Table of Contents
Living organisms are constantly converting energy. To understand how they can stay alive and ordered while the universe tends toward disorder, it is useful to focus on the concept of free energy.
Energy in Biology: A Short Reminder
In metabolism, many chemical reactions are coupled: one reaction releases energy, another consumes it. Cells care less about the total energy content of substances and more about whether a reaction can proceed in a given direction and do useful work. This is where free energy becomes important.
Free Energy and Spontaneity
In thermodynamics, the quantity that describes whether a process can proceed spontaneously under constant temperature and pressure (as in most cells) is the Gibbs free energy, usually written as $G$.
For a given reaction, the change in free energy is written as:
$$\Delta G = G_{\text{products}} - G_{\text{reactants}}$$
The sign of $\Delta G$ tells you how the reaction behaves:
- $\Delta G < 0$:
- The reaction is exergonic.
- It can proceed spontaneously in the forward direction.
- Free energy is released and can, in principle, be used to do work.
- $\Delta G > 0$:
- The reaction is endergonic.
- It is not spontaneous in the forward direction under the given conditions.
- It requires energy input.
- $\Delta G = 0$:
- The system is at equilibrium.
- There is no net change in reactants or products.
Biological systems are especially concerned with exergonic and endergonic reactions and how to connect them.
Why Organisms Depend on Free Energy
Maintaining Order Against the Trend to Disorder
The second law of thermodynamics states that in a closed system, overall disorder (entropy) tends to increase. Living organisms, however, are highly ordered: they have complex structures, maintain constant internal conditions, and build up large molecules from small ones.
They can do this because:
- They are open systems: matter and energy flow in and out.
- They take in high free-energy substances (such as organic molecules, light, or reduced inorganic compounds).
- They export heat and waste with lower free energy.
Locally (inside the organism), order can increase as long as the organism causes a greater increase in disorder in its surroundings. In other words, life is possible because organisms use free energy from the environment to keep their own internal structures far from equilibrium.
Life Away from Equilibrium
A system at true thermodynamic equilibrium does no net work and has no net chemical reactions progressing in one direction. Living cells must:
- Maintain concentration differences (e.g., ions across membranes).
- Sustain metabolic fluxes (constant flow through metabolic pathways).
- Keep gradients (such as proton gradients) that can drive processes.
All of these are non-equilibrium states. To keep them, organisms must continuously spend free energy. When free energy supply stops (no food, no light, no usable chemicals), systems tend toward equilibrium, and life processes cease.
Standard Free Energy vs. Actual Cellular Conditions
The free energy change of a reaction depends on conditions such as concentrations and temperature. Two related quantities are useful:
- $\Delta G^\circ$: the standard free energy change under defined standard conditions (concentration 1 mol/L, specified temperature and pressure).
- $\Delta G$: the actual free energy change under the conditions inside a cell, which are rarely “standard.”
The relationship between them includes the effect of concentrations. A reaction that is endergonic under standard conditions can become exergonic in a cell if product concentrations are very low and reactant concentrations are high. Cells exploit this by:
- Constantly removing products (for example, by using them in the next step of a pathway),
- Maintaining particular concentration ratios through transport processes.
Thus, whether a reaction actually “runs” in a cell depends on $\Delta G$ under real conditions, not only on $\Delta G^\circ$.
Free Energy and Coupled Reactions
Many essential biological processes are endergonic on their own (for example, synthesizing large molecules from smaller ones). To make them proceed, cells couple them to exergonic reactions so that the overall process has:
$$\Delta G_{\text{total}} = \Delta G_{\text{exergonic}} + \Delta G_{\text{endergonic}} < 0$$
Key points about coupling:
- Coupling usually occurs via shared intermediates or energy carriers, not by simply “adding” two unrelated reactions.
- A classic energy carrier is ATP, whose hydrolysis is strongly exergonic under cellular conditions.
- In effect, cells “pay” with free energy released from ATP or other exergonic processes to drive uphill, endergonic steps.
From the viewpoint of free energy, metabolism is a complex web of coupled reactions arranged so that necessary endergonic processes are always linked to adequate sources of free energy.
Sources of Free Energy for Organisms
Different organisms tap into different original sources of free energy from the environment:
- Photoautotrophs (such as plants, algae, many bacteria) use light as the primary free energy source, capturing it and storing it in chemical form.
- Chemoautotrophs use the free energy released when inorganic substances (such as hydrogen sulfide or ammonia) are oxidized.
- Heterotrophs (such as animals, fungi, many bacteria) obtain free energy by oxidizing organic substances (for example, sugars, fats, proteins).
In all cases:
- An external source with high free energy is tapped.
- Part of that free energy is captured in chemical forms usable by the cell (for example, ATP, reduced coenzymes).
- The rest is released as heat and waste, increasing the entropy of the surroundings.
Free Energy and Work in Cells
Organisms use free energy to perform different types of work:
- Chemical work:
- Synthesis of macromolecules, such as proteins, nucleic acids, and complex lipids.
- Transport work:
- Moving substances across membranes against concentration or electrical gradients.
- Mechanical work:
- Movement of cells, contraction of muscles, movement of chromosomes, and intracellular transport.
In all of these, the direction and feasibility of the underlying reactions or processes are governed by $\Delta G$. Free energy is the “budget” that must be managed: any work that decreases free energy of the cell-environment system can proceed; work that would increase it must be paid for by coupling to larger decreases elsewhere.
Summary: Why Life Depends on Free Energy
- Organisms do not just need “energy”; they need free energy that can drive specific, directed changes.
- They live far from thermodynamic equilibrium by continuously importing free energy and exporting heat and low free-energy waste.
- The sign and magnitude of $\Delta G$ determine whether a metabolic process can proceed and whether it can be harnessed to do work.
- By coupling exergonic and endergonic reactions, organisms ensure that essential but unfavorable steps still occur.
- Different types of organisms exploit different environmental free energy sources, but all share the same fundamental requirement: a continuous flow of free energy to maintain life processes.