Bioenergetics and Thermodynamics in Biological Systems
In 30 seconds:
Bioenergetics is the study of energy flow in living systems — your body's battery management system. The core idea: energy-releasing reactions (exergonic) are coupled to energy-requiring ones (endergonic) via ATP, the universal energy currency.
Key exam topics:
First and second laws of thermodynamics applied to biological systems
Gibbs free energy (ΔG) and reaction spontaneity
ATP structure, high-energy bonds, and energy charge concept
Most common trap:
Students confuse ΔG°' (standard conditions) with actual ΔG (cellular conditions). A reaction with positive ΔG°' can still proceed in cells if substrates/products are kept far from equilibrium.
Let's start with the big picture: your body is a non-stop energy conversion machine. Every heartbeat, thought, and muscle contraction runs on carefully coupled energy transfers. Here's what you need to know for the exam.
Introduction to Bioenergetics
Bioenergetics encompasses the flow of energy in living organisms.
The first law of thermodynamics (conservation of energy) states that energy cannot be created or destroyed, only converted between forms. The second law states that the entropy (disorder) of the universe increases in all spontaneous processes.
Living organisms maintain order by coupling exergonic reactions (energy-releasing, ΔG < 0) to endergonic reactions (energy-requiring, ΔG > 0) through common intermediates, with ATP serving as the universal energy currency.
Gibbs Free Energy
The Gibbs free energy equation (ΔG = ΔH − TΔS) determines whether a reaction is thermodynamically favorable. The standard free energy change (ΔG°') is defined at pH 7.0, 25°C, and 1 M concentrations. The actual ΔG for any reaction is related to ΔG°' by the equation ΔG = ΔG°' + RT ln([products]/[reactants]).
Reactions with a large negative ΔG°' are irreversible under cellular conditions and are common regulatory points in metabolic pathways.
Common regulatory points include hexokinase, PFK-1, and pyruvate kinase in glycolysis; pyruvate dehydrogenase; and citrate synthase in the TCA cycle. Students often forget that pyruvate dehydrogenase is a regulatory point even though it is not part of glycolysis or the TCA cycle itself.
ATP: The Universal Energy Currency
ATP (adenosine triphosphate) has a high free energy of hydrolysis (ΔG°' = −30.5 kJ/mol) due to electrostatic repulsion between the four negative charges on the triphosphate chain at pH 7, resonance stabilization of the products (ADP and Pi), and the favorable entropy increase.
Biosynthetic reactions — phosphorylation of substrates
Active transport — Na+/K+ ATPase, Ca2+ ATPase
Mechanical work — muscle contraction, myosin ATPase
Signal amplification — protein kinases, second messenger synthesis
Energy Coupling and Energy Charge
The concept of "energy charge" (Atkinson, 1968) ranges from 0 (all AMP) to 1 (all ATP). Most cells maintain an energy charge of 0.85‑0.95.
High energy charge inhibits ATP-generating catabolic pathways (glycolysis, TCA cycle) and stimulates ATP-utilizing anabolic pathways (gluconeogenesis, fatty acid synthesis).
The adenylate kinase reaction (2 ADP → ATP + AMP) provides a sensitive metabolic signal: AMP is a powerful allosteric activator of many catabolic enzymes.
AMPK (AMP-activated protein kinase) is a master metabolic regulator. Rising AMP levels during cellular energy stress activate AMPK, which promotes catabolic (ATP-generating) pathways and inhibits anabolic (ATP-consuming) pathways. Metformin and exercise exert some of their beneficial effects through AMPK activation.
High-yield NEET PG question: The creatine kinase reaction (creatine + ATP → creatine phosphate + ADP) has a ΔG°' of +12.6 kJ/mol. Why does it proceed forward in resting muscle? Answer: The high [ATP]/[ADP] ratio maintains a negative actual ΔG — illustrating ΔG°' vs actual ΔG under cellular conditions.
Cellular energy transduction: ATP synthesis and utilization
Enzyme Kinetics and Regulation
Enzyme Kinetics
Enzymes are protein catalysts that accelerate chemical reactions by lowering the activation energy without being consumed.
The Michaelis-Menten model describes enzyme kinetics: E + S ⇌ ES → E + P.
The Michaelis constant (Km) is the substrate concentration at which the reaction rate is half of Vmax, representing an apparent dissociation constant. A low Km indicates high affinity of the enzyme for its substrate.
The turnover number (kcat = Vmax / [Etotal]) is the number of substrate molecules converted per enzyme molecule per second at saturating substrate concentrations. The catalytic efficiency (kcat / Km) is used to compare different enzymes or the same enzyme with different substrates.
Lineweaver-Burk and Related Plots
The Lineweaver-Burk plot (double-reciprocal plot, 1/v vs 1/[S]) gives the y-intercept = 1/Vmax and x-intercept = −1/Km, but is susceptible to error at low substrate concentrations. The Eadie-Hofstee plot and Hanes-Woolf plot are more accurate alternatives.
Types of Enzyme Inhibition
Competitive inhibition — inhibitor binds reversibly to the active site, increasing the apparent Km (Vmax unchanged), overcome by high [S]
Uncompetitive inhibition — inhibitor binds only to the ES complex, decreasing both Vmax and Km
Noncompetitive (mixed) inhibition — inhibitor binds to both E and ES, decreasing Vmax without changing Km
Irreversible inhibition — inhibitor covalently modifies the active site (e.g., aspirin acetylates cyclooxygenase, organophosphates inhibit AChE)
Allosteric Regulation
Allosteric enzymes (e.g., aspartate transcarbamoylase, PFK-1) have multiple subunits and undergo conformational changes upon binding regulatory molecules.
Allosteric activators shift the enzyme to the R (relaxed, active) state; allosteric inhibitors shift it to the T (tense, inactive) state. The sigmoidal kinetics of allosteric enzymes result from cooperative substrate binding (similar to hemoglobin).
Homotropic allostery: the substrate itself acts as an allosteric activator (positive cooperativity). Heterotropic allostery: a different molecule (effector) modulates the enzyme's activity.
Covalent Modification
Phosphorylation (by protein kinases) is the most common and versatile regulatory mechanism. The kinase/phosphatase system provides rapid, reversible, and highly regulated control.
Glycogen phosphorylase is activated by phosphorylation; glycogen synthase is inactivated by phosphorylation. Students often reverse these — remember: phosphorylase needs phosphate to break down glycogen (phosphorolysis), so it is active when phosphorylated.
Other covalent modifications include acetylation (histone acetylation regulates gene expression), ubiquitination (targets proteins for proteasomal degradation), sumoylation, methylation, ADP-ribosylation, and proteolytic cleavage (zymogen activation — trypsinogen → trypsin by enteropeptidase). Zymogen activation cascades are critical in digestion, blood coagulation, and apoptosis.
Detailed Michaelis-Menten Kinetics
Michaelis-Menten equation: v = Vmax × [S] / (Km + [S]) — describes the hyperbolic relationship between initial velocity and substrate concentration for enzyme-catalyzed reactions.
Km (Michaelis constant)
= substrate concentration at which v = Vmax/2. Represents the apparent dissociation constant of the ES complex. Low Km = high affinity. Km is independent of enzyme concentration — a characteristic property of each enzyme-substrate pair.
Vmax
= maximum velocity when all enzyme active sites are saturated with substrate. Depends on enzyme concentration and kcat (turnover number). Vmax = kcat × [Etotal].
kcat (turnover number)
= number of substrate molecules converted to product per enzyme per second. Catalytic efficiency = kcat / Km — the best parameter to compare enzyme performance (units: M-1s-1).
Enzyme
Km (M)
kcat (s-1)
kcat/Km (M-1s-1)
Substrate
Catalase
2.5 × 10-2
1.0 × 107
4.0 × 108
H2O2
Acetylcholinesterase
9.0 × 10-5
1.4 × 104
1.6 × 108
ACh
Fumarase
5.0 × 10-6
8.0 × 102
1.6 × 108
Fumarate
Lineweaver-Burk Double-Reciprocal Plot
The Lineweaver-Burk plot (1/v vs 1/[S]) linearizes the Michaelis-Menten equation: 1/v = (Km/Vmax)(1/[S]) + 1/Vmax.
Limitations: (1) susceptible to error amplification at low [S] (large reciprocal values), (2) small errors at high [S] become magnified. Alternative linearizations: Eadie-Hofstee (v vs v/[S]), Hanes-Woolf ([S]/v vs [S]).
Lithium (inhibition of inositol monophosphatase — decreases Vmax)
Lithium (uncompetitive inhibition of GSK-3β — both Km and Vmax decrease)
Competitive: "C" for "Competes at the active site" → increased Km, Vmax unchanged. Non-competitive: "N" for "No change in Km" → Vmax decreases. Uncompetitive: "U" for "Underground (ES only)" → both Km and Vmax decrease.
Methotrexate (MTX) is a competitive inhibitor of dihydrofolate reductase (DHFR) — the Km for DHFR increases in the presence of MTX, but Vmax is unchanged. This competitive inhibition can be overcome by increasing the concentration of the natural substrate (dihydrofolate) — but clinically, MTX is tightly bound and the inhibition is essentially irreversible at the cellular level. Leucovorin (folinic acid) provides a reduced folate source that bypasses DHFR, acting as a "rescue" agent.
PYQ: On a Lineweaver-Burk plot, competitive inhibition shows lines that intersect at the Y-axis. What do non-competitive and uncompetitive patterns look like? Answer: Non-competitive: lines intersect on the X-axis. Uncompetitive: parallel lines with the same slope but different intercepts.
Protein Structure: Levels and Stabilizing Bonds
Proteins are linear polymers of amino acids that fold into specific three-dimensional structures determined by their amino acid sequence.
Level
Description
Stabilizing Bonds
Example
Primary (1°)
Linear sequence of amino acids (AA residues)
Covalent peptide bonds
(between α-COO- and α-NH3+)
Insulin A and B chains, hemoglobin α and β chains
Secondary (2°)
Local regular repeating structures: α-helix (right-handed, 3.6 AA/turn, H-bonds between n and n+4), β-sheet (parallel, antiparallel), β-turns, loops
Hydrogen bonds
between backbone C=O and N-H groups; proline breaks α-helices (rigid ring, no NH for H-bond); glycine common in turns (flexible)
Collagen is the most abundant protein in the human body — characterized by the repeating Gly-X-Y triplet where X is often proline and Y is often hydroxyproline (4-hydroxyproline, requires vitamin C for synthesis).
Structure:
Three polypeptide α-chains form a right-handed triple helix (tropocollagen) — every third residue is glycine (small enough to fit in the helical core). Hydroxyproline and hydroxylysine (modified post-translationally) form interchain H-bonds that stabilize the triple helix.
Synthesis:
Preprocollagen → propeptide cleavage → tropocollagen → covalent cross-linking (lysyl oxidase, Cu2+-dependent) → collagen fibrils. Vitamin C is a cofactor for prolyl and lysyl hydroxylases → scurvy impairs collagen synthesis (poor wound healing, fragile vessels).
Types:
Type I (bone, tendon, skin — most abundant), Type II (cartilage), Type III (reticular fibers — blood vessels, skin), Type IV (basement membrane — network-forming, non-fibrillar).
Hemoglobin (Hb) is a tetrameric heme protein (α2β2 in adults, HbA) that binds oxygen cooperatively — the hallmark of allosteric proteins.
Oxygen dissociation curve (ODC)
is sigmoidal due to cooperative binding: binding of O2 to one heme increases affinity of remaining hemes for O2 (T → R state transition). P50 (PO2 at 50% saturation) is ~26 mmHg for normal adult Hb — a measure of O2 affinity.
Factor
Effect on ODC
Mechanism
Clinical Significance
↓pH (Bohr effect)
Right shift (↓affinity)
H+ stabilizes T state (His146 — salt bridges)
Metabolically active tissues (lactic acid, CO2) → more O2 unloading
↑PCO2 (Bohr effect)
Right shift (↓affinity)
CO2 forms carbaminohemoglobin + H+
Same as above — enhances O2 delivery
↑2,3-BPG (Haldane effect)
Right shift (↓affinity)
2,3-BPG binds to β-chain cavity, stabilizes T state
Anemia, hypoxia, high altitude (↑2,3-BPG → ↑O2 unloading)
↑Temperature
Right shift (↓affinity)
Fever destabilizes R state
Exercise, fever → more O2 delivery
↓CO (carbon monoxide)
Left shift (↑affinity)
CO binds heme iron with 250× higher affinity than O2 + increases O2 affinity of remaining hemes
CO poisoning: left-shifted curve + reduced O2-carrying capacity → severe tissue hypoxia despite normal PO2
Fetal Hb (HbF, α2γ2)
Left shift (↑affinity)
γ chain lacks His143 → reduced 2,3-BPG binding
Facilitates O2 transfer from maternal to fetal circulation
Right shift (↓Hb-O2 affinity, ↑O2 unloading): "CADET, face right!" — CO2, Acid (↓pH), 2,3-DPG, Exercise, Temperature (↑). Left shift (↑affinity): "COLD" — CO, aLkalosis, DPG (↓), fetal Hb.
CO poisoning is a classic exam trap: Left-shifted ODC + decreased O2 carrying capacity. Pulse oximetry is falsely normal (COHb reads as oxyhemoglobin). ABG shows normal PaO2 but low O2 content. Treatment: 100% O2 or hyperbaric O2 therapy.
A 60-year-old with chronic kidney disease (anemia of chronic disease) has Hb 8 g/dL. The oxygen dissociation curve is right-shifted due to elevated 2,3-BPG — this compensates for anemia by improving O2 unloading. In contrast, stored blood for transfusion has depleted 2,3-BPG (left shift) → transfused RBCs initially have increased O2 affinity until 2,3-BPG regenerates over 24-48 hours.
PYQ: What is the P50 of normal adult hemoglobin? Answer: ~26 mmHg — the PO2 at which hemoglobin is 50% saturated with oxygen.
PYQ: A patient with CO poisoning has normal PaO2 on ABG but severe tissue hypoxia. Why? Answer: CO binds heme with 250× greater affinity than O2, reducing O2-carrying capacity AND shifting the ODC leftward (increased O2 affinity), further impairing O2 unloading to tissues.
Michaelis-Menten kinetics and allosteric regulation of enzymes
Hemoglobin structure and oxygen dissociation curve showing allosteric regulation by 2,3-BPG, pH, and CO2
Cellular Respiration: Glycolysis and the TCA Cycle
Glycolysis Overview
Glycolysis is the cytoplasmic pathway (Embden-Meyerhof-Parnas) that converts one molecule of glucose (6C) into two molecules of pyruvate (3C) with a net gain of 2 ATP and 2 NADH.
The pathway has two phases: the energy investment phase (consumes 2 ATP) and the energy payoff phase (produces 4 ATP).
Key Regulatory Enzymes of Glycolysis
Hexokinase — irreversible, inhibited by glucose-6-phosphate. Hexokinase IV (glucokinase) in the liver has a higher Km for glucose and is not inhibited by G6P, allowing the liver to phosphorylate glucose even at high blood glucose levels after a meal.
PFK-1 — the rate-limiting enzyme of glycolysis, activated by AMP and fructose-2,6-bisphosphate, and inhibited by ATP and citrate.
Pyruvate kinase — activated by fructose-1,6-bisphosphate, inhibited by ATP and alanine.
Fate of Pyruvate
The fate of pyruvate depends on oxygen availability. Under aerobic conditions, pyruvate is transported into the mitochondrial matrix and oxidatively decarboxylated by the pyruvate dehydrogenase complex (PDC).
PDC is a massive multi-enzyme complex (MW ~9.5 million Da) consisting of three catalytic components: E1 (pyruvate dehydrogenase, with thiamine pyrophosphate), E2 (dihydrolipoyl transacetylase, transfers the acetyl group to CoA forming acetyl-CoA), and E3 (dihydrolipoyl dehydrogenase, with FAD, regenerates oxidized lipoamide).
PDC is regulated by product inhibition (acetyl-CoA, NADH) and by phosphorylation (inactivation by PDK, activation by PDP). In thiamine (B1) deficiency, PDC activity is impaired because E1 requires TPP, leading to lactic acidosis and neurological symptoms (Wernicke-Korsakoff syndrome).
The Tricarboxylic Acid (TCA) Cycle
The tricarboxylic acid (TCA) cycle occurs in the mitochondrial matrix. Acetyl-CoA (2C) enters by condensing with oxaloacetate (4C) to form citrate (6C), catalyzed by citrate synthase.
Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase are the major regulatory enzymes of the TCA cycle.
3 NADH
1 FADH2
1 GTP
2 CO2
The TCA cycle is the central hub of metabolism, receiving inputs from carbohydrate, lipid, and amino acid catabolism and providing intermediates for biosynthesis via anaplerotic reactions.
Pyruvate carboxylase (biotin-dependent) converts pyruvate to oxaloacetate — a key anaplerotic reaction. Biotin deficiency impairs this reaction and the TCA cycle.
Overview of glycolysis and the citric acid cycle with energy yields
Oxidative Phosphorylation and the Electron Transport Chain
Electron Transport Chain Overview
The electron transport chain (ETC) is located in the inner mitochondrial membrane and consists of four multi-protein complexes (Complexes I‑IV) and two mobile electron carriers (ubiquinone/coenzyme Q and cytochrome c).
The chain transfers electrons from NADH and FADH2 to molecular oxygen, creating a proton gradient (proton motive force) that drives ATP synthesis by ATP synthase (Complex V).
Complexes of the ETC
Complex I (NADH:ubiquinone oxidoreductase) — accepts electrons from NADH, translocates 4 H+ per NADH. Inhibitors: rotenone, amytal
Complex II (succinate dehydrogenase) — accepts electrons from FADH2, does not pump H+
Complex III (cytochrome bc1) — uses the Q cycle to translocate 4 H+ per 2 electrons. Inhibitor: antimycin A
Complex IV (cytochrome c oxidase) — transfers electrons to O2, reducing it to 2 H2O, translocates 2 H+. Inhibitors: cyanide, carbon monoxide, azide (bind to heme a3-CuB binuclear center)
A common exam question asks: "Which complex of the ETC is inhibited by cyanide?" The answer is Complex IV (cytochrome c oxidase). Another frequent question: "Which complex does not pump protons?" — Complex II (succinate dehydrogenase).
ATP Synthase (Complex V)
ATP synthase (Complex V, F1FO-ATPase) uses the flow of H+ back into the matrix through the FO channel to drive rotation of the central stalk, causing conformational changes in the F1 catalytic head.
The binding change mechanism (Boyer, 1973): the three β subunits alternate between open (ADP + Pi release), loose (ADP + Pi binding), and tight (ATP synthesis) conformations. Each full rotation produces 3 ATP. Approximately 3‑4 H+ are required per ATP synthesized.
Chemiosmotic Theory
The chemiosmotic theory (Peter Mitchell, Nobel Prize 1978): the proton motive force has two components — the electrical gradient (ΔΨ, inside negative by 150‑180 mV) and the pH gradient (ΔpH, matrix alkaline by 0.5‑1.0 unit).
Uncouplers (dinitrophenol, thermogenin/UCP1 in brown fat, FCCP) dissipate the proton gradient, generating heat instead of ATP. UCP1 in brown adipose tissue is important for non-shivering thermogenesis in neonates and in adults with metabolically active brown fat.
NADH → Complex I → Q → Complex III → Cyt c → Complex IV → O2
FADH2 → Complex II → Q → Complex III → Cyt c → Complex IV → O2
High-yield ETC questions: (1) Rotenone inhibits Complex I → ATP depletion. (2) Antimycin A inhibits Complex III → superoxide production. (3) Cyanide, CO, azide inhibit Complex IV → histotoxic hypoxia. (4) Oligomycin inhibits ATP synthase (Complex V). (5) DNP uncouples ETC from ATP synthesis → heat generation.
ETC inhibitors: "ROtten Ants CAN't COpe" — Rotenone (CI), Antimycin A (CIII), Cyanide (CIV), Carbon monoxide (CIV), Oligomycin (CV).
Complex II (succinate dehydrogenase) is also part of the TCA cycle — the only membrane-bound TCA cycle enzyme. It does not pump protons, explaining why FADH2 yields fewer ATP than NADH (~1.5 vs ~2.5 ATP).
The mitochondrial electron transport chain and chemiosmotic ATP synthesis
