Thermodynamics and Bioenergetics
Understanding whether a cellular reaction can proceed on its own starts with how systems exchange energy and how Gibbs free energy predicts spontaneity.
Every reaction inside a cell either releases usable energy or requires it. Understanding whether a reaction can proceed on its own — and how fast — starts with a few core thermodynamic principles: how systems exchange energy, and how Gibbs free energy predicts spontaneity.
Key Takeaways
The first law of thermodynamics: energy is conserved, only transformed between forms.
Systems are isolated (no exchange), closed (energy only), or open (energy and matter); biological systems are open in principle but often treated as closed for experimental analysis.
Internal energy changes via heat and work: ΔU = q + w.
Bioenergetics is the study of energy storage, transfer, and use in living organisms.
Gibbs free energy (ΔG = ΔH − TΔS) determines spontaneity: negative is spontaneous, positive is nonspontaneous, zero is equilibrium.
Enthalpy and entropy combine with temperature to determine spontaneity across four possible sign combinations.
Standard conditions (ΔG°, or ΔG°′ with pH 7 in biochemistry) don't reflect actual cellular conditions; ΔG = ΔG° + RT ln Q accounts for real intracellular concentrations.
The First Law of Thermodynamics
The first law of thermodynamics states that energy is conserved: it cannot be created or destroyed, only transformed from one form to another. This principle underlies every energy transaction in biology, from ATP hydrolysis to the electron transport chain.
Classifying Thermodynamic Systems
To talk about energy transfer precisely, thermodynamics defines a system — the portion of the universe being studied — and its surroundings, everything outside that boundary. Energy can move between the system and its surroundings, and that exchange is central to biological processes.
Isolated, Closed, and Open Systems
Systems are classified into three categories based on what they can exchange with their surroundings:
System Type | Energy Exchange? | Matter Exchange? |
|---|---|---|
Isolated | No | No |
Closed | Yes | No |
Open | Yes | Yes |
Biological Systems: Open in Theory, Closed in Practice
Biological systems are generally considered open systems, since cells constantly exchange nutrients, gases, and waste products with their environment. However, biochemical experiments and studies typically occur at the cellular or subcellular level rather than across an entire organism. For analytical purposes, it's often more practical to treat the system as closed — energy exchange allowed, matter exchange restricted.
Internal Energy and Bioenergetics
The first law of thermodynamics tells us how a system's internal energy can change: through heat transfer and work interactions. This is written as:
ΔU = q + w
where ΔU is the change in internal energy, q is heat, and w is work. Internal energy can only change in these two ways — energy entering or leaving as heat, or entering or leaving as work.
When thermodynamics is applied specifically to biological systems, the resulting field is called bioenergetics — the study of how energy is stored, transferred, and utilized inside living organisms.
Gibbs Free Energy and Spontaneity
In biology, internal energy alone isn't usually the primary concern. What matters most is whether a reaction will proceed spontaneously under physiological conditions — and that determination depends on Gibbs free energy:
ΔG = ΔH − TΔS
In words: the change in Gibbs free energy equals the change in enthalpy minus temperature multiplied by the change in entropy.
Reading the Sign of Delta G
The sign of ΔG determines whether a reaction proceeds spontaneously:
ΔG < 0 — the reaction is spontaneous
ΔG > 0 — the reaction is nonspontaneous
ΔG = 0 — the system is at equilibrium
How Enthalpy and Entropy Determine Spontaneity
To understand spontaneity fully, it helps to look at enthalpy and entropy individually:
Enthalpy (ΔH) measures the overall heat change of a system during a reaction, reflecting the difference in bond energies between reactants and products.
Entropy (ΔS) measures the degree of disorder, or energy dispersion, in a system. A positive ΔS corresponds to greater disorder — more precisely, greater dispersal of energy.
Because ΔG = ΔH − TΔS, temperature determines how these two factors interact:
ΔH | ΔS | Spontaneity |
|---|---|---|
Positive | Positive | Spontaneous at high temperatures (TΔS eventually outweighs ΔH) |
Positive | Negative | Nonspontaneous at all temperatures |
Negative | Positive | Spontaneous at all temperatures |
Negative | Negative | Spontaneous at low temperatures (TΔS term stays small) |
Standard vs. Nonstandard Conditions in the Cell
The standard change in Gibbs free energy, written as ΔG°, refers to a defined set of reference conditions: 1 M concentrations, 1 atmosphere of pressure, and 25°C.
MCAT Callout — Biochemical Standard State: In biochemistry specifically, this standard is usually written as ΔG°′ (delta G naught prime) and adds one more condition — pH 7 — since cellular reactions occur in a buffered, near-neutral environment rather than the strongly acidic 1 M-proton conditions of a general chemistry standard state.
Cells, however, don't operate under standard conditions. Because of this, the actual change in free energy inside the cell is given by:
ΔG = ΔG° + RT ln Q
where R is the gas constant, T is temperature in Kelvin, and Q is the reaction quotient — the concentration of products (each raised to its stoichiometric coefficient, multiplied together) divided by the concentration of reactants (same treatment). This equation allows free energy to be calculated at any concentration and temperature, not just the standard reference state.
Common MCAT Mistakes
Confusing ΔG with ΔG°. ΔG° is the free energy change under fixed reference conditions (1 M, 1 atm, 25°C); ΔG is the actual free energy change at real cellular concentrations, related by ΔG = ΔG° + RT ln Q. A reaction with a positive ΔG° can still be spontaneous in the cell if ΔG is negative.
Assuming a spontaneous reaction (ΔG < 0) happens fast. Spontaneity says nothing about rate — a reaction can be thermodynamically favorable and still proceed slowly without a catalyst.
Mixing up ΔG°′ with ΔG°. ΔG°′ is the biochemical standard state used in cellular contexts and adds pH 7 to the general chemistry standard state's 1 M/1 atm/25°C — they aren't interchangeable.
Treating biological systems as strictly closed or strictly open with no nuance. Cells are open systems in principle (they exchange matter and energy with their environment), but biochemical analysis often treats them as closed for practical, experimental purposes.
MCAT-Style Concept Check
Question: A reaction has a positive ΔH and a positive ΔS. Under which condition will this reaction become spontaneous?
A) At low temperatures, because the TΔS term shrinks.
B) At high temperatures, because the TΔS term eventually exceeds ΔH.
C) The reaction is nonspontaneous at all temperatures regardless of ΔH and ΔS.
D) The reaction is spontaneous at all temperatures regardless of ΔH and ΔS.
Answer: B
Explanation: With ΔG = ΔH − TΔS, a positive ΔH and positive ΔS means the sign of ΔG depends on temperature. At high temperatures, the TΔS term grows large enough to outweigh the positive ΔH, making ΔG negative and the reaction spontaneous (B). Choice A has the temperature dependence backwards — low temperatures keep TΔS small, leaving ΔG positive. Choice C describes a positive ΔH/negative ΔS reaction, which is nonspontaneous at all temperatures. Choice D describes a negative ΔH/positive ΔS reaction, which is spontaneous at all temperatures.
FAQ
What's the difference between ΔG and ΔG°?
ΔG° is the free energy change measured under standard reference conditions (1 M concentrations, 1 atm, 25°C). ΔG is the actual free energy change at real, nonstandard concentrations, calculated as ΔG = ΔG° + RT ln Q.
Why do biochemists use ΔG°′ instead of ΔG°?
ΔG°′ adds pH 7 to the standard conditions, reflecting the near-neutral, buffered environment inside cells rather than the strongly acidic 1 M-proton conditions used in general chemistry's standard state.
Does a negative ΔG mean a reaction happens quickly?
No. ΔG only indicates whether a reaction is thermodynamically favorable (spontaneous), not how fast it proceeds. Reaction rate depends on factors like activation energy and catalysis, not spontaneity alone.
Are biological systems open or closed?
Biological systems are generally open systems, since cells exchange both matter and energy with their surroundings. In practice, biochemical experiments often treat the system as closed, restricting matter exchange for analytical simplicity.