Thermodynamics

🔥 Thermodynamics in Biological Systems

Essential concepts and high-yield notes for CSIR NET Life Sciences.

1. Laws of Thermodynamics (Theoretical Understanding)

Thermodynamics explains how energy flows in physical and biological systems.

➤ First Law of Thermodynamics (Energy Conservation)

• Theory: The total energy of the universe remains constant. Energy can change form (e.g., chemical → mechanical → heat), but cannot be created or destroyed.
• Biological Insight: During cellular respiration, chemical energy in glucose is converted into ATP. Heat is always released — no biological process is 100% efficient.

ΔU = Q - W

➤ Second Law of Thermodynamics (Entropy Law)

• Theory: Every spontaneous process increases the entropy (disorder) of the universe. Systems naturally move toward greater randomness.
• Biological Importance: Living cells maintain order by increasing the entropy of their surroundings (e.g., Glucose breakdown → CO₂ + H₂O).

ΔS_universe > 0

➤ Third Law of Thermodynamics

• Theory: The entropy of a perfectly ordered crystal at absolute zero (0 K) is exactly zero. Provides a theoretical reference point.

---

⚡ 2. Gibbs Free Energy (G)

Gibbs free energy represents the usable (or available) energy in a system to do work. It combines enthalpy (heat content, H) and entropy (disorder, S).

G = H - TS

➤ Change in Gibbs Free Energy (ΔG)

ΔG = ΔH - TΔS

ΔG Value	Meaning
ΔG < 0	Spontaneous (Exergonic) - Energy is released.
ΔG > 0	Non-spontaneous (Endergonic) - Energy input required.
ΔG = 0	System is at Equilibrium.

---

📘 3. Gibbs–Helmholtz Equation

Describes the temperature dependence of free energy and shows exactly how ΔG changes with temperature.

[∂(ΔG/T) / ∂T]_P = -ΔH / T²

---

📏 4. Standard Gibbs Free Energy

Chemical Standard (ΔG°)

• 1 M concentration
• 1 atm pressure, 25°C
• pH = 0 ([H⁺] = 1 M)

Biochemical Standard (ΔG°′)

• Physiological conditions
• Constant water concentration
• pH = 7 ([H⁺] = 10^-7 M)

➤ Relation with Equilibrium

ΔG°′ = -RT ln K′_eq

---

🔗 5. Coupled Reactions & Cellular Energy

Cells power unfavorable, endergonic reactions by coupling them directly with highly favorable, exergonic reactions (like ATP hydrolysis). Standard conditions do NOT exist inside living cells.

ΔG = ΔG°′ + RT ln ([Products]/[Reactants])

• The cellular ATP/ADP ratio is kept artificially high.
• Cellular ΔG of ATP hydrolysis ≈ -50 to -65 kJ/mol.
• Mg²⁺ binding heavily affects free energy by shielding negative charges.

---

📝 CSIR NET Life Sciences Level MCQs

Test your understanding with these 10 high-yield questions.

1. A biochemical reaction has a ΔG°′ of +15 kJ/mol. Which condition could make the actual cellular ΔG negative?

1. Adding an enzyme catalyst.
2. Keeping product concentration exceptionally high relative to reactants.
3. Keeping reactant concentration exceptionally high relative to products.
4. Increasing the activation energy of the forward reaction.

2. Why does the hydrolysis of Phosphoenolpyruvate (PEP) yield significantly more free energy than ATP?

1. PEP contains more phosphate groups than ATP.
2. The phosphate group in PEP is attached via a high-energy thioester bond.
3. The product (pyruvate) undergoes enol-keto tautomerization, pulling the reaction forward.
4. ATP is stabilized by resonance, whereas PEP has no resonance stabilization at all.

3. During protein folding, conformational entropy decreases drastically. How is protein folding thermodynamically possible (ΔG < 0)?

1. It is entirely driven by ATP hydrolysis.
2. The hydrophobic effect increases the entropy of the surrounding water molecules.
3. Folding is driven by a massive increase in the enthalpy (ΔH > 0) of the system.
4. The Second Law of Thermodynamics does not apply to macromolecules.

4. If a chemical reaction reaches thermodynamic equilibrium in a closed system, which is true?

1. The standard free energy change (ΔG°′) is zero.
2. The actual free energy change (ΔG) is zero.
3. Both the forward and reverse reaction rates drop to zero.
4. The entropy of the system reaches its minimum possible value.

5. The cellular free energy of ATP hydrolysis is roughly -50 to -65 kJ/mol, while standard is -30.5 kJ/mol. Why?

1. Cellular pH is much lower than the standard pH of 7.
2. Enzymes decrease the free energy of the products.
3. Cellular concentration of ATP is maintained much higher than ADP and Pi.
4. Intracellular temperature is significantly higher than standard temperature.

6. A pathway pairs Reaction A (ΔG°′ = +20 kJ/mol) with Reaction B (ΔG°′ = -30 kJ/mol). What is the theoretical ΔG°′ of the coupled process?

1. -10 kJ/mol; spontaneous
2. +10 kJ/mol; non-spontaneous
3. -50 kJ/mol; spontaneous
4. +50 kJ/mol; non-spontaneous

7. If a reaction has an equilibrium constant (K′_eq) of 10⁴, what can be inferred under standard conditions?

1. The reaction is highly endergonic.
2. At equilibrium, reactant concentration vastly exceeds products.
3. The standard free energy change (ΔG°′) is highly negative.
4. The reaction will not occur without ATP coupling.

8. If a reaction has a positive enthalpy change (ΔH > 0) and positive entropy change (ΔS > 0), it will be:

1. Spontaneous at all temperatures.
2. Non-spontaneous at all temperatures.
3. Spontaneous only at high temperatures.
4. Spontaneous only at low temperatures.

9. What is the role of Mg²⁺ ions in the cellular thermodynamics of ATP?

1. They bind irreversibly to prevent spontaneous hydrolysis.
2. They coordinate with oxygen atoms, reducing electrostatic repulsion.
3. They act as an electron donor to reduce ATP.
4. They increase the ΔG°′ of ATP hydrolysis to -100 kJ/mol.

10. If an isolated living cell is forced into true thermodynamic equilibrium (ΔG = 0), what is its state?

1. Dividing rapidly.
2. At optimal metabolic efficiency.
3. Dead.
4. In a state of dormancy.

✅ Answer Key & Explanations:

1: C (High reactant concentration pushes Q below K_eq)
2: C (Tautomerization of pyruvate provides a massive energetic driving force)
3: B (The hydrophobic effect releases ordered water cages, causing an entropy increase in the universe)
4: B (Actual ΔG = 0 at equilibrium)
5: C (The mass action ratio in a cell is kept far from equilibrium)
6: A (Free energies are additive: +20 + (-30) = -10)
7: C (K_eq > 1 makes the ln term positive, resulting in a negative ΔG°′)
8: C (At high T, the -TΔS term overcomes the positive ΔH)
9: B (Reduces repulsion and facilitates enzyme binding)
10: C (Living systems must maintain steady states far from equilibrium. Equilibrium = Death)