Explain the concept of energy coupling in metabolism with an example.

• A. Energy coupling is the process by which energy released from exergonic reactions powers endergonic processes, such as ATP hydrolysis powering active transport.
• B. Energy coupling occurs only in photosynthesis.
• C. Energy coupling uses energy created from heat to drive chemical reactions.
• D. Energy coupling violates thermodynamics by creating energy.

Answer: (C)

Energy coupling is the way cells use energy released by one reaction to drive another reaction that needs energy. The key idea is that an exergonic process, like ATP hydrolysis, releases free energy that powers an endergonic step, so the combined set of reactions has a negative overall free-energy change and can proceed.

A classic example is ATP hydrolysis powering active transport. When ATP is broken down to ADP and inorganic phosphate, a large amount of energy becomes available. That energy is harnessed by a transporter protein, often through phosphorylation or a conformational change, to move ions across a membrane against their electrochemical gradient. Without this coupling, moving ions uphill would not occur spontaneously.

It’s important to note that energy in biology isn’t sourced from heat. Cells operate at relatively stable temperatures, and heat would simply dissipate rather than provide directed work for specific tasks. Instead, energy is transferred through high-energy molecules like ATP, linking favorable and unfavorable reactions so metabolism can proceed efficiently.