Thermodynamics and Free Energy in Biology

Vocabulary flashcards covering bioenergetics, Gibbs free energy, spontaneity, equilibrium, exergonic and endergonic reactions, types of cellular work, and ATP coupling.

Spontaneous reaction

A chemical reaction that does not require an input of energy in order to occur and is energetically favorable.

Nonspontaneous reaction

A reaction that requires an input of energy in order to occur and cannot proceed to completion without an energy input.

Free energy ($G$)

The portion of a system's energy that is able to perform work when temperature and pressure are uniform and constant throughout the system.

Work

Energy utilized to oppose or act against a contradicting force, such as friction or gravity.

Change in enthalpy ($\Delta H$)

The change in total energy of the system being observed during a reaction.

Change in entropy ($\Delta S$)

The change in the randomness or entropy of the system being observed during a reaction.

Change in free energy ($\Delta G$)

The change calculated as $\Delta H - T\Delta S$ (where $T$ is in Kelvin), or as the free energy of the final state (products) minus the free energy of the initial state (reactants).

System stability

A state that is inversely proportional to free energy; systems with low stability possess high free energy, while systems with high stability possess low free energy.

Chemical equilibrium

The state in a reversible reaction where the rate of the forward reaction equals the rate of the reverse reaction, resulting in maximum stability and essentially nonexistent free energy.

Exergonic reaction

An energy-outward chemical reaction that releases free energy to the surroundings, possesses a negative $\Delta G$, stores less free energy in its products than reactants, and occurs spontaneously.

Endergonic reaction

An energy-inward chemical reaction that absorbs free energy from the surroundings, possesses a positive or zero $\Delta G$, stores more free energy in products than reactants, and is nonspontaneous.

Cellular respiration (energetics)

The catabolic breakdown of one mole of glucose that releases $686\,\text{kcal}$ of free energy available to perform work, leaving the products with $686\,\text{kcal}$ less free energy than the reactants.

Photosynthesis (energetics)

An endergonic process converting carbon dioxide and water into glucose and oxygen, requiring an input of $686\,\text{kcal/mol}$ of free energy derived from sunlight.

Chemical work

A category of cellular work that involves pushing endergonic reactions forward, such as using ATP to drive a nonspontaneous process.

Transport work

Cellular work involving the active pumping of substances across a membrane against their concentration gradient or direction of spontaneous movement.

Mechanical work

Cellular work that generates physical motion, such as the beating of cilia, movement of flagella, or contraction of muscle cells.

Energy coupling

The management of cellular energy resources by using the free energy released from exergonic reactions to drive endergonic reactions.

Adenosine triphosphate (ATP)

A nucleoside triphosphate composed of a ribose sugar, an adenine nitrogenous base, and three phosphate functional groups, used in RNA synthesis and energy coupling.

ATP hydrolysis

The process in which a water molecule breaks the terminal phosphate bond of ATP, producing ADP and an inorganic phosphate while releasing free energy to drive endergonic reactions.