Final answer:
Energetically unfavourable reactions can proceed when coupled with energetically favourable reactions, often using the energy released from ATP hydrolysis or similar exergonic processes to drive endergonic reactions in biological systems.
Step-by-step explanation:
Coupling energetically unfavourable reactions with energetically favourable reactions yields an overall reaction where the net energy change can be negative, making the coupled process more likely to occur. This is because the energetically favourable reaction releases enough energy to drive the energetically unfavourable reaction forward. This process is a fundamental concept in metabolic pathways wherein ATP hydrolysis is often used to drive other chemical reactions that would not otherwise occur spontaneously.
For example, in cellular respiration, the production of ATP is an energetically favourable reaction that is often coupled with the energy-requiring process of synthesizing large biomolecules. Similarly, the breakdown of glucose is an exergonic process (releases energy) that helps to drive the endergonic reactions (requires energy) within the cell.