Cellular Energetics: every key term you need (+ practice quiz)
99 flashcard terms for AP Biology Unit 3, written to match the course framework. Read them here, drill them as flashcards, or take the 51-question quiz. Free, no account needed.
The sum of all chemical reactions in a cell — building molecules (anabolism) and breaking them down (catabolism) to manage energy and matter.
Anabolism vs. Catabolism
Anabolic reactions build complex molecules and require energy (endergonic); catabolic reactions break molecules down and release energy (exergonic).
First Law of Thermodynamics
Energy cannot be created or destroyed, only transformed. Cells convert energy (e.g., light or chemical bonds) from one form to another, never gaining it from nothing.
Second Law of Thermodynamics
Every energy transfer increases the entropy (disorder) of the universe; some energy is always lost as heat. Cells must constantly take in energy to stay ordered.
Entropy
A measure of disorder. Living systems maintain internal order only by increasing the disorder of their surroundings — they are not exempt from the second law.
Free Energy
The portion of a system's energy available to do work. Reactions proceed spontaneously when they release free energy (negative ΔG).
Exergonic Reaction
Releases free energy (products have less energy than reactants); spontaneous. Cellular respiration is exergonic overall.
Endergonic Reaction
Absorbs free energy (products have more energy than reactants); not spontaneous, requires an energy input. Photosynthesis is endergonic.
Energy Coupling
Using the energy released by an exergonic reaction (like ATP hydrolysis) to drive an endergonic one. ATP is the cell's coupling agent.
ATP
Adenosine triphosphate — the cell's energy currency. Hydrolysis of its terminal phosphate bond (ATP → ADP + Pi) releases energy to power cellular work.
ATP–ADP Cycle
Cells recharge ADP + Pi back into ATP using energy from catabolism, then spend ATP on work — a continuous cycle of energy transfer.
Enzyme
A biological catalyst (usually a protein) that speeds up a reaction by lowering its activation energy, without being consumed.
Activation Energy
The energy barrier that must be overcome to start a reaction. Enzymes lower it, so reactions proceed faster at cellular temperatures.
Active Site
The specific region of an enzyme where the substrate binds and the reaction occurs. Its shape determines the enzyme's specificity.
Substrate
The reactant molecule an enzyme acts upon. It binds to the active site to form an enzyme-substrate complex.
Enzyme Specificity
Each enzyme catalyzes a specific reaction because only a matching substrate fits its active site — structure determines function.
Induced Fit Model
The active site changes shape slightly to fit the substrate snugly when it binds, improving catalysis — a refinement of the 'lock-and-key' idea.
Enzymes Are Not Consumed
An enzyme is released unchanged after catalysis and can be reused, so a small amount catalyzes many reactions.
Rate rises with temperature up to an optimum, then falls sharply as heat denatures the enzyme, destroying its active-site shape.
Effect of pH on Enzymes
Each enzyme has an optimal pH; deviations disrupt bonds in the active site, lowering activity or denaturing the enzyme.
Effect of Substrate Concentration
Rate increases with more substrate until enzymes are saturated (all active sites busy), after which rate plateaus at its maximum.
Denaturation of Enzymes
Loss of an enzyme's shape (from heat, pH, or salts) that destroys the active site and abolishes function — structure determines function.
Competitive Inhibition
An inhibitor resembles the substrate and blocks the active site. It can be overcome by adding more substrate.
Noncompetitive (Allosteric) Inhibition
An inhibitor binds a site other than the active site, changing the enzyme's shape so the substrate no longer fits. Adding substrate does not reverse it.
Allosteric Regulation
Molecules bind a regulatory (allosteric) site to activate or inhibit an enzyme by changing its shape — a way cells control metabolic pathways.
Feedback Inhibition
A pathway's final product inhibits an enzyme earlier in the pathway, shutting down its own production when levels are high — self-regulation.
Cofactors and Coenzymes
Nonprotein helpers required by some enzymes: cofactors are inorganic ions (like Mg²⁺); coenzymes are organic (often vitamins, like NAD⁺).
Cellular Respiration
The catabolic process that harvests energy from glucose to make ATP: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP. Overall exergonic.
Redox Reactions in Respiration
Energy is harvested by transferring electrons: glucose is oxidized (loses electrons) and oxygen is reduced (gains them), releasing energy step by step.
NAD⁺ / NADH
An electron carrier (coenzyme). NAD⁺ picks up electrons (and H) to become NADH, ferrying them to the electron transport chain.
FAD / FADH₂
Another electron carrier that delivers electrons from the Krebs cycle to the electron transport chain as FADH₂.
Glycolysis
Splits glucose (6C) into two pyruvate (3C) in the cytoplasm. Net yield: 2 ATP and 2 NADH. Anaerobic — no oxygen required.
Location of Glycolysis
Occurs in the cytosol (cytoplasm) of all cells — the universal, ancient first step of glucose breakdown.
Pyruvate Oxidation
Each pyruvate enters the mitochondrion and is converted to acetyl-CoA, releasing CO₂ and making NADH — the link between glycolysis and the Krebs cycle.
Krebs Cycle (Citric Acid Cycle)
In the mitochondrial matrix, acetyl-CoA is fully oxidized, releasing CO₂ and generating ATP, NADH, and FADH₂ (per turn); runs twice per glucose.
Location of Krebs Cycle
The mitochondrial matrix — the fluid inside the inner membrane.
The stage that makes most ATP: the electron transport chain plus chemiosmosis, using electrons from NADH and FADH₂.
Electron Transport Chain (ETC)
A series of proteins in the inner mitochondrial membrane that passes electrons down an energy gradient, pumping H⁺ into the intermembrane space.
Chemiosmosis
H⁺ ions flow back through ATP synthase down their gradient, and the energy released drives ATP synthesis — the mechanism of oxidative phosphorylation.
ATP Synthase
An enzyme in the inner membrane that acts like a turbine: protons flowing through it spin it, powering the phosphorylation of ADP into ATP.
Oxygen as Final Electron Acceptor
At the end of the ETC, oxygen accepts the spent electrons and combines with H⁺ to form water. Without O₂, the chain backs up and stops.
Proton (H⁺) Gradient
The ETC pumps H⁺ into the intermembrane space, storing potential energy as an electrochemical gradient that powers ATP synthase.
ATP Yield of Respiration
Aerobic respiration yields roughly 30–38 ATP per glucose — the vast majority from oxidative phosphorylation, only 4 (net 2+2) from glycolysis and Krebs directly.
Anaerobic Respiration & Fermentation
Without oxygen, cells use fermentation to regenerate NAD⁺ so glycolysis can continue, producing only the 2 ATP from glycolysis.
Lactic Acid Fermentation
In muscle and some microbes, pyruvate is reduced to lactate to regenerate NAD⁺ — no extra ATP, but glycolysis keeps running (causes muscle fatigue).
Alcoholic Fermentation
In yeast, pyruvate is converted to ethanol and CO₂ to regenerate NAD⁺ — the basis of bread, beer, and wine.
Why Fermentation Matters
Its purpose is to recycle NAD⁺, not to make ATP. Without regenerated NAD⁺, glycolysis (and all ATP production) would halt.
Photosynthesis
The anabolic process by which plants, algae, and cyanobacteria convert light energy into chemical energy: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂.
Autotroph vs. Heterotroph
Autotrophs (producers) make their own food from inorganic sources (photosynthesis); heterotrophs (consumers) obtain energy by eating other organisms.
Chloroplast Structure
Photosynthesis occurs in chloroplasts: light reactions in the thylakoid membranes (stacked into grana); the Calvin cycle in the surrounding stroma.
Chlorophyll & Pigments
Pigments in the thylakoid membrane (chlorophyll a, b, carotenoids) absorb light energy, exciting electrons to begin the light reactions.
Light-Dependent Reactions
In the thylakoid membrane, light energizes electrons; water is split (releasing O₂); the ETC pumps H⁺ and produces ATP and NADPH.
Photolysis of Water
Light-driven splitting of water (H₂O) in the light reactions, supplying electrons to replace those lost by chlorophyll and releasing O₂ as a byproduct.
Source of Photosynthetic Oxygen
The O₂ released comes from splitting water, not from CO₂ — a key experimental fact.
Clusters of pigments and proteins in the thylakoid that capture light. Photosystem II supplies the ETC; Photosystem I re-energizes electrons to make NADPH.
NADPH
An electron carrier made in the light reactions that delivers high-energy electrons (reducing power) to the Calvin cycle.
Light-Independent Reactions (Calvin Cycle)
In the stroma, ATP and NADPH from the light reactions power the fixation of CO₂ into sugar (G3P). No light directly required, but depends on light-reaction products.
Carbon Fixation
The incorporation of inorganic CO₂ into an organic molecule — the first step of the Calvin cycle, catalyzed by the enzyme RuBisCO.
RuBisCO
The enzyme that catalyzes carbon fixation in the Calvin cycle; the most abundant protein on Earth.
G3P (Glyceraldehyde-3-Phosphate)
The three-carbon sugar product of the Calvin cycle; two G3P combine to form glucose and other carbohydrates.
Link Between Light and Calvin Reactions
The Calvin cycle requires the ATP and NADPH made by the light reactions; in the dark those run out, so the Calvin cycle stops.
Respiration vs. Photosynthesis
They are roughly opposite: photosynthesis stores light energy in glucose (uses CO₂/H₂O, releases O₂); respiration releases that energy (uses glucose/O₂, releases CO₂/H₂O).
Chemiosmosis in Both Processes
Both photosynthesis and respiration use an electron transport chain to build an H⁺ gradient that ATP synthase uses to make ATP — a shared, ancient mechanism.
Mitochondria and Chloroplasts Compared
Mitochondria harvest energy (respiration) in nearly all eukaryotes; chloroplasts capture energy (photosynthesis) in plants/algae. Both have their own DNA (endosymbiosis).
Energy Flow Through Life
Energy enters ecosystems as sunlight, is captured by photosynthesis, passed along by respiration, and ultimately lost as heat — obeying the laws of thermodynamics.
Metabolic Pathways
Sequences of enzyme-catalyzed reactions where each product is the next reaction's substrate; regulated (e.g., by feedback inhibition) to control energy and materials.
Gibbs Free Energy Change (ΔG)
ΔG = ΔH − TΔS. Negative ΔG means a spontaneous, exergonic reaction; ATP hydrolysis has ΔG ≈ −7.3 kcal/mol under standard conditions.
Enzyme Kinetics: Vmax and Saturation
Reaction rate rises with substrate concentration until all active sites are occupied (Vmax); adding more substrate no longer increases rate.
Competitive vs. Noncompetitive on a Graph
Competitive inhibitors raise apparent Km but leave Vmax unchanged (overcome by more substrate); noncompetitive inhibitors lower Vmax.
Enzyme Cofactor Example
Mg²⁺ is required by many ATP-using enzymes; coenzymes such as NAD⁺ and FAD are organic carriers derived from vitamins B3 and B2.
Substrate-Level Phosphorylation
An enzyme transfers a phosphate directly from a substrate to ADP, as in glycolysis and the Krebs cycle — no membrane gradient needed.
Glycolysis Bookkeeping
Per glucose: invests 2 ATP, produces 4 ATP (net 2), 2 NADH, and 2 pyruvate. Occurs in cytosol, works with or without O₂.
Per glucose (2 turns): 6 NADH, 2 FADH₂, 2 ATP (GTP), 4 CO₂. Acetyl-CoA enters by joining oxaloacetate to form citrate.
Why NADH Yields More Than FADH₂
NADH donates electrons at Complex I, pumping protons at three sites (~2.5 ATP); FADH₂ enters at Complex II, bypassing one pump (~1.5 ATP).
Uncouplers
Molecules such as DNP or the protein thermogenin let H⁺ leak across the inner membrane, so energy is released as heat rather than ATP — brown fat thermogenesis.
Cyanide Poisoning
Cyanide binds cytochrome c oxidase (Complex IV), stopping electron flow to O₂; the ETC backs up, NADH accumulates, and ATP synthesis halts.
Absorption vs. Action Spectrum
Absorption spectrum shows wavelengths a pigment absorbs; action spectrum shows photosynthetic rate at each wavelength. Their close match (Engelmann's experiment) proved chlorophyll drives photosynthesis.
Electrons from PSI cycle back to the cytochrome complex, producing extra ATP without NADPH or O₂ to meet the Calvin cycle's 3:2 ATP:NADPH demand.
Calvin Cycle Bookkeeping
To make one G3P (3 carbons): 3 CO₂ fixed, 9 ATP and 6 NADPH consumed, 5 G3P recycled to regenerate 3 RuBP.
Photorespiration
RuBisCO can bind O₂ instead of CO₂ on hot, dry days when stomata close, wasting carbon and energy — a problem C4 and CAM plants solve.
C4 and CAM Strategies
C4 plants (corn) fix CO₂ into 4-carbon compounds in mesophyll and shuttle it to bundle-sheath cells; CAM plants (cacti) open stomata at night and store CO₂ as malate.
Chemiosmosis Comparison
In mitochondria H⁺ is pumped into the intermembrane space; in chloroplasts H⁺ accumulates in the thylakoid lumen. Both drive ATP synthase as H⁺ flows back.
Energy Coupling
An exergonic reaction such as ATP hydrolysis is linked to an endergonic one, usually by transferring a phosphate to make a higher-energy intermediate. The coupled sum must be negative ΔG.
Activation Energy Is Independent of ΔG
Enzymes lower Ea and speed the approach to equilibrium, but they never change the free-energy difference between reactants and products or the position of equilibrium.
Allosteric Regulation
A regulatory molecule binds a site other than the active site, shifting the enzyme between more-active and less-active conformations. It allows fine, reversible metabolic control.
Cooperativity
Substrate binding at one subunit changes the shape of others so they bind more readily, producing a sigmoidal rate curve — as in hemoglobin's oxygen binding.
Feedback Inhibition Example
Isoleucine allosterically inhibits threonine deaminase, the first enzyme of its own synthesis pathway, so the cell stops making what it already has.
Enzyme pH Optima
Pepsin works near pH 2 in the stomach; trypsin works near pH 8 in the small intestine. Optimum reflects the environment where the enzyme's ionizable R groups hold the active shape.
Km as an Affinity Measure
Km is the substrate concentration giving half of Vmax. A low Km means high affinity — the enzyme saturates at low substrate levels.
Reaction rate roughly doubles or triples per 10 °C rise until denaturation, because more collisions exceed the activation energy. Beyond the optimum, rate collapses sharply.
Proton Motive Force
The combined pH and charge gradient across a membrane stores potential energy. It powers ATP synthase, bacterial flagella, and some active transport.
ATP Synthase Mechanism
Protons flowing down their gradient spin a rotor, and the mechanical rotation forces conformational changes in the catalytic head that join ADP + Pi. It is a molecular turbine.
Where the Gradients Sit
In mitochondria protons accumulate in the intermembrane space; in chloroplasts they accumulate in the thylakoid lumen. In both, ATP is made on the side where the enzyme's head projects into the matrix or stroma.
Respiratory Quotient (RQ)
RQ = CO2 produced ÷ O2 consumed. Carbohydrate oxidation gives about 1.0, fat about 0.7 — a respirometer can therefore reveal the fuel being burned.
Why Fermentation Exists
Fermentation regenerates NAD+ from NADH so glycolysis can continue producing 2 ATP per glucose when no oxygen is available. It makes no ATP itself.
Electron Carrier Ordering
Carriers in the transport chain are arranged by increasing electron affinity, so electrons fall stepwise to oxygen, releasing energy in usable increments rather than one destructive burst.
Rubisco's Dual Nature
Rubisco can fix CO2 (productive) or O2 (photorespiration, wasteful). Its relative affinity for the two gases shapes plant adaptations to hot, dry climates.
Floating Leaf Disk Assay
Vacuum-infiltrated leaf disks sink; as photosynthesis releases O2 into the airspaces they rise. The time for half the disks to float (ET50) measures photosynthetic rate.