Source: UCF Molecular Biology I, Chapters 1–3
Tags: weak bonds, strong bonds, covalent, hydrogen bonds, van der Waals, hydrophobic effect, Gibbs free energy, enthalpy, entropy, equilibrium, activation energy, ATP, high-energy bonds, phosphoanhydride, peptide bond, protein folding, enzymes, biosynthetic pathways, coupled reactions
Difficulty: Intermediate Prerequisites: Chapter 2 notes (DNA/RNA structure, protein synthesis basics). General chemistry background in bond types and thermodynamics is helpful but not required.
This chapter bridges chemistry and biology. Every molecular process you will study in this course, from DNA replication to protein folding to signal transduction, depends on the principles covered here: how bonds form and break, why some reactions happen spontaneously and others need energy input, and how cells use ATP to drive otherwise unfavourable reactions. If the thermodynamics feels abstract, anchor it to the concrete biological examples (protein synthesis, DNA synthesis, protein phosphorylation) that the exam tests heavily.
Biological molecules are held together by strong bonds (covalent, including peptide bonds) and shaped by weak bonds (hydrogen bonds, van der Waals, hydrophobic interactions, ionic). Gibbs free energy (ΔG) determines whether a reaction is spontaneous. ATP hydrolysis provides the energy to drive biosynthetic reactions forward. Enzymes lower activation energy but do not change the equilibrium.
Strong bonds (covalent bonds)
Bonds formed by sharing electrons between atoms. They require a large amount of energy to break and release a large amount of energy when they form. Covalent bonds make up the primary structure of proteins (peptide bonds) and the backbone of DNA/RNA (phosphodiester bonds).
In simple terms, these are the bonds that hold molecules together permanently, until an enzyme or significant energy input breaks them.
Weak bonds
Non-covalent interactions including hydrogen bonds, van der Waals forces, ionic bonds, and hydrophobic interactions. Key properties: they require multiplicity (many of them working together), they require complementarity (the interacting surfaces must fit), they are reversible, and their individual energies are only slightly greater than the kinetic energy of thermal motion (~0.6 kcal/mol at 298 K).
Peptide bond
The covalent bond between the carboxyl group of one amino acid and the amino group of the next. It is planar due to partial double bond character (resonance between the C=O and C–N gives the bond ~40% double bond character). This planarity restricts rotation. Peptide bonds are not easily broken after formation because the activation energy for hydrolysis is high.
Hydrogen bond
A weak interaction between a hydrogen atom covalently bonded to an electronegative atom (N or O) and another electronegative atom. C–H groups cannot serve as hydrogen bond donors (C is not electronegative enough). N–H···O=C and O–H···O=P are valid hydrogen bonds.
Van der Waals forces
Weak, short-range attractions between all atoms, arising from transient fluctuations in electron distribution. They are longer in distance than covalent bonds and are a type of intermolecular interaction. They occur between all molecules (polar and nonpolar). They are strongest when electron shells are at optimal distance (not overlapping). Van der Waals forces are not covalent bonds.
Hydrophobic effect / hydrophobic bonds
The tendency of nonpolar molecules to aggregate in aqueous solution, not because they attract each other strongly, but because their clustering increases water's ability to hydrogen-bond with itself (increases water's self-interaction and the overall entropy of the system). This is a major driving force in protein folding.
In simple terms, nonpolar molecules are pushed together by water molecules preferring to interact with each other.
Gibbs free energy (ΔG)
The energy available to do work in a system at constant temperature and pressure. ΔG = ΔH − TΔS. A negative ΔG means the reaction is spontaneous (thermodynamically favourable). At equilibrium, ΔG = 0. ΔG depends on enthalpy (ΔH), entropy (ΔS), and temperature, but not activation energy.
ΔG° (standard free energy change)
The free energy change under standard conditions (1 M concentrations, 25°C, 1 atm). ΔG°' is the biochemical standard, adjusted to pH 7.
ΔG (actual free energy change)
Takes into account the actual concentrations of reactants and products in the cell. Even if ΔG° is positive, the actual ΔG can be negative if reactant concentrations are high enough relative to products (or vice versa).
Equilibrium constant (Keq)
At equilibrium, the ratio of product concentrations to reactant concentrations. When ΔG is negative, Keq > 1 (products favoured). When ΔG° is positive, the reaction still may proceed forward under cellular conditions if ΔG is negative.
Activation energy
The energy barrier that must be overcome for a reaction to proceed. A reaction can be thermodynamically favourable (negative ΔG) yet kinetically slow if the activation energy is high. This is why hydrogen and oxygen gas do not spontaneously combine into water at room temperature despite the reaction being energetically favourable.
Enzymes
Biological catalysts (nonrepetitive heteropolymers, i.e. proteins with unique sequences). They lower the activation energy of a reaction but do not alter the equilibrium or the ΔG. Enzymes rely on weak bonds (not strong bonds) for substrate interaction and recognition.
ATP (adenosine triphosphate)
The primary energy currency of the cell. ATP contains phosphoanhydride bonds between its phosphate groups. These are "high-energy bonds" because their hydrolysis yields a large negative ΔG (not because the bonds themselves are strong). ATP is used more frequently than AMP for energy because ATP contains phosphoanhydride bonds, while AMP's only phosphate linkage is the lower-energy ester bond.
High-energy bonds
Bonds that yield a large negative ΔG upon hydrolysis. They are thermodynamically unstable (not to be confused with kinetically stable). The phosphoanhydride bonds in ATP are the classic example. High-energy bonds are not especially strong; they are simply bonds whose products are at much lower energy than the reactants.
Phosphoanhydride bond
The bond between phosphate groups in ATP (the bonds connecting the alpha-beta and beta-gamma phosphates). These are the high-energy bonds. The ester bond connecting the alpha phosphate to the ribose is not a high-energy bond.
Anhydride bond
A bond formed by the condensation of two acid groups (e.g. two carboxylic acids, or two phosphoric acids). The anhydride bond is high energy upon hydrolysis because hydrolysis relieves competing resonance between the two carbonyl (or phosphoryl) groups.
PEP (phosphoenolpyruvate)
A high-energy molecule used in cells to directly generate ATP via substrate-level phosphorylation. PEP has a higher phosphoryl transfer potential than ATP.
Substrate-level phosphorylation
The synthesis of ATP by direct transfer of a phosphate group from a high-energy substrate to ADP, using the energy from a coupled reaction. Distinct from oxidative phosphorylation (which uses the electron transport chain) and photophosphorylation (which uses light energy).
Coupled reactions
Two reactions linked together so that the energy released by the favourable (exergonic) reaction drives the unfavourable (endergonic) one. Biosynthetic pathways in cells use this principle: the overall ΔG must be negative for the coupled process.
L-form and D-form amino acids
Biological proteins use exclusively L-form amino acids. A peptide made of D-form amino acids has very low toxicity in humans because human enzymes cannot recognise or degrade D-amino acid peptides (our proteases are shaped for L-forms). This is also why D-amino acid peptides can function as antibiotics.
Strong (covalent) bonds define the primary structure of macromolecules: the peptide bonds in proteins, the phosphodiester bonds in DNA/RNA.
Weak bonds define the higher-order structure: protein folding (secondary, tertiary, quaternary structure), DNA double helix stability, enzyme-substrate recognition, antibody-antigen binding, and protein-protein interactions.
Individual weak bonds have energies comparable to thermal kinetic energy (~0.6 kcal/mol at 298 K). Any single weak bond can be broken by thermal motion. Molecular stability comes from the multiplicity of weak bonds acting together.
When a strong bond forms, a large amount of energy must be added to break it (this is what "strong" means in this context).
Planar due to partial double bond character from resonance. This is not because of single bonds; it is specifically because the C–N bond has partial double bond character.
The peptide bond is between the carbon of one amino acid's carboxyl group and the nitrogen of the next amino acid's amino group.
Proteins are not immediately degraded after synthesis because the activation energy for peptide bond hydrolysis is high, even though hydrolysis is thermodynamically favourable.
Require an electronegative atom (N, O) bonded to hydrogen as the donor, and another electronegative atom as the acceptor.
C–H cannot be a hydrogen bond donor because carbon is not sufficiently electronegative.
Valid examples: N–H···O=C, O–H···O=P.
Occur between all atoms and molecules, not just polar ones.
They are intermolecular interactions, not covalent bonds.
Strength is distance-dependent: strongest at optimal distance, weakens rapidly with increasing distance, and becomes repulsive when electron shells overlap.
They are longer-range than covalent bonds (the interaction distance is greater).
A molecule is polar if it has an asymmetric distribution of charge. CH₃Cl is polar (the chlorine creates an asymmetric dipole). CO₂, CCl₄, O₂, and CH₄ are nonpolar (their geometries cancel out any individual bond dipoles).
Nonpolar molecules in water aggregate not because they attract each other, but because their clustering lets water molecules maximise their own hydrogen bonding.
This aggregation increases the entropy of water (fewer ordered water cages around nonpolar surfaces), making the process thermodynamically favourable.
Hydrophobic amino acids contribute to protein folding by burying themselves in the protein interior, away from water. They interact with each other via van der Waals forces and simultaneously increase water's self-interaction. All of these effects contribute.
Alanine has an extra CH₃ group compared to glycine. When binding a nonpolar molecule in aqueous solution, the extra CH₃ group of alanine forms additional van der Waals attractions with the nonpolar molecule.
ΔG = ΔH − TΔS.
Spontaneous reaction: ΔG < 0.
At equilibrium: ΔG = 0; the rates of forward and reverse reactions are equal (not the concentrations of products and reactants).
Negative ΔG corresponds to Keq > 1.
When both ΔH and ΔS are positive, the reaction is spontaneous only at high temperatures (the TΔS term must outweigh ΔH).
Salt dissolving in water absorbs heat (endothermic, positive ΔH) but is still favourable because the entropy increase from the crystal breaking apart (positive TΔS) outweighs the enthalpy cost.
ΔG takes into account actual cellular concentrations of reactants and products. ΔG° and ΔG°' do not.
A reaction with a positive ΔG°' can still proceed forward in the cell if the actual ΔG is made negative by high reactant concentrations relative to products.
A reaction with a negative ΔG° can go towards reactants under physiological conditions if the cellular concentrations push ΔG positive.
At equilibrium, forward and reverse reaction rates are equal. This does not mean concentrations are equal.
Enzymes do not alter the equilibrium of a reaction. They lower the activation energy and speed up the approach to equilibrium.
Most reactions in the cell are not at equilibrium. Cells use energy to maintain themselves far from equilibrium.
For the reaction C + D → A + B, the forward rate = Kf[C][D] and the equilibrium constant Keq = [A][B]/[C][D].
A thermodynamically favourable reaction (negative ΔG) may still not occur at an appreciable rate if the activation energy is too high.
Example: hydrogen + oxygen → water is highly favourable, but the activation energy barrier prevents it from happening spontaneously at room temperature.
Catalysts (including enzymes) lower the activation energy. Adding more reactant can increase the reaction rate but does not lower the activation energy.
If a reaction diagram shows the products (C) at higher free energy than the reactants (A + B), the reaction is endergonic (ΔG > 0), non-spontaneous, and the Keq favours reactants.
ΔG° = Gf(products) − Gf(reactants). This formula is correct regardless of direction.
In a multi-step pathway (A → B → C → D → E), individual steps can be unfavourable as long as the overall ΔG for the entire pathway is negative. Some steps may require energy coupling.
The conversion of B to C might be unfavourable (uphill on the diagram), yet the overall A → E conversion can still be favourable if the net free energy change is negative.
ATP's phosphoanhydride bonds (between the phosphate groups) are the high-energy bonds. The ester bond connecting the first phosphate to the ribose is not high-energy.
High-energy bonds yield a large negative ΔG upon hydrolysis. This does not mean they are strong bonds; it means their hydrolysis products are at much lower energy.
High-energy bonds are thermodynamically unstable but kinetically stable (they do not hydrolyse spontaneously at a fast rate without an enzyme).
The anhydride bond is high energy upon hydrolysis because hydrolysis relieves the competing resonance between the two carbonyl groups.
ATP is used more than AMP because ATP has phosphoanhydride bonds; AMP has only an ester bond.
PEP (phosphoenolpyruvate) is a high-energy molecule that directly generates ATP in the cell via substrate-level phosphorylation.
Biosynthetic pathways (building macromolecules) are not spontaneous on their own. They require an external energy source, typically ATP or GTP.
Protein synthesis: amino acids are first activated by ATP (forming aminoacyl-AMP), then transferred to tRNA. The energy from ATP hydrolysis drives the otherwise unfavourable peptide bond formation, consistent with the second law of thermodynamics.
DNA synthesis: nucleotide triphosphates (dNTPs) are used as substrates. The incorporation of a nucleotide releases pyrophosphate (PPi), which is then hydrolysed by pyrophosphatase. The hydrolysis of PPi pulls the overall reaction forward (making the net ΔG negative), even though the polymerisation step alone has a slightly positive free energy.
The reverse reaction of DNA synthesis is prevented in cells by metabolising the byproducts (PPi → 2Pi), which removes products and keeps the reaction far from equilibrium.
When ATP phosphorylates a protein, the added phosphate group can cause conformational changes (by forming new interactions or disrupting existing ones with surrounding amino acid side chains), block ligand binding, or create new binding interfaces via the negative charge. All of these effects are possible.
This is a nuanced point. Strong bonds give off less energy than weak bonds when broken? This seems counterintuitive, but the exam question states: strong bonds give off less energy when broken than weak bonds. This phrasing is about the energy released to the surroundings. Breaking a strong bond requires energy input (endothermic). The question likely refers to: strong bonds store more energy, and when broken, that energy is captured, but the process of breaking them absorbs energy from the surroundings. The answer given is (a) more, meaning strong bonds give off more energy when broken, which aligns with the concept that breaking a high-energy covalent bond releases more energy to products (as in ATP hydrolysis). However, if the question is about the act of breaking a bond (which requires input), the phrasing can be confusing. For the exam: strong bonds release more energy than weak bonds when hydrolysed, because the products are at much lower energy states.
Gibbs free energy: ΔG = ΔH − TΔS
Relationship between ΔG° and Keq: ΔG° = −RT ln(Keq) If ΔG° < 0, then Keq > 1 (products favoured). If ΔG° > 0, then Keq < 1 (reactants favoured).
Actual free energy: ΔG = ΔG° + RT ln([products]/[reactants])
Equilibrium constant: For A + B → C + D: Keq = [C][D] / [A][B]
Standard free energy of formation: ΔG° = Gf(products) − Gf(reactants)
ATP-driven phosphorylation is how cells regulate nearly every signalling pathway: kinases add phosphate groups to proteins, switching them on or off. This is the molecular basis of cell signalling in everything from insulin response to immune activation. The thermodynamic principles here also explain why fever (elevated temperature) destabilises proteins, and why organisms that live at extreme temperatures need specially adapted enzymes.
Students often think enzymes change the equilibrium of a reaction. They do not. Enzymes lower the activation energy and increase the rate at which equilibrium is reached, but the final ratio of products to reactants is unchanged.
"High-energy bond" does not mean the bond is strong. It means hydrolysis of that bond releases a large amount of free energy. ATP's phosphoanhydride bonds are thermodynamically unstable.
Students confuse ΔG° with ΔG. A positive ΔG° does not mean the reaction cannot proceed in the cell. The actual ΔG, which accounts for cellular concentrations, might still be negative.
The hydrophobic effect is driven by entropy (water's increased disorder when nonpolar groups cluster), not by direct attraction between nonpolar molecules.
⚠️ Know the four properties of weak bonds: multiplicity, complementarity, reversibility, energy near thermal kinetic energy.
⚠️ Be able to apply ΔG = ΔH − TΔS to predict spontaneity under different temperature conditions.
⚠️ Understand why a positive ΔG°' does not prevent a reaction from occurring in vivo (actual concentrations matter).
⚠️ Know which bonds in ATP are high-energy (phosphoanhydride between phosphates) and which are not (the ester bond to ribose).
⚠️ Pyrophosphate hydrolysis pulling DNA synthesis forward is a heavily tested concept.
⚠️ The peptide bond is planar due to partial double bond character, not single bond character.
⚠️ C–H cannot donate a hydrogen bond. This specific pair is a favourite exam question.
True or false: Enzymes alter the equilibrium constant of a reaction. ___
Fill in the blank: ΔG = ΔH − ___.
True or false: High-energy bonds are the strongest bonds in a molecule. ___
Fill in the blank: The hydrophobic effect is primarily driven by an increase in ___ of the water molecules.
True or false: At equilibrium, the concentrations of products and reactants are always equal. ___
Answers: 1. False. 2. TΔS. 3. False (they are thermodynamically unstable, not necessarily strong). 4. Entropy. 5. False (the rates are equal, not the concentrations).
Q: Why does the synthesis of water from hydrogen and oxygen gas not occur readily at room temperature, even though it is energetically favourable?
A: The activation energy is too high. The reaction has a negative ΔG (favourable) but requires a significant energy input to get started, which thermal energy at room temperature cannot provide.
Q: A reaction has positive ΔH and positive ΔS. Under what conditions is it spontaneous?
A: At high temperatures. The TΔS term must exceed ΔH to make ΔG negative.
Q: Why is the dissolving of salt in water thermodynamically favourable even though it absorbs heat?
A: The large positive entropy change (TΔS) from dispersing the ions outweighs the positive enthalpy (ΔH), making the overall ΔG negative.
Q: Which bonds in ATP are considered high-energy, and why?
A: The phosphoanhydride bonds between the phosphate groups (alpha-beta and beta-gamma). Their hydrolysis yields a large negative ΔG because the products (the separated phosphates) are at much lower energy due to relief of electrostatic repulsion and increased resonance stabilisation.
Q: How does pyrophosphate cleavage drive DNA synthesis forward?
A: DNA polymerisation releases pyrophosphate (PPi). Pyrophosphatase then hydrolyses PPi into two inorganic phosphates (2Pi). Removing the byproduct shifts the equilibrium forward, making the overall ΔG of DNA synthesis negative, even though the polymerisation step alone has a slightly positive free energy.
Q: Why are proteins not immediately degraded after synthesis?
A: Although hydrolysis of peptide bonds is thermodynamically favourable, the activation energy for breaking a peptide bond is high. Without an enzyme (protease), degradation is extremely slow.
The thermodynamic principles here underpin every metabolic pathway you will study: glycolysis, the citric acid cycle, and oxidative phosphorylation all follow the same logic of coupled reactions and energy currency. Protein folding (covered in detail in structural biochemistry) depends directly on the interplay between hydrophobic effects, hydrogen bonding, and van der Waals forces introduced in this chapter. DNA replication and transcription (upcoming chapters) both depend on nucleotide triphosphate hydrolysis and pyrophosphate cleavage to drive polymerisation forward.
weak bonds, strong bonds, covalent bonds, peptide bond, hydrogen bond, van der Waals forces, hydrophobic effect, hydrophobic interactions, polar, nonpolar, Gibbs free energy, delta G, enthalpy, entropy, equilibrium, equilibrium constant, Keq, activation energy, catalyst, enzyme, ATP, ADP, AMP, phosphoanhydride, high-energy bond, anhydride bond, ester bond, PEP, substrate-level phosphorylation, coupled reactions, biosynthesis, protein folding, pyrophosphate, pyrophosphatase, DNA synthesis energy, protein phosphorylation, L-amino acids, D-amino acids, thermodynamics, second law, spontaneous reaction, endergonic, exergonic, exothermic, endothermic