PSII) | Produced (at end of ETC) |
| Glucose | Produced | Consumed |
| Location (eukaryotes) | Chloroplast | Cytosol (glycolysis) + Mitochondria |
| ATP synthase location | Thylakoid membrane | Inner mitochondrial membrane |
| H⁺ gradient direction | Into thylakoid lumen (↑ in lumen) | Into intermembrane space (↑ in IMS) |
Where each stage occurs: Glycolysis = cytosol. Pyruvate oxidation + Krebs = mitochondrial matrix. ETC + ATP synthase = inner mitochondrial membrane. This is tested constantly — especially "where is ATP produced?" (all four stages produce some ATP, but most comes from oxidative phosphorylation at the inner membrane).
Fermentation regenerates NAD⁺, NOT ATP. The purpose of fermentation is to keep glycolysis running. No extra ATP is made. Ethanol and lactate are just waste products of NAD⁺ regeneration.
Plants perform both photosynthesis AND cellular respiration. During the day, photosynthesis rate > respiration rate (net O₂ release). At night, only respiration occurs (net CO₂ release, O₂ consumption). This is a classic misconception the exam tests.
Uncoupling proteins / decoupling: If the inner mitochondrial membrane becomes leaky to H⁺ (e.g., due to uncoupling proteins in brown fat), the proton gradient dissipates as heat rather than ATP. This generates body heat — used by hibernating mammals and newborns for thermogenesis.
A researcher adds a chemical that makes the inner mitochondrial membrane freely permeable to H⁺ ions, but does not directly inhibit any of the ETC protein complexes or ATP synthase. What would be the most likely immediate effect on the cell?
- (A) ATP production increases because H⁺ ions can flow more freely through the membrane.
- (B) The Krebs cycle stops immediately because it requires the ETC to be functional.
- (C) ATP production decreases because the proton gradient is dissipated, so H⁺ flow through ATP synthase is reduced, and heat is released instead.
- (D) Glycolysis stops because it depends on the mitochondrial membrane potential.
Mixed Practice Questions
In an experiment, a student labels carbon dioxide with radioactive ¹⁴C and supplies it to a plant performing photosynthesis. After 30 minutes, the student would expect to find ¹⁴C most concentrated in which of the following molecules?
- (A) O₂ released by the plant
- (B) H₂O in the stroma
- (C) Glucose (carbohydrates) synthesized in the Calvin cycle
- (D) ATP produced by the light reactions
Compare the role of the electron transport chain (ETC) in photosynthesis to its role in aerobic cellular respiration. Describe the source of electrons, the location of the ETC, and the direction of H⁺ pumping in each process.
• Source of electrons: Water (H₂O), split in Photosystem II. The electrons released replace those excited and passed to the ETC from PSII.
• Location: Thylakoid membrane in the chloroplast (grana).
• Direction of H⁺ pumping: H⁺ is pumped FROM the stroma INTO the thylakoid lumen, creating a high H⁺ concentration inside the thylakoid. H⁺ then flows back through ATP synthase into the stroma → photophosphorylation (ATP synthesis).
• End product of electrons: Electrons passed to NADP⁺, reducing it to NADPH at Photosystem I.
Cellular Respiration ETC:
• Source of electrons: NADH and FADH₂, produced during glycolysis, pyruvate oxidation, and the Krebs cycle.
• Location: Inner mitochondrial membrane.
• Direction of H⁺ pumping: H⁺ is pumped FROM the matrix INTO the intermembrane space, creating a high H⁺ concentration in the IMS. H⁺ flows back through ATP synthase into the matrix → oxidative phosphorylation (ATP synthesis).
• End product of electrons: Electrons passed to O₂ (terminal electron acceptor), reducing it to H₂O.
High-Frequency Errors to Avoid
- 🌿Plants don't do cellular respiration — WRONGPlants perform both photosynthesis AND cellular respiration. Photosynthesis occurs in chloroplasts during daylight; cellular respiration occurs in mitochondria 24/7. At night, plants only respire (consume O₂, release CO₂). During the day in bright light, net photosynthesis exceeds respiration.
- 💧O₂ in photosynthesis comes from CO₂ — WRONGOxygen produced during photosynthesis comes from the splitting of WATER (H₂O) in Photosystem II — not from CO₂. The oxygen in CO₂ ends up in carbohydrates (glucose). This was proven by heavy oxygen (¹⁸O) isotope labeling experiments.
- 🔋Fermentation produces extra ATP — WRONGFermentation produces NO additional ATP beyond the 2 ATP from glycolysis. Its only purpose is to regenerate NAD⁺ from NADH so that glycolysis can continue. The ethanol or lactic acid are simply byproducts of NAD⁺ regeneration.
- ⚡Most ATP in respiration comes from glycolysis or Krebs — WRONG~90% of ATP comes from oxidative phosphorylation (ETC + ATP synthase) at the inner mitochondrial membrane. Glycolysis yields only 2 ATP; Krebs only 2 ATP directly. The NADH and FADH₂ from all stages then produce ~26–28 ATP via the ETC.
- 🧪Competitive and noncompetitive inhibitors both reduce Vmax — WRONGOnly noncompetitive inhibitors reduce Vmax (because even saturating the enzyme with substrate can't overcome the structural distortion). Competitive inhibitors leave Vmax unchanged — they only increase the apparent Km (more substrate needed to reach the same Vmax).
- 🔄The Calvin cycle happens in the dark — WRONGThe Calvin cycle is called "light-independent" because it doesn't directly use light — but it requires ATP and NADPH from the light reactions. In the dark, the light reactions stop, ATP and NADPH are depleted, and the Calvin cycle halts. "Dark reactions" is a misleading historical term.
- 🔬Glycolysis requires mitochondria — WRONGGlycolysis occurs in the CYTOSOL and requires no mitochondria. This is why prokaryotes (no mitochondria) can perform glycolysis. It's the only ATP-producing pathway available to anaerobic organisms.
Unit 3 — Key Takeaways
Biological catalysts that lower activation energy. Induced-fit model: active site reshapes to bind substrate. Specific; reusable; do not change ΔG or equilibrium position.
Optimal temp and pH; denaturation above optimum. Competitive inhibitor: binds active site, ↑ Km, same Vmax. Noncompetitive: binds allosteric site, ↓ Vmax. Feedback inhibition controls metabolic pathways.
ATP = universal energy currency; hydrolysis to ADP+Pᵢ releases energy. OIL RIG: oxidation = electron loss, reduction = gain. NADH, FADH₂ carry electrons to ETC. NAD⁺/NADP⁺ are electron acceptors.
Light reactions (thylakoid): H₂O split → O₂; ATP + NADPH produced by ETC + chemiosmosis. Calvin cycle (stroma): CO₂ + ATP + NADPH → G3P → glucose. O₂ from H₂O, not CO₂.
Glycolysis (cytosol) → Krebs (matrix) → ETC + ATP synthase (inner membrane) → ~30–32 ATP. Fermentation: no O₂, only 2 ATP, regenerates NAD⁺. O₂ = terminal electron acceptor → H₂O.
Unit 3 = 12–16% of the AP Biology Exam — the single highest-weighted unit. The top exam topics are: enzyme inhibition (competitive vs. noncompetitive, graph interpretation), inputs/outputs of all 4 stages of cellular respiration (with locations), chemiosmosis and ATP synthase in both photosynthesis and respiration, the source of O₂ in photosynthesis, fermentation purpose (NAD⁺ regeneration), and factors affecting photosynthesis rate. Do NOT memorize specific Krebs cycle or glycolysis intermediate names — the exclusion statements are clear. Focus on concept, not names.