12.2 Hydrogen/Electron Carriers & Thermodynamics/Free Energy

12.2 Hydrogen/Electron Carriers & Thermodynamics/Free Energy

Quick Answer: Cells transfer energy in two currencies: phosphoanhydride bonds (ATP) for immediate mechanical/chemical work, and reducing power carried by NAD⁺/NADH and FAD/FADH₂ for moving electrons to the electron transport chain. Whether any reaction can happen is decided by Gibbs free energy, ΔG = ΔH − TΔS: negative ΔG = exergonic (spontaneous), positive ΔG = endergonic (needs energy). Cells survive by coupling exergonic reactions to endergonic ones.

NAD⁺/NADH and FAD/FADH₂

Nicotinamide adenine dinucleotide (NAD⁺) is a dinucleotide coenzyme derived from niacin (vitamin B₃). It accepts a hydride ion (H⁻)—two electrons plus one proton—at its nicotinamide ring, becoming NADH; the second proton escapes to solution as H⁺. The half-reaction is:

NAD⁺ + 2e⁻ + H⁺ → NADH (reduction potential E°' ≈ −0.32 V)

Flavin adenine dinucleotide (FAD), derived from riboflavin (vitamin B₂), accepts two hydrogens (two electrons and two protons) to become FADH₂. FAD is typically prosthetic—tightly bound to its enzyme (e.g., succinate dehydrogenase in the citric acid cycle)—whereas NAD⁺ is a freely diffusible carrier. Both deliver electrons to the inner mitochondrial membrane's electron transport chain: NADH feeds complex I (yielding ~2.5 ATP per NADH by modern P/O ratios), and FADH₂ feeds complex II (yielding ~1.5 ATP per FADH₂). This difference is why, on the PA-CAT, a question about why NADH yields more ATP than FADH₂ has a concrete thermodynamic answer: NADH enters the chain one complex earlier, so its electrons pump more protons.

A related carrier is NADP⁺/NADPH, identical in structure to NAD⁺ except a phosphate on the adenosine ribose; it is used mostly in anabolic (biosynthetic) and antioxidant pathways rather than energy harvest. The cell keeps the NAD⁺/NADH ratio high (favoring oxidation of fuels) and the NADPH/NADP⁺ ratio high (favoring reduction of biosynthetic substrates and glutathione), which lets the two pools serve opposite purposes despite their structural similarity.

Free Energy (ΔG) and Spontaneity

Gibbs free energy is defined as G = H − TS, where H is enthalpy, T is temperature, and S is entropy. The change for a reaction is:

ΔG = ΔH − TΔS

• ΔG < 0 → exergonic — releases free energy; spontaneous in the forward direction.
• ΔG > 0 → endergonic — requires free energy input; non-spontaneous unless coupled.
• ΔG = 0 → equilibrium — no net change.

The standard transformed free energy, ΔG°', is defined at pH 7, 25°C, 1 M concentrations, and is tabulated for many biochemical reactions. The actual cellular ΔG uses reactant/product ratios:

ΔG = ΔG°' + RT ln Q

where Q is the mass-action ratio. Because cells keep ATP/ADP ratios far above equilibrium (≈ 10³–10⁴), the in vivo ΔG of ATP hydrolysis is roughly −50 to −60 kJ/mol—far more negative than −30.5 kJ/mol.

Laws of Thermodynamics Applied to Cells

First Law (energy conservation): the universe's energy is constant. Cells do not create energy; they transform chemical bond energy (food) into work and heat. Photosynthesis converts photon energy to bond energy; respiration converts bond energy to ATP and ultimately heat.

Second Law (entropy increases): every transfer increases the universe's entropy. Cells maintain local order (low entropy) only by exporting heat and disorder to the surroundings. This is why metabolism produces heat and why a constant energy input (food or sunlight) is required to sustain life.

Coupled Reactions

A reaction with positive ΔG cannot proceed alone, but if it is paired with a strongly exergonic reaction so the summed ΔG is negative, the pair proceeds. Example: glutamate + NH₃ → glutamine has ΔG°' ≈ +14.2 kJ/mol; coupling to ATP hydrolysis (−30.5 kJ/mol) gives a net −16.3 kJ/mol. In practice this occurs via a phosphorylated intermediate (γ-glutamyl phosphate), so the phosphate from ATP is transferred to glutamate before NH₃ attacks. Coupling is the universal strategy cells use to drive biosynthesis, active transport, and mechanical work.

Redox in Metabolism

Oxidation is loss of electrons; reduction is gain. Biological oxidations of fuel molecules transfer electrons to NAD⁺ and FAD, which carry them to the ETC where O₂ is the final electron acceptor (reduced to H₂O). The redox potential difference between NADH/NAD⁺ (−0.32 V) and O₂/H₂O (+0.82 V) drives proton pumping across the inner mitochondrial membrane; the resulting electrochemical gradient powers ATP synthase. Understanding this chain of conversions—bond energy → reducing power → proton gradient → ATP—is the central narrative of Bioenergetics on the PA-CAT Bulletin of Information (rev. 20240815).