How does the coupling of reactions drive metabolic pathways?

The coupling of reactions drives metabolic pathways by using the energy released from exergonic reactions to fuel endergonic reactions.

In metabolic pathways, reactions are often coupled so that a thermodynamically favourable reaction (an exergonic reaction that releases energy) can help to drive a thermodynamically unfavourable reaction (an endergonic reaction that requires energy). This coupling of reactions is a fundamental principle in bioenergetics and is essential for the functioning of all living organisms.

Exergonic reactions, such as the breakdown of glucose in cellular respiration, release energy. This energy is often captured in the form of adenosine triphosphate (ATP), the main energy currency of the cell. ATP is a high-energy molecule that can be used to fuel endergonic reactions, which require an input of energy. This is achieved by the transfer of a phosphate group from ATP to another molecule, a process known as phosphorylation. This transfer of energy allows endergonic reactions to proceed, which would otherwise not occur spontaneously.

The coupling of exergonic and endergonic reactions is facilitated by enzymes, biological catalysts that speed up chemical reactions. Enzymes lower the activation energy of reactions, allowing them to proceed at a faster rate and at lower energy levels. This is crucial in metabolic pathways, where a series of reactions must occur in a specific order to achieve a particular outcome, such as the synthesis of a complex molecule or the breakdown of a nutrient.

In summary, the coupling of reactions is a key mechanism by which metabolic pathways are driven. By harnessing the energy released from exergonic reactions, cells can fuel endergonic reactions and carry out essential metabolic processes. This coupling of reactions, facilitated by enzymes, allows life as we know it to exist and function.