AP Syllabus focus:
‘Life maintains high levels of order without violating thermodynamic laws by ensuring energy input exceeds energy loss.’
Living cells appear highly ordered, yet they obey thermodynamics. This page explains how organisms maintain and build biological order by continually acquiring, transforming, and dissipating energy.
Thermodynamics in Living Systems
Thermodynamics describes how energy moves and changes form. Cells are open systems: they exchange energy and matter with their surroundings, so cellular order must be understood in that environmental context.
The First Law: Energy Is Conserved
Energy cannot be created or destroyed, only transferred or transformed. In biology, chemical energy in bonds is frequently transformed into work and heat.
Cells transform energy to perform:
Chemical work (building macromolecules)
Transport work (moving substances across membranes)
Mechanical work (movement)
Because energy is conserved, any increase in cellular order must be “paid for” by energy taken in from outside the cell (ultimately from sunlight or chemical sources).
The Second Law: Entropy Tends to Increase
The Second Law states that every energy transfer increases the entropy of the universe.
Entropy: A measure of energy dispersal or disorder; spontaneous processes tend to increase total entropy of a system plus its surroundings.
A key implication is that energy conversions are never 100% efficient: some energy becomes heat, dispersing into the environment and raising entropy.
This diagram compares particle arrangement in solids, liquids, and gases to illustrate increasing entropy from highly ordered (solid) to highly disordered (gas). It supports the idea that dispersal of energy (often as heat) is associated with greater randomness in the surroundings. Source
Biological Order Without Violating Thermodynamics
Cells can increase internal order (lower internal entropy) because they are not isolated. They maintain low entropy locally by increasing entropy elsewhere—primarily by releasing heat and low-energy waste molecules.
Connecting the Syllabus Statement
“Life maintains high levels of order without violating thermodynamic laws by ensuring energy input exceeds energy loss.”
This means:
Organisms continually import energy (and often matter) from the environment.
They use some of that energy to build/maintain ordered structures (cells, tissues, gradients).
They lose energy to the surroundings as heat and less-useful chemical forms, increasing environmental entropy.
Net effect: organismal order is sustained while the universe’s entropy increases.
Free Energy and Spontaneity
To predict whether a cellular process can proceed, biologists use Gibbs free energy.
Gibbs free energy (): The portion of a system’s energy available to do work at constant temperature and pressure; processes with negative are energetically favourable.
Cells often drive non-spontaneous ordering processes by coupling them to spontaneous, energy-releasing reactions.
= change in free energy (kJ/mol)
= change in enthalpy/total energy (kJ/mol)
= absolute temperature (K)
= change in entropy (kJ/mol·K)
In cells, ordering reactions often decrease system entropy (), so they require compensating energy changes (for example, energy released elsewhere) to make the overall negative for the coupled process.
How Cells Maintain Order Day-to-Day
Biological order is dynamic: it requires constant energy expenditure to counteract spontaneous disordering.
Energy Input Must Exceed Energy Loss
Cells maintain order by continuously:
Capturing energy from the environment (light or chemical energy)
Transferring energy through metabolic pathways
Dissipating energy as heat during transformations (inevitable entropy increase)
Exporting waste to prevent internal accumulation of low-energy, high-entropy products
If energy input drops below energy loss over time, ordered structures degrade, gradients collapse, and the organism cannot maintain homeostasis.
FAQ
By exporting entropy to the surroundings.
Heat released from energy transformations increases environmental entropy.
Waste products carry dispersed energy/matter away from the organism.
Molecular energy transfers involve many steps with friction-like losses (random collisions).
Some energy inevitably spreads into random kinetic motion of molecules, which is heat, raising entropy.
Negative $\Delta G$ indicates a process is energetically favourable, not fast.
Reaction rate depends on kinetic factors (e.g., energy barriers and molecular rearrangements), so a favourable process may still proceed very slowly.
Increasing $T$ increases the magnitude of $T\Delta S$.
This can make entropy-increasing processes more favourable at higher temperatures, and entropy-decreasing processes less favourable unless compensated by energy changes elsewhere.
Biological order is local and maintained.
Organisms create structured arrangements (local order) while simultaneously increasing disorder externally through heat and exported products, so total entropy still rises.
Practice Questions
Explain how a cell can become more ordered while still obeying the Second Law of Thermodynamics. (2 marks)
Mentions the cell is an open system that takes in energy/matter (1)
States increased order in the cell is offset by increased entropy of the surroundings (e.g., heat/waste release) (1)
Using the idea of Gibbs free energy, explain why continuous energy input is required to maintain biological order in organisms. (5 marks)
Defines/links to energy available to do work and spontaneity (negative feasible) (1)
States that building/maintaining ordered structures tends to decrease entropy and is not spontaneous alone (1)
Explains that such processes require energy input or coupling to energy-releasing reactions to achieve overall negative (2)
Links to continual energy loss as heat/waste increasing environmental entropy, so ongoing input is needed to prevent decay of order (1)
