Question

If endergonic reactions are not spontaneous, how do they occur?

Answer 1

Hint : We know that the spontaneous reaction supports the formation of products at the conditions under which the reaction is occurring. Spontaneous reaction supports decrease in energy and an increase in entropy which means that combustion reactions occur spontaneously. It is exothermic (decrease in the energy of the system as energy is released to heat the surrounding). We can take an example of combustion reaction.

Complete Step By Step Answer:
We know about the Gibbs free energy which is a thermodynamic potential that is used to calculate the maximum of reversible work performed by a thermodynamic system at a constant temperature and pressure. Exergonic reactions have a negative release of free energy.
Exergonic means energy is Exiting the system. A negative means that the reactants, or initial state, have more free energy than the products, or final state. So, exergonic reactions are also spontaneous reactions, because they can occur without the addition of energy.
An endergonic reaction occurs by coupling with an even more exergonic reaction.
An exergonic reaction is one in which $\Delta G$ increases, and an endergonic process is one in which $\Delta G$ decreases. The only way that an endergonic reaction can occur spontaneously is if it is coupled with an even more exergonic reaction. The sum of the $\Delta G$ G values of the two reactions is then negative. The overall reaction becomes exergonic and spontaneous. The process is called energy coupling. For example, glutamate and ammonium ions react to form the amino acid glutamine. The reaction is endergonic:
$glutamate+NH_{4}^{-}\to glutamine;\Delta G=+14.2kJ/mol.$ The conversion of ATP to ADP is strongly exergonic: $ATP+HO\to ADP+Pi;~\Delta G~=-30.5\text{ }kJ/mol.$ If we add the two reactions, we get;
$glutamate+NH+ATP+HO\text{ }\to \text{ }glutamine+ADP+~Pi;~\Delta G~=-16.3\text{ }kJ/mol$
In the body, the enzyme glutamine synthetase catalyzes the reaction. It proceeds by way of an intermediate phosphate.

Note :
Note that they have solved this problem by recalling the concepts of the Gibbs free energy. Positive and negative values of shows the type of reaction that is either exergonic reactions or endergonic reactions.