endergonic

Endergonic refers to a type of chemical reaction or process that requires an input of energy to proceed. Thermodynamically, endergonic reactions have a positive Gibbs free energy change (ΔG > 0) under standard conditions. In biological contexts, the actual change in free energy is given by ΔG = ΔG°' + RT ln([products]/[reactants]); endergonic steps typically have a positive ΔG under cellular conditions unless coupled to energy-releasing processes.

In biology, many essential synthetic and biosynthetic processes are endergonic. Examples include the formation of complex molecules such as carbohydrates, proteins, and nucleic acids, as well as the activation of substrates for metabolic pathways. In photosynthesis, for instance, the construction of glucose from carbon dioxide and water requires energy absorption from light. Cellularly, energy is often supplied by energy-rich molecules such as ATP or by reducing equivalents like NADPH to drive these endergonic steps.

Energy coupling is a central theme for enabling endergonic reactions. Cells accomplish this by linking an energetically favorable (exergonic) reaction to an unfavorable one, so the overall process has a negative ΔG. Common coupling partners include ATP hydrolysis (ATP → ADP + Pi) and the transfer of high-energy phosphate groups or electrons in redox reactions. Enzymes can catalyze endergonic steps by lowering the activation energy, increasing rate, but they do not alter the intrinsic ΔG of the overall reaction.

Endergonic is contrasted with exergonic (ΔG < 0), which proceeds spontaneously under the given conditions. A reaction’s classification depends on ΔG under the relevant conditions, not merely on whether it occurs quickly.